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| -rw-r--r-- | Documentation/filesystems/00-INDEX | 4 | ||||
| -rw-r--r-- | Documentation/filesystems/relay.txt | 479 | ||||
| -rw-r--r-- | Documentation/filesystems/relayfs.txt | 442 |
3 files changed, 481 insertions, 444 deletions
diff --git a/Documentation/filesystems/00-INDEX b/Documentation/filesystems/00-INDEX index 66fdc0744fe0..16dec61d7671 100644 --- a/Documentation/filesystems/00-INDEX +++ b/Documentation/filesystems/00-INDEX @@ -62,8 +62,8 @@ ramfs-rootfs-initramfs.txt - info on the 'in memory' filesystems ramfs, rootfs and initramfs. reiser4.txt - info on the Reiser4 filesystem based on dancing tree algorithms. -relayfs.txt - - info on relayfs, for efficient streaming from kernel to user space. +relay.txt + - info on relay, for efficient streaming from kernel to user space. romfs.txt - description of the ROMFS filesystem. smbfs.txt diff --git a/Documentation/filesystems/relay.txt b/Documentation/filesystems/relay.txt new file mode 100644 index 000000000000..d6788dae0349 --- /dev/null +++ b/Documentation/filesystems/relay.txt @@ -0,0 +1,479 @@ +relay interface (formerly relayfs) +================================== + +The relay interface provides a means for kernel applications to +efficiently log and transfer large quantities of data from the kernel +to userspace via user-defined 'relay channels'. + +A 'relay channel' is a kernel->user data relay mechanism implemented +as a set of per-cpu kernel buffers ('channel buffers'), each +represented as a regular file ('relay file') in user space. Kernel +clients write into the channel buffers using efficient write +functions; these automatically log into the current cpu's channel +buffer. User space applications mmap() or read() from the relay files +and retrieve the data as it becomes available. The relay files +themselves are files created in a host filesystem, e.g. debugfs, and +are associated with the channel buffers using the API described below. + +The format of the data logged into the channel buffers is completely +up to the kernel client; the relay interface does however provide +hooks which allow kernel clients to impose some structure on the +buffer data. The relay interface doesn't implement any form of data +filtering - this also is left to the kernel client. The purpose is to +keep things as simple as possible. + +This document provides an overview of the relay interface API. The +details of the function parameters are documented along with the +functions in the relay interface code - please see that for details. + +Semantics +========= + +Each relay channel has one buffer per CPU, each buffer has one or more +sub-buffers. Messages are written to the first sub-buffer until it is +too full to contain a new message, in which case it it is written to +the next (if available). Messages are never split across sub-buffers. +At this point, userspace can be notified so it empties the first +sub-buffer, while the kernel continues writing to the next. + +When notified that a sub-buffer is full, the kernel knows how many +bytes of it are padding i.e. unused space occurring because a complete +message couldn't fit into a sub-buffer. Userspace can use this +knowledge to copy only valid data. + +After copying it, userspace can notify the kernel that a sub-buffer +has been consumed. + +A relay channel can operate in a mode where it will overwrite data not +yet collected by userspace, and not wait for it to be consumed. + +The relay channel itself does not provide for communication of such +data between userspace and kernel, allowing the kernel side to remain +simple and not impose a single interface on userspace. It does +provide a set of examples and a separate helper though, described +below. + +The read() interface both removes padding and internally consumes the +read sub-buffers; thus in cases where read(2) is being used to drain +the channel buffers, special-purpose communication between kernel and +user isn't necessary for basic operation. + +One of the major goals of the relay interface is to provide a low +overhead mechanism for conveying kernel data to userspace. While the +read() interface is easy to use, it's not as efficient as the mmap() +approach; the example code attempts to make the tradeoff between the +two approaches as small as possible. + +klog and relay-apps example code +================================ + +The relay interface itself is ready to use, but to make things easier, +a couple simple utility functions and a set of examples are provided. + +The relay-apps example tarball, available on the relay sourceforge +site, contains a set of self-contained examples, each consisting of a +pair of .c files containing boilerplate code for each of the user and +kernel sides of a relay application. When combined these two sets of +boilerplate code provide glue to easily stream data to disk, without +having to bother with mundane housekeeping chores. + +The 'klog debugging functions' patch (klog.patch in the relay-apps +tarball) provides a couple of high-level logging functions to the +kernel which allow writing formatted text or raw data to a channel, +regardless of whether a channel to write into exists or not, or even +whether the relay interface is compiled into the kernel or not. These +functions allow you to put unconditional 'trace' statements anywhere +in the kernel or kernel modules; only when there is a 'klog handler' +registered will data actually be logged (see the klog and kleak +examples for details). + +It is of course possible to use the relay interface from scratch, +i.e. without using any of the relay-apps example code or klog, but +you'll have to implement communication between userspace and kernel, +allowing both to convey the state of buffers (full, empty, amount of +padding). The read() interface both removes padding and internally +consumes the read sub-buffers; thus in cases where read(2) is being +used to drain the channel buffers, special-purpose communication +between kernel and user isn't necessary for basic operation. Things +such as buffer-full conditions would still need to be communicated via +some channel though. + +klog and the relay-apps examples can be found in the relay-apps +tarball on http://relayfs.sourceforge.net + +The relay interface user space API +================================== + +The relay interface implements basic file operations for user space +access to relay channel buffer data. Here are the file operations +that are available and some comments regarding their behavior: + +open() enables user to open an _existing_ channel buffer. + +mmap() results in channel buffer being mapped into the caller's + memory space. Note that you can't do a partial mmap - you + must map the entire file, which is NRBUF * SUBBUFSIZE. + +read() read the contents of a channel buffer. The bytes read are + 'consumed' by the reader, i.e. they won't be available + again to subsequent reads. If the channel is being used + in no-overwrite mode (the default), it can be read at any + time even if there's an active kernel writer. If the + channel is being used in overwrite mode and there are + active channel writers, results may be unpredictable - + users should make sure that all logging to the channel has + ended before using read() with overwrite mode. Sub-buffer + padding is automatically removed and will not be seen by + the reader. + +sendfile() transfer data from a channel buffer to an output file + descriptor. Sub-buffer padding is automatically removed + and will not be seen by the reader. + +poll() POLLIN/POLLRDNORM/POLLERR supported. User applications are + notified when sub-buffer boundaries are crossed. + +close() decrements the channel buffer's refcount. When the refcount + reaches 0, i.e. when no process or kernel client has the + buffer open, the channel buffer is freed. + +In order for a user application to make use of relay files, the +host filesystem must be mounted. For example, + + mount -t debugfs debugfs /debug + +NOTE: the host filesystem doesn't need to be mounted for kernel + clients to create or use channels - it only needs to be + mounted when user space applications need access to the buffer + data. + + +The relay interface kernel API +============================== + +Here's a summary of the API the relay interface provides to in-kernel clients: + +TBD(curr. line MT:/API/) + channel management functions: + + relay_open(base_filename, parent, subbuf_size, n_subbufs, + callbacks) + relay_close(chan) + relay_flush(chan) + relay_reset(chan) + + channel management typically called on instigation of userspace: + + relay_subbufs_consumed(chan, cpu, subbufs_consumed) + + write functions: + + relay_write(chan, data, length) + __relay_write(chan, data, length) + relay_reserve(chan, length) + + callbacks: + + subbuf_start(buf, subbuf, prev_subbuf, prev_padding) + buf_mapped(buf, filp) + buf_unmapped(buf, filp) + create_buf_file(filename, parent, mode, buf, is_global) + remove_buf_file(dentry) + + helper functions: + + relay_buf_full(buf) + subbuf_start_reserve(buf, length) + + +Creating a channel +------------------ + +relay_open() is used to create a channel, along with its per-cpu +channel buffers. Each channel buffer will have an associated file +created for it in the host filesystem, which can be and mmapped or +read from in user space. The files are named basename0...basenameN-1 +where N is the number of online cpus, and by default will be created +in the root of the filesystem (if the parent param is NULL). If you +want a directory structure to contain your relay files, you should +create it using the host filesystem's directory creation function, +e.g. debugfs_create_dir(), and pass the parent directory to +relay_open(). Users are responsible for cleaning up any directory +structure they create, when the channel is closed - again the host +filesystem's directory removal functions should be used for that, +e.g. debugfs_remove(). + +In order for a channel to be created and the host filesystem's files +associated with its channel buffers, the user must provide definitions +for two callback functions, create_buf_file() and remove_buf_file(). +create_buf_file() is called once for each per-cpu buffer from +relay_open() and allows the user to create the file which will be used +to represent the corresponding channel buffer. The callback should +return the dentry of the file created to represent the channel buffer. +remove_buf_file() must also be defined; it's responsible for deleting +the file(s) created in create_buf_file() and is called during +relay_close(). + +Here are some typical definitions for these callbacks, in this case +using debugfs: + +/* + * create_buf_file() callback. Creates relay file in debugfs. + */ +static struct dentry *create_buf_file_handler(const char *filename, + struct dentry *parent, + int mode, + struct rchan_buf *buf, + int *is_global) +{ + return debugfs_create_file(filename, mode, parent, buf, + &relay_file_operations); +} + +/* + * remove_buf_file() callback. Removes relay file from debugfs. + */ +static int remove_buf_file_handler(struct dentry *dentry) +{ + debugfs_remove(dentry); + + return 0; +} + +/* + * relay interface callbacks + */ +static struct rchan_callbacks relay_callbacks = +{ + .create_buf_file = create_buf_file_handler, + .remove_buf_file = remove_buf_file_handler, +}; + +And an example relay_open() invocation using them: + + chan = relay_open("cpu", NULL, SUBBUF_SIZE, N_SUBBUFS, &relay_callbacks); + +If the create_buf_file() callback fails, or isn't defined, channel +creation and thus relay_open() will fail. + +The total size of each per-cpu buffer is calculated by multiplying the +number of sub-buffers by the sub-buffer size passed into relay_open(). +The idea behind sub-buffers is that they're basically an extension of +double-buffering to N buffers, and they also allow applications to +easily implement random-access-on-buffer-boundary schemes, which can +be important for some high-volume applications. The number and size +of sub-buffers is completely dependent on the application and even for +the same application, different conditions will warrant different +values for these parameters at different times. Typically, the right +values to use are best decided after some experimentation; in general, +though, it's safe to assume that having only 1 sub-buffer is a bad +idea - you're guaranteed to either overwrite data or lose events +depending on the channel mode being used. + +The create_buf_file() implementation can also be defined in such a way +as to allow the creation of a single 'global' buffer instead of the +default per-cpu set. This can be useful for applications interested +mainly in seeing the relative ordering of system-wide events without +the need to bother with saving explicit timestamps for the purpose of +merging/sorting per-cpu files in a postprocessing step. + +To have relay_open() create a global buffer, the create_buf_file() +implementation should set the value of the is_global outparam to a +non-zero value in addition to creating the file that will be used to +represent the single buffer. In the case of a global buffer, +create_buf_file() and remove_buf_file() will be called only once. The +normal channel-writing functions, e.g. relay_write(), can still be +used - writes from any cpu will transparently end up in the global +buffer - but since it is a global buffer, callers should make sure +they use the proper locking for such a buffer, either by wrapping +writes in a spinlock, or by copying a write function from relay.h and +creating a local version that internally does the proper locking. + +Channel 'modes' +--------------- + +relay channels can be used in either of two modes - 'overwrite' or +'no-overwrite'. The mode is entirely determined by the implementation +of the subbuf_start() callback, as described below. The default if no +subbuf_start() callback is defined is 'no-overwrite' mode. If the +default mode suits your needs, and you plan to use the read() +interface to retrieve channel data, you can ignore the details of this +section, as it pertains mainly to mmap() implementations. + +In 'overwrite' mode, also known as 'flight recorder' mode, writes +continuously cycle around the buffer and will never fail, but will +unconditionally overwrite old data regardless of whether it's actually +been consumed. In no-overwrite mode, writes will fail, i.e. data will +be lost, if the number of unconsumed sub-buffers equals the total +number of sub-buffers in the channel. It should be clear that if +there is no consumer or if the consumer can't consume sub-buffers fast +enough, data will be lost in either case; the only difference is +whether data is lost from the beginning or the end of a buffer. + +As explained above, a relay channel is made of up one or more +per-cpu channel buffers, each implemented as a circular buffer +subdivided into one or more sub-buffers. Messages are written into +the current sub-buffer of the channel's current per-cpu buffer via the +write functions described below. Whenever a message can't fit into +the current sub-buffer, because there's no room left for it, the +client is notified via the subbuf_start() callback that a switch to a +new sub-buffer is about to occur. The client uses this callback to 1) +initialize the next sub-buffer if appropriate 2) finalize the previous +sub-buffer if appropriate and 3) return a boolean value indicating +whether or not to actually move on to the next sub-buffer. + +To implement 'no-overwrite' mode, the userspace client would provide +an implementation of the subbuf_start() callback something like the +following: + +static int subbuf_start(struct rchan_buf *buf, + void *subbuf, + void *prev_subbuf, + unsigned int prev_padding) +{ + if (prev_subbuf) + *((unsigned *)prev_subbuf) = prev_padding; + + if (relay_buf_full(buf)) + return 0; + + subbuf_start_reserve(buf, sizeof(unsigned int)); + + return 1; +} + +If the current buffer is full, i.e. all sub-buffers remain unconsumed, +the callback returns 0 to indicate that the buffer switch should not +occur yet, i.e. until the consumer has had a chance to read the +current set of ready sub-buffers. For the relay_buf_full() function +to make sense, the consumer is reponsible for notifying the relay +interface when sub-buffers have been consumed via +relay_subbufs_consumed(). Any subsequent attempts to write into the +buffer will again invoke the subbuf_start() callback with the same +parameters; only when the consumer has consumed one or more of the +ready sub-buffers will relay_buf_full() return 0, in which case the +buffer switch can continue. + +The implementation of the subbuf_start() callback for 'overwrite' mode +would be very similar: + +static int subbuf_start(struct rchan_buf *buf, + void *subbuf, + void *prev_subbuf, + unsigned int prev_padding) +{ + if (prev_subbuf) + *((unsigned *)prev_subbuf) = prev_padding; + + subbuf_start_reserve(buf, sizeof(unsigned int)); + + return 1; +} + +In this case, the relay_buf_full() check is meaningless and the +callback always returns 1, causing the buffer switch to occur +unconditionally. It's also meaningless for the client to use the +relay_subbufs_consumed() function in this mode, as it's never +consulted. + +The default subbuf_start() implementation, used if the client doesn't +define any callbacks, or doesn't define the subbuf_start() callback, +implements the simplest possible 'no-overwrite' mode, i.e. it does +nothing but return 0. + +Header information can be reserved at the beginning of each sub-buffer +by calling the subbuf_start_reserve() helper function from within the +subbuf_start() callback. This reserved area can be used to store +whatever information the client wants. In the example above, room is +reserved in each sub-buffer to store the padding count for that +sub-buffer. This is filled in for the previous sub-buffer in the +subbuf_start() implementation; the padding value for the previous +sub-buffer is passed into the subbuf_start() callback along with a +pointer to the previous sub-buffer, since the padding value isn't +known until a sub-buffer is filled. The subbuf_start() callback is +also called for the first sub-buffer when the channel is opened, to +give the client a chance to reserve space in it. In this case the +previous sub-buffer pointer passed into the callback will be NULL, so +the client should check the value of the prev_subbuf pointer before +writing into the previous sub-buffer. + +Writing to a channel +-------------------- + +Kernel clients write data into the current cpu's channel buffer using +relay_write() or __relay_write(). relay_write() is the main logging +function - it uses local_irqsave() to protect the buffer and should be +used if you might be logging from interrupt context. If you know +you'll never be logging from interrupt context, you can use +__relay_write(), which only disables preemption. These functions +don't return a value, so you can't determine whether or not they +failed - the assumption is that you wouldn't want to check a return +value in the fast logging path anyway, and that they'll always succeed +unless the buffer is full and no-overwrite mode is being used, in +which case you can detect a failed write in the subbuf_start() +callback by calling the relay_buf_full() helper function. + +relay_reserve() is used to reserve a slot in a channel buffer which +can be written to later. This would typically be used in applications +that need to write directly into a channel buffer without having to +stage data in a temporary buffer beforehand. Because the actual write +may not happen immediately after the slot is reserved, applications +using relay_reserve() can keep a count of the number of bytes actually +written, either in space reserved in the sub-buffers themselves or as +a separate array. See the 'reserve' example in the relay-apps tarball +at http://relayfs.sourceforge.net for an example of how this can be +done. Because the write is under control of the client and is +separated from the reserve, relay_reserve() doesn't protect the buffer +at all - it's up to the client to provide the appropriate +synchronization when using relay_reserve(). + +Closing a channel +----------------- + +The client calls relay_close() when it's finished using the channel. +The channel and its associated buffers are destroyed when there are no +longer any references to any of the channel buffers. relay_flush() +forces a sub-buffer switch on all the channel buffers, and can be used +to finalize and process the last sub-buffers before the channel is +closed. + +Misc +---- + +Some applications may want to keep a channel around and re-use it +rather than open and close a new channel for each use. relay_reset() +can be used for this purpose - it resets a channel to its initial +state without reallocating channel buffer memory or destroying +existing mappings. It should however only be called when it's safe to +do so, i.e. when the channel isn't currently being written to. + +Finally, there are a couple of utility callbacks that can be used for +different purposes. buf_mapped() is called whenever a channel buffer +is mmapped from user space and buf_unmapped() is called when it's +unmapped. The client can use this notification to trigger actions +within the kernel application, such as enabling/disabling logging to +the channel. + + +Resources +========= + +For news, example code, mailing list, etc. see the relay interface homepage: + + http://relayfs.sourceforge.net + + +Credits +======= + +The ideas and specs for the relay interface came about as a result of +discussions on tracing involving the following: + +Michel Dagenais <michel.dagenais@polymtl.ca> +Richard Moore <richardj_moore@uk.ibm.com> +Bob Wisniewski <bob@watson.ibm.com> +Karim Yaghmour <karim@opersys.com> +Tom Zanussi <zanussi@us.ibm.com> + +Also thanks to Hubertus Franke for a lot of useful suggestions and bug +reports. diff --git a/Documentation/filesystems/relayfs.txt b/Documentation/filesystems/relayfs.txt deleted file mode 100644 index 5832377b7340..000000000000 --- a/Documentation/filesystems/relayfs.txt +++ /dev/null @@ -1,442 +0,0 @@ - -relayfs - a high-speed data relay filesystem -============================================ - -relayfs is a filesystem designed to provide an efficient mechanism for -tools and facilities to relay large and potentially sustained streams -of data from kernel space to user space. - -The main abstraction of relayfs is the 'channel'. A channel consists -of a set of per-cpu kernel buffers each represented by a file in the -relayfs filesystem. Kernel clients write into a channel using -efficient write functions which automatically log to the current cpu's -channel buffer. User space applications mmap() the per-cpu files and -retrieve the data as it becomes available. - -The format of the data logged into the channel buffers is completely -up to the relayfs client; relayfs does however provide hooks which -allow clients to impose some structure on the buffer data. Nor does -relayfs implement any form of data filtering - this also is left to -the client. The purpose is to keep relayfs as simple as possible. - -This document provides an overview of the relayfs API. The details of -the function parameters are documented along with the functions in the -filesystem code - please see that for details. - -Semantics -========= - -Each relayfs channel has one buffer per CPU, each buffer has one or -more sub-buffers. Messages are written to the first sub-buffer until -it is too full to contain a new message, in which case it it is -written to the next (if available). Messages are never split across -sub-buffers. At this point, userspace can be notified so it empties -the first sub-buffer, while the kernel continues writing to the next. - -When notified that a sub-buffer is full, the kernel knows how many -bytes of it are padding i.e. unused. Userspace can use this knowledge -to copy only valid data. - -After copying it, userspace can notify the kernel that a sub-buffer -has been consumed. - -relayfs can operate in a mode where it will overwrite data not yet -collected by userspace, and not wait for it to consume it. - -relayfs itself does not provide for communication of such data between -userspace and kernel, allowing the kernel side to remain simple and -not impose a single interface on userspace. It does provide a set of -examples and a separate helper though, described below. - -klog and relay-apps example code -================================ - -relayfs itself is ready to use, but to make things easier, a couple -simple utility functions and a set of examples are provided. - -The relay-apps example tarball, available on the relayfs sourceforge -site, contains a set of self-contained examples, each consisting of a -pair of .c files containing boilerplate code for each of the user and -kernel sides of a relayfs application; combined these two sets of -boilerplate code provide glue to easily stream data to disk, without -having to bother with mundane housekeeping chores. - -The 'klog debugging functions' patch (klog.patch in the relay-apps -tarball) provides a couple of high-level logging functions to the -kernel which allow writing formatted text or raw data to a channel, -regardless of whether a channel to write into exists or not, or -whether relayfs is compiled into the kernel or is configured as a -module. These functions allow you to put unconditional 'trace' -statements anywhere in the kernel or kernel modules; only when there -is a 'klog handler' registered will data actually be logged (see the -klog and kleak examples for details). - -It is of course possible to use relayfs from scratch i.e. without -using any of the relay-apps example code or klog, but you'll have to -implement communication between userspace and kernel, allowing both to -convey the state of buffers (full, empty, amount of padding). - -klog and the relay-apps examples can be found in the relay-apps -tarball on http://relayfs.sourceforge.net - - -The relayfs user space API -========================== - -relayfs implements basic file operations for user space access to -relayfs channel buffer data. Here are the file operations that are -available and some comments regarding their behavior: - -open() enables user to open an _existing_ buffer. - -mmap() results in channel buffer being mapped into the caller's - memory space. Note that you can't do a partial mmap - you must - map the entire file, which is NRBUF * SUBBUFSIZE. - -read() read the contents of a channel buffer. The bytes read are - 'consumed' by the reader i.e. they won't be available again - to subsequent reads. If the channel is being used in - no-overwrite mode (the default), it can be read at any time - even if there's an active kernel writer. If the channel is - being used in overwrite mode and there are active channel - writers, results may be unpredictable - users should make - sure that all logging to the channel has ended before using - read() with overwrite mode. - -poll() POLLIN/POLLRDNORM/POLLERR supported. User applications are - notified when sub-buffer boundaries are crossed. - -close() decrements the channel buffer's refcount. When the refcount - reaches 0 i.e. when no process or kernel client has the buffer - open, the channel buffer is freed. - - -In order for a user application to make use of relayfs files, the -relayfs filesystem must be mounted. For example, - - mount -t relayfs relayfs /mnt/relay - -NOTE: relayfs doesn't need to be mounted for kernel clients to create - or use channels - it only needs to be mounted when user space - applications need access to the buffer data. - - -The relayfs kernel API -====================== - -Here's a summary of the API relayfs provides to in-kernel clients: - - - channel management functions: - - relay_open(base_filename, parent, subbuf_size, n_subbufs, - callbacks) - relay_close(chan) - relay_flush(chan) - relay_reset(chan) - relayfs_create_dir(name, parent) - relayfs_remove_dir(dentry) - relayfs_create_file(name, parent, mode, fops, data) - relayfs_remove_file(dentry) - - channel management typically called on instigation of userspace: - - relay_subbufs_consumed(chan, cpu, subbufs_consumed) - - write functions: - - relay_write(chan, data, length) - __relay_write(chan, data, length) - relay_reserve(chan, length) - - callbacks: - - subbuf_start(buf, subbuf, prev_subbuf, prev_padding) - buf_mapped(buf, filp) - buf_unmapped(buf, filp) - create_buf_file(filename, parent, mode, buf, is_global) - remove_buf_file(dentry) - - helper functions: - - relay_buf_full(buf) - subbuf_start_reserve(buf, length) - - -Creating a channel ------------------- - -relay_open() is used to create a channel, along with its per-cpu -channel buffers. Each channel buffer will have an associated file -created for it in the relayfs filesystem, which can be opened and -mmapped from user space if desired. The files are named -basename0...basenameN-1 where N is the number of online cpus, and by -default will be created in the root of the filesystem. If you want a -directory structure to contain your relayfs files, you can create it -with relayfs_create_dir() and pass the parent directory to -relay_open(). Clients are responsible for cleaning up any directory -structure they create when the channel is closed - use -relayfs_remove_dir() for that. - -The total size of each per-cpu buffer is calculated by multiplying the -number of sub-buffers by the sub-buffer size passed into relay_open(). -The idea behind sub-buffers is that they're basically an extension of -double-buffering to N buffers, and they also allow applications to -easily implement random-access-on-buffer-boundary schemes, which can -be important for some high-volume applications. The number and size -of sub-buffers is completely dependent on the application and even for -the same application, different conditions will warrant different -values for these parameters at different times. Typically, the right -values to use are best decided after some experimentation; in general, -though, it's safe to assume that having only 1 sub-buffer is a bad -idea - you're guaranteed to either overwrite data or lose events -depending on the channel mode being used. - -Channel 'modes' ---------------- - -relayfs channels can be used in either of two modes - 'overwrite' or -'no-overwrite'. The mode is entirely determined by the implementation -of the subbuf_start() callback, as described below. In 'overwrite' -mode, also known as 'flight recorder' mode, writes continuously cycle -around the buffer and will never fail, but will unconditionally -overwrite old data regardless of whether it's actually been consumed. -In no-overwrite mode, writes will fail i.e. data will be lost, if the -number of unconsumed sub-buffers equals the total number of -sub-buffers in the channel. It should be clear that if there is no -consumer or if the consumer can't consume sub-buffers fast enought, -data will be lost in either case; the only difference is whether data -is lost from the beginning or the end of a buffer. - -As explained above, a relayfs channel is made of up one or more -per-cpu channel buffers, each implemented as a circular buffer -subdivided into one or more sub-buffers. Messages are written into -the current sub-buffer of the channel's current per-cpu buffer via the -write functions described below. Whenever a message can't fit into -the current sub-buffer, because there's no room left for it, the -client is notified via the subbuf_start() callback that a switch to a -new sub-buffer is about to occur. The client uses this callback to 1) -initialize the next sub-buffer if appropriate 2) finalize the previous -sub-buffer if appropriate and 3) return a boolean value indicating -whether or not to actually go ahead with the sub-buffer switch. - -To implement 'no-overwrite' mode, the userspace client would provide -an implementation of the subbuf_start() callback something like the -following: - -static int subbuf_start(struct rchan_buf *buf, - void *subbuf, - void *prev_subbuf, - unsigned int prev_padding) -{ - if (prev_subbuf) - *((unsigned *)prev_subbuf) = prev_padding; - - if (relay_buf_full(buf)) - return 0; - - subbuf_start_reserve(buf, sizeof(unsigned int)); - - return 1; -} - -If the current buffer is full i.e. all sub-buffers remain unconsumed, -the callback returns 0 to indicate that the buffer switch should not -occur yet i.e. until the consumer has had a chance to read the current -set of ready sub-buffers. For the relay_buf_full() function to make -sense, the consumer is reponsible for notifying relayfs when -sub-buffers have been consumed via relay_subbufs_consumed(). Any -subsequent attempts to write into the buffer will again invoke the -subbuf_start() callback with the same parameters; only when the -consumer has consumed one or more of the ready sub-buffers will -relay_buf_full() return 0, in which case the buffer switch can -continue. - -The implementation of the subbuf_start() callback for 'overwrite' mode -would be very similar: - -static int subbuf_start(struct rchan_buf *buf, - void *subbuf, - void *prev_subbuf, - unsigned int prev_padding) -{ - if (prev_subbuf) - *((unsigned *)prev_subbuf) = prev_padding; - - subbuf_start_reserve(buf, sizeof(unsigned int)); - - return 1; -} - -In this case, the relay_buf_full() check is meaningless and the -callback always returns 1, causing the buffer switch to occur -unconditionally. It's also meaningless for the client to use the -relay_subbufs_consumed() function in this mode, as it's never -consulted. - -The default subbuf_start() implementation, used if the client doesn't -define any callbacks, or doesn't define the subbuf_start() callback, -implements the simplest possible 'no-overwrite' mode i.e. it does -nothing but return 0. - -Header information can be reserved at the beginning of each sub-buffer -by calling the subbuf_start_reserve() helper function from within the -subbuf_start() callback. This reserved area can be used to store -whatever information the client wants. In the example above, room is -reserved in each sub-buffer to store the padding count for that -sub-buffer. This is filled in for the previous sub-buffer in the -subbuf_start() implementation; the padding value for the previous -sub-buffer is passed into the subbuf_start() callback along with a -pointer to the previous sub-buffer, since the padding value isn't -known until a sub-buffer is filled. The subbuf_start() callback is -also called for the first sub-buffer when the channel is opened, to -give the client a chance to reserve space in it. In this case the -previous sub-buffer pointer passed into the callback will be NULL, so -the client should check the value of the prev_subbuf pointer before -writing into the previous sub-buffer. - -Writing to a channel --------------------- - -kernel clients write data into the current cpu's channel buffer using -relay_write() or __relay_write(). relay_write() is the main logging -function - it uses local_irqsave() to protect the buffer and should be -used if you might be logging from interrupt context. If you know -you'll never be logging from interrupt context, you can use -__relay_write(), which only disables preemption. These functions -don't return a value, so you can't determine whether or not they -failed - the assumption is that you wouldn't want to check a return -value in the fast logging path anyway, and that they'll always succeed -unless the buffer is full and no-overwrite mode is being used, in -which case you can detect a failed write in the subbuf_start() -callback by calling the relay_buf_full() helper function. - -relay_reserve() is used to reserve a slot in a channel buffer which -can be written to later. This would typically be used in applications -that need to write directly into a channel buffer without having to -stage data in a temporary buffer beforehand. Because the actual write -may not happen immediately after the slot is reserved, applications -using relay_reserve() can keep a count of the number of bytes actually -written, either in space reserved in the sub-buffers themselves or as -a separate array. See the 'reserve' example in the relay-apps tarball -at http://relayfs.sourceforge.net for an example of how this can be -done. Because the write is under control of the client and is -separated from the reserve, relay_reserve() doesn't protect the buffer -at all - it's up to the client to provide the appropriate -synchronization when using relay_reserve(). - -Closing a channel ------------------ - -The client calls relay_close() when it's finished using the channel. -The channel and its associated buffers are destroyed when there are no -longer any references to any of the channel buffers. relay_flush() -forces a sub-buffer switch on all the channel buffers, and can be used -to finalize and process the last sub-buffers before the channel is -closed. - -Creating non-relay files ------------------------- - -relay_open() automatically creates files in the relayfs filesystem to -represent the per-cpu kernel buffers; it's often useful for -applications to be able to create their own files alongside the relay -files in the relayfs filesystem as well e.g. 'control' files much like -those created in /proc or debugfs for similar purposes, used to -communicate control information between the kernel and user sides of a -relayfs application. For this purpose the relayfs_create_file() and -relayfs_remove_file() API functions exist. For relayfs_create_file(), -the caller passes in a set of user-defined file operations to be used -for the file and an optional void * to a user-specified data item, -which will be accessible via inode->u.generic_ip (see the relay-apps -tarball for examples). The file_operations are a required parameter -to relayfs_create_file() and thus the semantics of these files are -completely defined by the caller. - -See the relay-apps tarball at http://relayfs.sourceforge.net for -examples of how these non-relay files are meant to be used. - -Creating relay files in other filesystems ------------------------------------------ - -By default of course, relay_open() creates relay files in the relayfs -filesystem. Because relay_file_operations is exported, however, it's -also possible to create and use relay files in other pseudo-filesytems -such as debugfs. - -For this purpose, two callback functions are provided, -create_buf_file() and remove_buf_file(). create_buf_file() is called -once for each per-cpu buffer from relay_open() to allow the client to -create a file to be used to represent the corresponding buffer; if -this callback is not defined, the default implementation will create -and return a file in the relayfs filesystem to represent the buffer. -The callback should return the dentry of the file created to represent -the relay buffer. Note that the parent directory passed to -relay_open() (and passed along to the callback), if specified, must -exist in the same filesystem the new relay file is created in. If -create_buf_file() is defined, remove_buf_file() must also be defined; -it's responsible for deleting the file(s) created in create_buf_file() -and is called during relay_close(). - -The create_buf_file() implementation can also be defined in such a way -as to allow the creation of a single 'global' buffer instead of the -default per-cpu set. This can be useful for applications interested -mainly in seeing the relative ordering of system-wide events without -the need to bother with saving explicit timestamps for the purpose of -merging/sorting per-cpu files in a postprocessing step. - -To have relay_open() create a global buffer, the create_buf_file() -implementation should set the value of the is_global outparam to a -non-zero value in addition to creating the file that will be used to -represent the single buffer. In the case of a global buffer, -create_buf_file() and remove_buf_file() will be called only once. The -normal channel-writing functions e.g. relay_write() can still be used -- writes from any cpu will transparently end up in the global buffer - -but since it is a global buffer, callers should make sure they use the -proper locking for such a buffer, either by wrapping writes in a -spinlock, or by copying a write function from relayfs_fs.h and -creating a local version that internally does the proper locking. - -See the 'exported-relayfile' examples in the relay-apps tarball for -examples of creating and using relay files in debugfs. - -Misc ----- - -Some applications may want to keep a channel around and re-use it -rather than open and close a new channel for each use. relay_reset() -can be used for this purpose - it resets a channel to its initial -state without reallocating channel buffer memory or destroying -existing mappings. It should however only be called when it's safe to -do so i.e. when the channel isn't currently being written to. - -Finally, there are a couple of utility callbacks that can be used for -different purposes. buf_mapped() is called whenever a channel buffer -is mmapped from user space and buf_unmapped() is called when it's -unmapped. The client can use this notification to trigger actions -within the kernel application, such as enabling/disabling logging to -the channel. - - -Resources -========= - -For news, example code, mailing list, etc. see the relayfs homepage: - - http://relayfs.sourceforge.net - - -Credits -======= - -The ideas and specs for relayfs came about as a result of discussions -on tracing involving the following: - -Michel Dagenais <michel.dagenais@polymtl.ca> -Richard Moore <richardj_moore@uk.ibm.com> -Bob Wisniewski <bob@watson.ibm.com> -Karim Yaghmour <karim@opersys.com> -Tom Zanussi <zanussi@us.ibm.com> - -Also thanks to Hubertus Franke for a lot of useful suggestions and bug -reports. |