lib/bitmap.c: enhance bitmap syntax

Today there are platforms with many CPUs (up to 4K).  Trying to boot only
part of the CPUs may result in too long string.

For example lets take NPS platform that is part of arch/arc.  This
platform have SMP system with 256 cores each with 16 HW threads (SMT
machine) where HW thread appears as CPU to the kernel.  In this example
there is total of 4K CPUs.  When one tries to boot only part of the HW
threads from each core the string representing the map may be long...  For
example if for sake of performance we decided to boot only first half of
HW threads of each core the map will look like:
0-7,16-23,32-39,...,4080-4087

This patch introduce new syntax to accommodate with such use case.  I
added an optional postfix to a range of CPUs which will choose according
to given modulo the desired range of reminders i.e.:

    <cpus range>:sed_size/group_size

For example, above map can be described in new syntax like this:
0-4095:8/16

Note that this patch is backward compatible with current syntax.

[akpm@linux-foundation.org: rework documentation]
Link: http://lkml.kernel.org/r/1473579629-4283-1-git-send-email-noamca@mellanox.com
Signed-off-by: Noam Camus <noamca@mellanox.com>
Cc: David Decotigny <decot@googlers.com>
Cc: Ben Hutchings <ben@decadent.org.uk>
Cc: David S. Miller <davem@davemloft.net>
Cc: Pan Xinhui <xinhui@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 36 insertions, 14 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 705fb915cbf7..a1489e14f8ee 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -33,6 +33,37 @@ can also be entered as
 Double-quotes can be used to protect spaces in values, e.g.:
 	param="spaces in here"
 
+cpu lists:
+----------
+
+Some kernel parameters take a list of CPUs as a value, e.g.  isolcpus,
+nohz_full, irqaffinity, rcu_nocbs.  The format of this list is:
+
+	<cpu number>,...,<cpu number>
+
+or
+
+	<cpu number>-<cpu number>
+	(must be a positive range in ascending order)
+
+or a mixture
+
+<cpu number>,...,<cpu number>-<cpu number>
+
+Note that for the special case of a range one can split the range into equal
+sized groups and for each group use some amount from the beginning of that
+group:
+
+	<cpu number>-cpu number>:<used size>/<group size>
+
+For example one can add to the command line following parameter:
+
+	isolcpus=1,2,10-20,100-2000:2/25
+
+where the final item represents CPUs 100,101,125,126,150,151,...
+
+
+
 This document may not be entirely up to date and comprehensive. The command
 "modinfo -p ${modulename}" shows a current list of all parameters of a loadable
 module. Loadable modules, after being loaded into the running kernel, also
@@ -1789,13 +1820,7 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			See Documentation/filesystems/nfs/nfsroot.txt.
 
 	irqaffinity=	[SMP] Set the default irq affinity mask
-			Format:
-			<cpu number>,...,<cpu number>
-			or
-			<cpu number>-<cpu number>
-			(must be a positive range in ascending order)
-			or a mixture
-			<cpu number>,...,<cpu number>-<cpu number>
+			The argument is a cpu list, as described above.
 
 	irqfixup	[HW]
 			When an interrupt is not handled search all handlers
@@ -1812,13 +1837,7 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			Format: <RDP>,<reset>,<pci_scan>,<verbosity>
 
 	isolcpus=	[KNL,SMP] Isolate CPUs from the general scheduler.
-			Format:
-			<cpu number>,...,<cpu number>
-			or
-			<cpu number>-<cpu number>
-			(must be a positive range in ascending order)
-			or a mixture
-			<cpu number>,...,<cpu number>-<cpu number>
+			The argument is a cpu list, as described above.
 
 			This option can be used to specify one or more CPUs
 			to isolate from the general SMP balancing and scheduling
@@ -2680,6 +2699,7 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			Default: on
 
 	nohz_full=	[KNL,BOOT]
+			The argument is a cpu list, as described above.
 			In kernels built with CONFIG_NO_HZ_FULL=y, set
 			the specified list of CPUs whose tick will be stopped
 			whenever possible. The boot CPU will be forced outside
@@ -3285,6 +3305,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			See Documentation/blockdev/ramdisk.txt.
 
 	rcu_nocbs=	[KNL]
+			The argument is a cpu list, as described above.
+
 			In kernels built with CONFIG_RCU_NOCB_CPU=y, set
 			the specified list of CPUs to be no-callback CPUs.
 			Invocation of these CPUs' RCU callbacks will