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author | Waiman Long <longman@redhat.com> | 2019-05-24 15:42:22 -0400 |
---|---|---|
committer | Linus Torvalds <torvalds@linux-foundation.org> | 2019-05-24 14:17:18 -0700 |
commit | 51816e9e113934281b44f1a352852ef7631e75ea (patch) | |
tree | 2f2d6cc2f33a22e3a0603d844396bf3f5d5b6592 /kernel/locking | |
parent | 0a72ef89901409847036664c23ba6eee7cf08e0e (diff) | |
download | linux-stable-51816e9e113934281b44f1a352852ef7631e75ea.tar.gz linux-stable-51816e9e113934281b44f1a352852ef7631e75ea.tar.bz2 linux-stable-51816e9e113934281b44f1a352852ef7631e75ea.zip |
locking/lock_events: Use this_cpu_add() when necessary
The kernel test robot has reported that the use of __this_cpu_add()
causes bug messages like:
BUG: using __this_cpu_add() in preemptible [00000000] code: ...
Given the imprecise nature of the count and the possibility of resetting
the count and doing the measurement again, this is not really a big
problem to use the unprotected __this_cpu_*() functions.
To make the preemption checking code happy, the this_cpu_*() functions
will be used if CONFIG_DEBUG_PREEMPT is defined.
The imprecise nature of the locking counts are also documented with
the suggestion that we should run the measurement a few times with the
counts reset in between to get a better picture of what is going on
under the hood.
Fixes: a8654596f0371 ("locking/rwsem: Enable lock event counting")
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'kernel/locking')
-rw-r--r-- | kernel/locking/lock_events.h | 42 |
1 files changed, 40 insertions, 2 deletions
diff --git a/kernel/locking/lock_events.h b/kernel/locking/lock_events.h index feb1acc54611..46b71af8eef2 100644 --- a/kernel/locking/lock_events.h +++ b/kernel/locking/lock_events.h @@ -31,12 +31,50 @@ enum lock_events { DECLARE_PER_CPU(unsigned long, lockevents[lockevent_num]); /* + * The purpose of the lock event counting subsystem is to provide a low + * overhead way to record the number of specific locking events by using + * percpu counters. It is the percpu sum that matters, not specifically + * how many of them happens in each cpu. + * + * It is possible that the same percpu counter may be modified in both + * the process and interrupt contexts. For architectures that perform + * percpu operation with multiple instructions, it is possible to lose + * count if a process context percpu update is interrupted in the middle + * and the same counter is updated in the interrupt context. Therefore, + * the generated percpu sum may not be precise. The error, if any, should + * be small and insignificant. + * + * For those architectures that do multi-instruction percpu operation, + * preemption in the middle and moving the task to another cpu may cause + * a larger error in the count. Again, this will be few and far between. + * Given the imprecise nature of the count and the possibility of resetting + * the count and doing the measurement again, this is not really a big + * problem. + * + * To get a better picture of what is happening under the hood, it is + * suggested that a few measurements should be taken with the counts + * reset in between to stamp out outliner because of these possible + * error conditions. + * + * To minimize overhead, we use __this_cpu_*() in all cases except when + * CONFIG_DEBUG_PREEMPT is defined. In this particular case, this_cpu_*() + * will be used to avoid the appearance of unwanted BUG messages. + */ +#ifdef CONFIG_DEBUG_PREEMPT +#define lockevent_percpu_inc(x) this_cpu_inc(x) +#define lockevent_percpu_add(x, v) this_cpu_add(x, v) +#else +#define lockevent_percpu_inc(x) __this_cpu_inc(x) +#define lockevent_percpu_add(x, v) __this_cpu_add(x, v) +#endif + +/* * Increment the PV qspinlock statistical counters */ static inline void __lockevent_inc(enum lock_events event, bool cond) { if (cond) - __this_cpu_inc(lockevents[event]); + lockevent_percpu_inc(lockevents[event]); } #define lockevent_inc(ev) __lockevent_inc(LOCKEVENT_ ##ev, true) @@ -44,7 +82,7 @@ static inline void __lockevent_inc(enum lock_events event, bool cond) static inline void __lockevent_add(enum lock_events event, int inc) { - __this_cpu_add(lockevents[event], inc); + lockevent_percpu_add(lockevents[event], inc); } #define lockevent_add(ev, c) __lockevent_add(LOCKEVENT_ ##ev, c) |