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locking/lock_events: Use this_cpu_add() when necessary

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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: a8654596f0 ("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>
This commit is contained in:
Waiman Long 2019-05-24 15:42:22 -04:00 committed by Linus Torvalds
parent 0a72ef8990
commit 51816e9e11
1 changed files with 40 additions and 2 deletions
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42
kernel/locking/lock_events.h
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@ -30,13 +30,51 @@ enum lock_events {
*/ */
DECLARE_PER_CPU(unsigned long, lockevents[lockevent_num]); 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 * Increment the PV qspinlock statistical counters
*/ */
static inline void __lockevent_inc(enum lock_events event, bool cond) static inline void __lockevent_inc(enum lock_events event, bool cond)
{ {
if (cond) if (cond)
__this_cpu_inc(lockevents[event]); lockevent_percpu_inc(lockevents[event]);
} }
#define lockevent_inc(ev) __lockevent_inc(LOCKEVENT_ ##ev, true) #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) 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) #define lockevent_add(ev, c) __lockevent_add(LOCKEVENT_ ##ev, c)