diff options
| -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) |