Re: [PATCH 1/4] perf: Fix system-wide events miscounting during cgroup monitoring

["Liang, Kan" <kan.liang@linux.intel.com>]

On 4/29/2019 11:04 AM, Mark Rutland wrote:
> On Mon, Apr 29, 2019 at 07:44:02AM -0700, kan.liang@linux.intel.com wrote:
>> From: Kan Liang <kan.liang@linux.intel.com>
>>
>> When counting system-wide events and cgroup events simultaneously, the
>> value of system-wide events are miscounting. For example,
>>
>>      perf stat -e cycles,instructions -e cycles,instructions -G
>> cgroup1,cgroup1,cgroup2,cgroup2 -a -e cycles,instructions -I 1000
>>
>>       1.096265502     12,375,398,872      cycles              cgroup1
>>       1.096265502      8,396,184,503      instructions        cgroup1
>>   #    0.10  insn per cycle
>>       1.096265502    109,609,027,112      cycles              cgroup2
>>       1.096265502     11,533,690,148      instructions        cgroup2
>>   #    0.14  insn per cycle
>>       1.096265502    121,672,937,058      cycles
>>       1.096265502     19,331,496,727      instructions               #
>> 0.24  insn per cycle
>>
>> The events are identical events for system-wide and cgroup. The
>> value of system-wide events is less than the sum of cgroup events,
>> which is wrong.
>>
>> Both system-wide and cgroup are per-cpu. They share the same cpuctx
>> groups, cpuctx->flexible_groups/pinned_groups.
>> In context switch, cgroup switch tries to schedule all the events in
>> the cpuctx groups. The unmatched cgroup events can be filtered by its
>> event->cgrp. However, system-wide events, which event->cgrp is NULL, are
>> unconditionally switched, which causes miscounting.
> 
> Why exactly does that cause mis-counting?
> 
> Are the system-wide events not switched back in? Or filtered
> erroneously?


The small period between the prev cgroup sched_out and the new cgroup 
sched_in is missed for system-wide events.
Current code mistakenly sched_out of system-wide events during that period.

> 
>> Introduce cgrp_switch in cpuctx to indicate the cgroup context switch.
>> If it's system-wide event in context switch, don't try to switch it.
>>
>> Fixes: e5d1367f17ba ("perf: Add cgroup support")
>> Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
>> ---
>>   include/linux/perf_event.h |  1 +
>>   kernel/events/core.c       | 30 ++++++++++++++++++++++++++++--
>>   2 files changed, 29 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
>> index e47ef76..039e2f2 100644
>> --- a/include/linux/perf_event.h
>> +++ b/include/linux/perf_event.h
>> @@ -795,6 +795,7 @@ struct perf_cpu_context {
>>   #ifdef CONFIG_CGROUP_PERF
>>   	struct perf_cgroup		*cgrp;
>>   	struct list_head		cgrp_cpuctx_entry;
>> +	unsigned int			cgrp_switch		:1;
>>   #endif
>>   
>>   	struct list_head		sched_cb_entry;
>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>> index dc7dead..388dd42 100644
>> --- a/kernel/events/core.c
>> +++ b/kernel/events/core.c
>> @@ -809,6 +809,7 @@ static void perf_cgroup_switch(struct task_struct *task, int mode)
>>   
>>   		perf_ctx_lock(cpuctx, cpuctx->task_ctx);
>>   		perf_pmu_disable(cpuctx->ctx.pmu);
>> +		cpuctx->cgrp_switch = true;
>>   
>>   		if (mode & PERF_CGROUP_SWOUT) {
>>   			cpu_ctx_sched_out(cpuctx, EVENT_ALL);
>> @@ -832,6 +833,7 @@ static void perf_cgroup_switch(struct task_struct *task, int mode)
>>   							     &cpuctx->ctx);
>>   			cpu_ctx_sched_in(cpuctx, EVENT_ALL, task);
>>   		}
>> +		cpuctx->cgrp_switch = false;
>>   		perf_pmu_enable(cpuctx->ctx.pmu);
>>   		perf_ctx_unlock(cpuctx, cpuctx->task_ctx);
>>   	}
>> @@ -2944,13 +2946,25 @@ static void ctx_sched_out(struct perf_event_context *ctx,
>>   
>>   	perf_pmu_disable(ctx->pmu);
>>   	if (is_active & EVENT_PINNED) {
>> -		list_for_each_entry_safe(event, tmp, &ctx->pinned_active, active_list)
>> +		list_for_each_entry_safe(event, tmp, &ctx->pinned_active, active_list) {
>> +#ifdef CONFIG_CGROUP_PERF
>> +			/* Don't sched system-wide event when cgroup context switch */
>> +			if (cpuctx->cgrp_switch && !event->cgrp)
>> +				continue;
>> +#endif
> 
> This pattern is duplicated several times, and should probably be a
> helper like:
> 
> static bool skip_cgroup_switch(struct perf_cpu_context *cpuctx,
> 			       struct perf_event *event);
> {
> 	return IS_ENABLED(CONFIG_CGROUP_PERF) &&
> 	       cpuctx->cgrp_switch &&
> 	       !event->cgrp;
> }
> 
> ... allowing the above to be an unconditional:
> 
> 	if (skip_cgroup_switch(cpuctx, event))
> 		continue;
> 
> ... and likewise for the other cases.
> 

I will change it in V2.

Thanks,
Kan

> Thanks,
> Mark.
> 
>>   			group_sched_out(event, cpuctx, ctx);
>> +		}
>>   	}
>>   
>>   	if (is_active & EVENT_FLEXIBLE) {
>> -		list_for_each_entry_safe(event, tmp, &ctx->flexible_active, active_list)
>> +		list_for_each_entry_safe(event, tmp, &ctx->flexible_active, active_list) {
>> +#ifdef CONFIG_CGROUP_PERF
>> +			/* Don't sched system-wide event when cgroup context switch */
>> +			if (cpuctx->cgrp_switch && !event->cgrp)
>> +				continue;
>> +#endif
>>   			group_sched_out(event, cpuctx, ctx);
>> +		}
>>   	}
>>   	perf_pmu_enable(ctx->pmu);
>>   }
>> @@ -3280,6 +3294,12 @@ static int pinned_sched_in(struct perf_event *event, void *data)
>>   	if (event->state <= PERF_EVENT_STATE_OFF)
>>   		return 0;
>>   
>> +#ifdef CONFIG_CGROUP_PERF
>> +	/* Don't sched system-wide event when cgroup context switch */
>> +	if (sid->cpuctx->cgrp_switch && !event->cgrp)
>> +		return 0;
>> +#endif
>> +
>>   	if (!event_filter_match(event))
>>   		return 0;
>>   
>> @@ -3305,6 +3325,12 @@ static int flexible_sched_in(struct perf_event *event, void *data)
>>   	if (event->state <= PERF_EVENT_STATE_OFF)
>>   		return 0;
>>   
>> +#ifdef CONFIG_CGROUP_PERF
>> +	/* Don't sched system-wide event when cgroup context switch */
>> +	if (sid->cpuctx->cgrp_switch && !event->cgrp)
>> +		return 0;
>> +#endif
>> +
>>   	if (!event_filter_match(event))
>>   		return 0;
>>   
>> -- 
>> 2.7.4
>>