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

[Mark Rutland]

On Mon, Apr 29, 2019 at 07:44:02AM -0700, kan.liang@xxxxxxxxxxxxxxx wrote:
> From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> 
> 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?

> 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@xxxxxxxxxxxxxxx>
> ---
>  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.

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
>