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Message-ID: <20190429150443.GB2182@lakrids.cambridge.arm.com>
Date: Mon, 29 Apr 2019 16:04:43 +0100
From: Mark Rutland <mark.rutland@....com>
To: kan.liang@...ux.intel.com
Cc: peterz@...radead.org, tglx@...utronix.de, mingo@...hat.com,
linux-kernel@...r.kernel.org, eranian@...gle.com, tj@...nel.org,
ak@...ux.intel.com
Subject: Re: [PATCH 1/4] perf: Fix system-wide events miscounting during
cgroup monitoring
On Mon, Apr 29, 2019 at 07:44:02AM -0700, kan.liang@...ux.intel.com wrote:
> From: Kan Liang <kan.liang@...ux.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?
> 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@...ux.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.
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
>
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