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Message-ID: <85d377a6-ef36-64f6-6574-7270bf9c4b23@linux.intel.com>
Date: Mon, 29 Apr 2019 11:27:47 -0400
From: "Liang, Kan" <kan.liang@...ux.intel.com>
To: Mark Rutland <mark.rutland@....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 4/29/2019 11:04 AM, Mark Rutland wrote:
> 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?
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@...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.
>
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
>>
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