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SubjectRe: [PATCH] perf/core: Fix cgroup events tracking
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On 06-Dec-22 8:20 AM, Chengming Zhou wrote:
> We encounter perf warnings when using cgroup events like:
> ```
> cd /sys/fs/cgroup
> mkdir test
> perf stat -e cycles -a -G test
> ```
>
> WARNING: CPU: 0 PID: 690 at kernel/events/core.c:849 perf_cgroup_switch+0xb2/0xc0
> [ 91.393417] Call Trace:
> [ 91.393772] <TASK>
> [ 91.394080] __schedule+0x4ae/0x9f0
> [ 91.394535] ? _raw_spin_unlock_irqrestore+0x23/0x40
> [ 91.395145] ? __cond_resched+0x18/0x20
> [ 91.395622] preempt_schedule_common+0x2d/0x70
> [ 91.396163] __cond_resched+0x18/0x20
> [ 91.396621] wait_for_completion+0x2f/0x160
> [ 91.397137] ? cpu_stop_queue_work+0x9e/0x130
> [ 91.397665] affine_move_task+0x18a/0x4f0

nit: These timestamps can be removed in commit log.

>
> WARNING: CPU: 0 PID: 690 at kernel/events/core.c:829 ctx_sched_in+0x1cf/0x1e0
> [ 91.430151] Call Trace:
> [ 91.430490] <TASK>
> [ 91.430793] ? ctx_sched_out+0xb7/0x1b0
> [ 91.431274] perf_cgroup_switch+0x88/0xc0
> [ 91.431778] __schedule+0x4ae/0x9f0
> [ 91.432215] ? _raw_spin_unlock_irqrestore+0x23/0x40
> [ 91.432825] ? __cond_resched+0x18/0x20
> [ 91.433299] preempt_schedule_common+0x2d/0x70
> [ 91.433839] __cond_resched+0x18/0x20
> [ 91.434298] wait_for_completion+0x2f/0x160
> [ 91.434808] ? cpu_stop_queue_work+0x9e/0x130
> [ 91.435334] affine_move_task+0x18a/0x4f0
>
> The above two warnings are not complete here since I remove other
> unimportant information. The problem is caused by the perf cgroup
> events tracking:
>
> CPU0 CPU1
> perf_event_open()
> perf_event_alloc()
> account_event()
> account_event_cpu()
> atomic_inc(perf_cgroup_events)
> __perf_event_task_sched_out()
> if (atomic_read(perf_cgroup_events))
> perf_cgroup_switch()
> // kernel/events/core.c:849
> WARN_ON_ONCE(cpuctx->ctx.nr_cgroups == 0)
> if (READ_ONCE(cpuctx->cgrp) == cgrp) // false
> return
> perf_ctx_lock()
> ctx_sched_out()
> cpuctx->cgrp = cgrp
> ctx_sched_in()
> perf_cgroup_set_timestamp()
> // kernel/events/core.c:829
> WARN_ON_ONCE(!ctx->nr_cgroups)
> perf_ctx_unlock()
> perf_install_in_context()
> add_event_to_ctx()
> list_add_event()
> perf_cgroup_event_enable()
> ctx->nr_cgroups++
> cpuctx->cgrp = X

IIUC, since it's a cgroup event, perf_install_in_context() will do:
cpu_function_call(cpu, __perf_install_in_context, event). And thus,
callchain starting with add_event_to_ctx() will be executed on CPU1,
not on CPU0.

> We can see from above that we wrongly use percpu atomic perf_cgroup_events
> to check if we need to perf_cgroup_switch(), which should only be used
> when we know this CPU has cgroup events enabled.
>
> The commit bd2756811766 ("perf: Rewrite core context handling") change
> to have only one context per-CPU, so we can just use cpuctx->cgrp to
> check if this CPU has cgroup events enabled.
>
> So percpu atomic perf_cgroup_events is not needed.
>
> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>

Fixes: bd2756811766 ("perf: Rewrite core context handling")

Otherwise looks good.
Tested-by: Ravi Bangoria <ravi.bangoria@amd.com>

Thanks,
Ravi

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