Messages in this thread | | | Date | Wed, 7 Dec 2022 16:11:05 +0530 | Subject | Re: [PATCH] perf/core: Fix cgroup events tracking | From | Ravi Bangoria <> |
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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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