Messages in this thread | | | Date | Wed, 7 Dec 2022 19:19:45 +0800 | Subject | Re: [PATCH] perf/core: Fix cgroup events tracking | From | Chengming Zhou <> |
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On 2022/12/7 18:41, Ravi Bangoria wrote: > 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.
Ok, will remove.
> >> >> 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.
Right, will fix it next version.
> >> 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>
Ok, will add Fixes tag next version.
Thanks!
> > Thanks, > Ravi
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