Re: [PATCH v2] perf/core: Fix cgroup events tracking

[Chengming Zhou]

Hello, ping :-)


On 2022/12/7 20:40, 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
> Call Trace:
>  <TASK>
>  __schedule+0x4ae/0x9f0
>  ? _raw_spin_unlock_irqrestore+0x23/0x40
>  ? __cond_resched+0x18/0x20
>  preempt_schedule_common+0x2d/0x70
>  __cond_resched+0x18/0x20
>  wait_for_completion+0x2f/0x160
>  ? cpu_stop_queue_work+0x9e/0x130
>  affine_move_task+0x18a/0x4f0
> 
> WARNING: CPU: 0 PID: 690 at kernel/events/core.c:829 ctx_sched_in+0x1cf/0x1e0
> Call Trace:
>  <TASK>
>  ? ctx_sched_out+0xb7/0x1b0
>  perf_cgroup_switch+0x88/0xc0
>  __schedule+0x4ae/0x9f0
>  ? _raw_spin_unlock_irqrestore+0x23/0x40
>  ? __cond_resched+0x18/0x20
>  preempt_schedule_common+0x2d/0x70
>  __cond_resched+0x18/0x20
>  wait_for_completion+0x2f/0x160
>  ? cpu_stop_queue_work+0x9e/0x130
>  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()
>     cpu_function_call()
> 					__perf_install_in_context()
> 					  add_event_to_ctx()
> 					    list_add_event()
> 					      perf_cgroup_event_enable()
> 					        ctx->nr_cgroups++
> 					        cpuctx->cgrp = X
> 
> 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.
> 
> Fixes: bd2756811766 ("perf: Rewrite core context handling")
> Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
> Tested-by: Ravi Bangoria <ravi.bangoria@xxxxxxx>
> ---
> v2:
>  - Remove timestamps and fix race graph in commit log per Ravi Bangoria.
>  - Add Fixes tag and Tested-by tag.
> ---
>  kernel/events/core.c | 42 ++++++++++--------------------------------
>  1 file changed, 10 insertions(+), 32 deletions(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index eacc3702654d..5d97a9f26003 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -380,7 +380,6 @@ enum event_type_t {
>  
>  /*
>   * perf_sched_events : >0 events exist
> - * perf_cgroup_events: >0 per-cpu cgroup events exist on this cpu
>   */
>  
>  static void perf_sched_delayed(struct work_struct *work);
> @@ -389,7 +388,6 @@ static DECLARE_DELAYED_WORK(perf_sched_work, perf_sched_delayed);
>  static DEFINE_MUTEX(perf_sched_mutex);
>  static atomic_t perf_sched_count;
>  
> -static DEFINE_PER_CPU(atomic_t, perf_cgroup_events);
>  static DEFINE_PER_CPU(struct pmu_event_list, pmu_sb_events);
>  
>  static atomic_t nr_mmap_events __read_mostly;
> @@ -844,9 +842,16 @@ static void perf_cgroup_switch(struct task_struct *task)
>  	struct perf_cpu_context *cpuctx = this_cpu_ptr(&perf_cpu_context);
>  	struct perf_cgroup *cgrp;
>  
> -	cgrp = perf_cgroup_from_task(task, NULL);
> +	/*
> +	 * cpuctx->cgrp is set when the first cgroup event enabled,
> +	 * and is cleared when the last cgroup event disabled.
> +	 */
> +	if (READ_ONCE(cpuctx->cgrp) == NULL)
> +		return;
>  
>  	WARN_ON_ONCE(cpuctx->ctx.nr_cgroups == 0);
> +
> +	cgrp = perf_cgroup_from_task(task, NULL);
>  	if (READ_ONCE(cpuctx->cgrp) == cgrp)
>  		return;
>  
> @@ -3631,8 +3636,7 @@ void __perf_event_task_sched_out(struct task_struct *task,
>  	 * to check if we have to switch out PMU state.
>  	 * cgroup event are system-wide mode only
>  	 */
> -	if (atomic_read(this_cpu_ptr(&perf_cgroup_events)))
> -		perf_cgroup_switch(next);
> +	perf_cgroup_switch(next);
>  }
>  
>  static bool perf_less_group_idx(const void *l, const void *r)
> @@ -4974,15 +4978,6 @@ static void unaccount_pmu_sb_event(struct perf_event *event)
>  		detach_sb_event(event);
>  }
>  
> -static void unaccount_event_cpu(struct perf_event *event, int cpu)
> -{
> -	if (event->parent)
> -		return;
> -
> -	if (is_cgroup_event(event))
> -		atomic_dec(&per_cpu(perf_cgroup_events, cpu));
> -}
> -
>  #ifdef CONFIG_NO_HZ_FULL
>  static DEFINE_SPINLOCK(nr_freq_lock);
>  #endif
> @@ -5048,8 +5043,6 @@ static void unaccount_event(struct perf_event *event)
>  			schedule_delayed_work(&perf_sched_work, HZ);
>  	}
>  
> -	unaccount_event_cpu(event, event->cpu);
> -
>  	unaccount_pmu_sb_event(event);
>  }
>  
> @@ -11679,15 +11672,6 @@ static void account_pmu_sb_event(struct perf_event *event)
>  		attach_sb_event(event);
>  }
>  
> -static void account_event_cpu(struct perf_event *event, int cpu)
> -{
> -	if (event->parent)
> -		return;
> -
> -	if (is_cgroup_event(event))
> -		atomic_inc(&per_cpu(perf_cgroup_events, cpu));
> -}
> -
>  /* Freq events need the tick to stay alive (see perf_event_task_tick). */
>  static void account_freq_event_nohz(void)
>  {
> @@ -11775,8 +11759,6 @@ static void account_event(struct perf_event *event)
>  	}
>  enabled:
>  
> -	account_event_cpu(event, event->cpu);
> -
>  	account_pmu_sb_event(event);
>  }
>  
> @@ -12822,13 +12804,11 @@ static void __perf_pmu_remove(struct perf_event_context *ctx,
>  
>  	perf_event_groups_for_cpu_pmu(event, groups, cpu, pmu) {
>  		perf_remove_from_context(event, 0);
> -		unaccount_event_cpu(event, cpu);
>  		put_pmu_ctx(event->pmu_ctx);
>  		list_add(&event->migrate_entry, events);
>  
>  		for_each_sibling_event(sibling, event) {
>  			perf_remove_from_context(sibling, 0);
> -			unaccount_event_cpu(sibling, cpu);
>  			put_pmu_ctx(sibling->pmu_ctx);
>  			list_add(&sibling->migrate_entry, events);
>  		}
> @@ -12847,7 +12827,6 @@ static void __perf_pmu_install_event(struct pmu *pmu,
>  
>  	if (event->state >= PERF_EVENT_STATE_OFF)
>  		event->state = PERF_EVENT_STATE_INACTIVE;
> -	account_event_cpu(event, cpu);
>  	perf_install_in_context(ctx, event, cpu);
>  }
>  
> @@ -13742,8 +13721,7 @@ static int __perf_cgroup_move(void *info)
>  	struct task_struct *task = info;
>  
>  	preempt_disable();
> -	if (atomic_read(this_cpu_ptr(&perf_cgroup_events)))
> -		perf_cgroup_switch(task);
> +	perf_cgroup_switch(task);
>  	preempt_enable();
>  
>  	return 0;