Re: [External] Re: [PATCH v2 2/6] perf/core: Introduce percpu perf_cgroup

[Chengming Zhou <zhouchengming@bytedance.com>]

On 2022/3/23 8:51 下午, Peter Zijlstra wrote:
> On Tue, Mar 22, 2022 at 08:08:30PM +0800, Chengming Zhou wrote:
> 
>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>> index 8b5cf2aedfe6..848a3bfa9513 100644
>> --- a/kernel/events/core.c
>> +++ b/kernel/events/core.c
> 
>> @@ -843,11 +845,21 @@ static void perf_cgroup_switch(struct task_struct *task)
>>  	 */
>>  	local_irq_save(flags);
>>  
>> +	cgrp = perf_cgroup_from_task(task, NULL);
>> +	if (cgrp == __this_cpu_read(cpu_perf_cgroup))
>> +		goto out;
>> +
>> +	__this_cpu_write(cpu_perf_cgroup, cgrp);
>> +
>>  	list = this_cpu_ptr(&cgrp_cpuctx_list);
>>  	list_for_each_entry_safe(cpuctx, tmp, list, cgrp_cpuctx_entry) {
>>  		WARN_ON_ONCE(cpuctx->ctx.nr_cgroups == 0);
>>  
>>  		perf_ctx_lock(cpuctx, cpuctx->task_ctx);
>> +
>> +		if (cpuctx->cgrp == cgrp)
>> +			continue;
>> +
>>  		perf_pmu_disable(cpuctx->ctx.pmu);
>>  
>>  		cpu_ctx_sched_out(cpuctx, EVENT_ALL);
> 
> This is just straight up broken.. you can't continue after taking a
> lock, that'll miss unlock.

Yes, Namhyung has pointed it out, I will fix it next version.

> 
> Also, I really don't see the point of cpu_perf_cgroup, cpuctx->cgrp
> should be able to do this.

But the problem is that we have two cpuctx on the percpu list, do you
mean we should use perf_cgroup of the first cpuctx to compare with
the next task's perf_cgroup ?

Or we should delete the cgrp in cpuctx, and use this new percpu cpu_perf_cgroup?

> 
> Also, perhaps merge this in the previous patch, I'm not sure it makes
> sense to split this.

Ok, will do. I put it in one patch in v1 RFC, then split it for easier review.
I will put it together in the next version.

Thanks.

> 
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -833,6 +833,7 @@ static DEFINE_PER_CPU(struct list_head,
>   */
>  static void perf_cgroup_switch(struct task_struct *task)
>  {
> +	struct perf_cgroup *cgrp;
>  	struct perf_cpu_context *cpuctx, *tmp;
>  	struct list_head *list;
>  	unsigned long flags;
> @@ -843,11 +844,20 @@ static void perf_cgroup_switch(struct ta
>  	 */
>  	local_irq_save(flags);
>  
> +	cgrp = perf_cgroup_from_task(task, NULL);
> +
>  	list = this_cpu_ptr(&cgrp_cpuctx_list);
>  	list_for_each_entry_safe(cpuctx, tmp, list, cgrp_cpuctx_entry) {
>  		WARN_ON_ONCE(cpuctx->ctx.nr_cgroups == 0);
>  
> +		if (READ_ONCE(cpuctx->cgrp == cgrp))
> +			continue
> +
>  		perf_ctx_lock(cpuctx, cpuctx->task_ctx);
> +
> +		if (cpuctx->cgrp == cgrp)
> +			goto next;
> +
>  		perf_pmu_disable(cpuctx->ctx.pmu);
>  
>  		cpu_ctx_sched_out(cpuctx, EVENT_ALL);
> @@ -855,50 +865,22 @@ static void perf_cgroup_switch(struct ta
>  		 * must not be done before ctxswout due
>  		 * to event_filter_match() in event_sched_out()
>  		 */
> -		cpuctx->cgrp = perf_cgroup_from_task(task,
> -						     &cpuctx->ctx);
> +		WRITE_ONCE(cpuctx->cgrp, cgrp);
>  		/*
>  		 * set cgrp before ctxsw in to allow
>  		 * event_filter_match() to not have to pass
>  		 * task around
> -		 * we pass the cpuctx->ctx to perf_cgroup_from_task()
> -		 * because cgroup events are only per-cpu
>  		 */
>  		cpu_ctx_sched_in(cpuctx, EVENT_ALL, task);
>  
>  		perf_pmu_enable(cpuctx->ctx.pmu);
> +next:
>  		perf_ctx_unlock(cpuctx, cpuctx->task_ctx);
>  	}
>  
>  	local_irq_restore(flags);
>  }
>  
> -static inline void perf_cgroup_sched_switch(struct task_struct *task,
> -					    struct task_struct *next)
> -{
> -	struct perf_cgroup *cgrp1;
> -	struct perf_cgroup *cgrp2 = NULL;
> -
> -	rcu_read_lock();
> -	/*
> -	 * we come here when we know perf_cgroup_events > 0
> -	 * we do not need to pass the ctx here because we know
> -	 * we are holding the rcu lock
> -	 */
> -	cgrp1 = perf_cgroup_from_task(task, NULL);
> -	cgrp2 = perf_cgroup_from_task(next, NULL);
> -
> -	/*
> -	 * only schedule out current cgroup events if we know
> -	 * that we are switching to a different cgroup. Otherwise,
> -	 * do no touch the cgroup events.
> -	 */
> -	if (cgrp1 != cgrp2)
> -		perf_cgroup_switch(task);
> -
> -	rcu_read_unlock();
> -}
> -
>  static int perf_cgroup_ensure_storage(struct perf_event *event,
>  				struct cgroup_subsys_state *css)
>  {
> @@ -1062,11 +1044,6 @@ static inline void update_cgrp_time_from
>  {
>  }
>  
> -static inline void perf_cgroup_sched_switch(struct task_struct *task,
> -					    struct task_struct *next)
> -{
> -}
> -
>  static inline int perf_cgroup_connect(pid_t pid, struct perf_event *event,
>  				      struct perf_event_attr *attr,
>  				      struct perf_event *group_leader)
> @@ -1080,11 +1057,6 @@ perf_cgroup_set_timestamp(struct task_st
>  {
>  }
>  
> -static inline void
> -perf_cgroup_sched_switch(struct task_struct *task, struct task_struct *next)
> -{
> -}
> -
>  static inline u64 perf_cgroup_event_time(struct perf_event *event)
>  {
>  	return 0;
> @@ -1104,6 +1076,10 @@ static inline void
>  perf_cgroup_event_disable(struct perf_event *event, struct perf_event_context *ctx)
>  {
>  }
> +
> +static void perf_cgroup_switch(struct task_struct *task)
> +{
> +}
>  #endif
>  
>  /*
> @@ -3625,7 +3601,7 @@ void __perf_event_task_sched_out(struct
>  	 * cgroup event are system-wide mode only
>  	 */
>  	if (atomic_read(this_cpu_ptr(&perf_cgroup_events)))
> -		perf_cgroup_sched_switch(task, next);
> +		perf_cgroup_switch(next);
>  }
>  
>  /*