Re: [PATCH v2] perf/core: install cgroup events to correct cpuctx

[Song Liu]

Hi Peter, 

> On Mar 5, 2020, at 11:48 PM, Song Liu <songliubraving@xxxxxx> wrote:
> 
> 
> 
>> On Jan 24, 2020, at 1:15 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>> 
>> On Wed, Jan 22, 2020 at 11:50:27AM -0800, Song Liu wrote:
>>> cgroup events are always installed in the cpuctx. However, when it is not
>>> installed via IPI, list_update_cgroup_event() adds it to cpuctx of current
>>> CPU, which triggers the following with CONFIG_DEBUG_LIST:
>>> 
>> 
>>> [   31.777570] list_add double add: new=ffff888ff7cf0db0, prev=ffff888ff7ce82f0, next=ffff888ff7cf0db0.
>> 
>>> To reproduce this, we can simply run:
>>> perf stat -e cs -a &
>>> perf stat -e cs -G anycgroup
>>> 
>>> Fix this by installing it to cpuctx that contains event->ctx, and the
>>> proper cgrp_cpuctx_list.
>>> 
>>> Fixes: db0503e4f675 ("perf/core: Optimize perf_install_in_event()")
>>> Suggested-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
>>> Cc: Andi Kleen <andi@xxxxxxxxxxxxxx>
>>> Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
>>> Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
>>> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
>>> Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
>>> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
>>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>>> Signed-off-by: Song Liu <songliubraving@xxxxxx>
>> 
>> Thanks!
> 
> I just realized this won't fully fix the problem, because later in 
> list_update_cgroup_event() we use "current":
> 
> 	struct perf_cgroup *cgrp = perf_cgroup_from_task(current, ctx);

Could you please share your thoughts on this? I think we cannot use current
in list_update_cgroup_event(), unless we call it on the target CPU. 

Thanks,
Song


> 
> I don't have a good idea to fix this cleanly. How about we just use IPI 
> to install cgroup events (like v1):
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index a1f8bde19b56..36e8fe27e2a1 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -2682,14 +2682,18 @@ perf_install_in_context(struct perf_event_context *ctx,
> 	smp_store_release(&event->ctx, ctx);
> 
> 	/*
> -	 * perf_event_attr::disabled events will not run and can be initialized
> -	 * without IPI. Except when this is the first event for the context, in
> -	 * that case we need the magic of the IPI to set ctx->is_active.
> +	 * perf_event_attr::disabled events will not run and can be
> +	 * initialized without IPI. Except:
> +	 *   1. when this is the first event for the context, in that case
> +	 *      we need the magic of the IPI to set ctx->is_active;
> +	 *   2. cgroup event in OFF state, because it is installed in the
> +	 *      cpuctx.
> 	 *
> 	 * The IOC_ENABLE that is sure to follow the creation of a disabled
> 	 * event will issue the IPI and reprogram the hardware.
> 	 */
> -	if (__perf_effective_state(event) == PERF_EVENT_STATE_OFF && ctx->nr_events) {
> +	if (__perf_effective_state(event) == PERF_EVENT_STATE_OFF &&
> +	    !is_cgroup_event(event) && ctx->nr_events) {
> 		raw_spin_lock_irq(&ctx->lock);
> 		if (ctx->task == TASK_TOMBSTONE) {
> 			raw_spin_unlock_irq(&ctx->lock);
> 
> Thanks,
> Song
> 
> 
>