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

[Song Liu]

> 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);

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