Re: [PATCH] perf/core: Install cgroup event via IPI

[Song Liu <songliubraving@fb.com>]

> On Jan 20, 2020, at 12:50 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> 
> On Thu, Jan 16, 2020 at 09:25:55AM -0800, Song Liu wrote:
>> cgroup events in OFF state cannot be installed without IPI, otherwise, it
>> may trigger the following calltrace with CONFIG_DEBUG_LIST:
>> 
>> [   31.776974] ------------[ cut here ]------------
>> [   31.777570] list_add double add: new=ffff888ff7cf0db0, prev=ffff888ff7ce82f0, next=ffff888ff7cf0db0.
>> [   31.778737] WARNING: CPU: 3 PID: 1186 at lib/list_debug.c:31 __list_add_valid+0x67/0x70
>> [   31.779745] Modules linked in:
>> [   31.780138] CPU: 3 PID: 1186 Comm: perf Tainted: G        W         5.5.0-rc6+ #3962
>> [   31.781125] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-2.el7 04/01/2014
>> [   31.782199] RIP: 0010:__list_add_valid+0x67/0x70
> 
>> 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);
> 
> I don't think this is right. Because cgroup events are always per-cpu
> events, ctx == &cpuctx->ctx, so the locking should work out just fine.
> 
> What does appear to be the problem is that:
> 
>  add_event_to_ctx()
>    list_update_cgroup_event()
>      cpuctx = __get_cpu_context(ctx)
> 
> uses this_cpu_ptr() and we're now calling it from the 'wrong' CPU.

Yes, this is because add_event_to_ctx() is called from wrong CPU. 

> 
> But I'm thinking the below should also work just fine, hmm?

And yes, this also fixes it. Thanks!

Would you send official PATCH?

Song