linux-kernel - Re: [PATCH] perf_counter: cleanup for __perf_event_sched

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Date:	Wed, 23 Sep 2009 13:32:51 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] perf_counter: cleanup for __perf_event_sched_in()

Xiao Guangrong writes:

> I only find three places in __perf_event_sched_in/out, could you tell me
> where is the fourth place?

My mistake, it is just those three.  I saw the list_for_each_entry_rcu
followed by if (event != event->group_leader) in perf_event_init_task
and missed the fact that it is iterating parent_ctx->event_list rather
than parent_ctx->group_list.  But as you point out:

> I also noticed that all group leader is the top level of ctx->group_list,
> if I not missed, the perf_event_init_task() function can be optimized,
> like this:
> 
> int perf_event_init_task(struct task_struct *child)
> {
> 	......
> 	/* We can only look at parent_ctx->group_list to get group leader */
> 	list_for_each_entry_rcu(event, &parent_ctx->event_list, event_entry) {
> 		if (event != event->group_leader)
> 			continue;
> 	......
> 	}
> 	......
> }

we would in fact be better off using group_list rather than event_list
anyway.  That should be safe since we hold parent_ctx->mutex.

> I'll fix those if you are not mind :-)

Please do. :)

Paul.
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