linux-kernel - Re: [PATCH] perf_event: Adjust frequency and unthrottle for non-group-leader events

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Date:	Wed, 14 Oct 2009 12:54:12 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	"Mark W. Krentel" <krentel@...rice.edu>,
	linux-kernel@...r.kernel.org,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>
Subject: Re: [PATCH] perf_event: Adjust frequency and unthrottle for
 non-group-leader events

On Wed, 2009-10-14 at 16:58 +1100, Paul Mackerras wrote:
> The loop in perf_ctx_adjust_freq checks the frequency of sampling
> event counters, and adjusts the event interval and unthrottles the
> event if required, and resets the interrupt count for the event.
> However, at present it only looks at group leaders.
> 
> This means that a sampling event that is not a group leader will
> eventually get throttled, once its interrupt count reaches
> sysctl_perf_event_sample_rate/HZ --- and that is guaranteed to
> happen, if the event is active for long enough, since the interrupt
> count never gets reset.  Once it is throttled it never gets
> unthrottled, so it basically just stops working at that point.
> 
> This fixes it by making perf_ctx_adjust_freq use ctx->event_list
> rather than ctx->group_list.  The existing spin_lock/spin_unlock
> around the loop makes it unnecessary to put rcu_read_lock/
> rcu_read_unlock around the list_for_each_entry_rcu().
> 
> Reported-by: Mark W. Krentel <krentel@...rice.edu>
> Signed-off-by: Paul Mackerras <paulus@...ba.org>

Looks good, thanks Paul!

Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>

> ---
> Apparently this bug was only seen on powerpc, and not on x86.  I have
> no idea why.

Weird, should work the same way on both of them.


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