linux-kernel - Re: [PATCH] perf_events: fix transaction recovery in group_sched

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Date:	Fri, 15 Oct 2010 19:29:57 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	eranian@...gle.com
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, paulus@...ba.org,
	davem@...emloft.net, fweisbec@...il.com,
	perfmon2-devel@...ts.sf.net, eranian@...il.com,
	robert.richter@....com
Subject: Re: [PATCH] perf_events: fix transaction recovery in
 group_sched_in()

On Fri, 2010-10-15 at 16:54 +0200, Stephane Eranian wrote:
> The group_sched_in() function uses a transactional approach to schedule
> a group of events. In a group, either all events can be scheduled or
> none are. To schedule each event in, the function calls event_sched_in().
> In case of error, event_sched_out() is called on each event in the group.
> 
> The problem is that event_sched_out() does not completely cancel the
> effects of event_sched_in(). Furthermore event_sched_out() changes the
> state of the event as if it had run which is not true is this particular
> case.
> 
> Those inconsistencies impact time tracking fields and may lead to events
> in a group not all reporting the same time_enabled and time_running values.
> This is demonstrated with the example below:
> 
> $ task -eunhalted_core_cycles,baclears,baclears -e unhalted_core_cycles,baclears,baclears sleep 5
> 1946101 unhalted_core_cycles (32.85% scaling, ena=829181, run=556827)
>   11423 baclears (32.85% scaling, ena=829181, run=556827)
>    7671 baclears (0.00% scaling, ena=556827, run=556827)
> 
> 2250443 unhalted_core_cycles (57.83% scaling, ena=962822, run=405995)
>   11705 baclears (57.83% scaling, ena=962822, run=405995)
>   11705 baclears (57.83% scaling, ena=962822, run=405995)
> 
> Notice that in the first group, the last baclears event does not
> report the same timings as its siblings.
> 
> This issue comes from the fact that tstamp_stopped is updated
> by event_sched_out() as if the event had actually run.
> 
> To solve the issue, we must ensure that, in case of error, there is
> no change in the event state whatsoever. That means timings must
> remain as they were when entering group_sched_in().
> 
> To do this we defer updating tstamp_running until we know the
> transaction succeeded. Therefore, we have split event_sched_in()
> in two parts separating the update to tstamp_running.
> 
> Similarly, in case of error, we do not want to update tstamp_stopped.
> Therefore, we have split event_sched_out() in two parts separating
> the update to tstamp_stopped. 
> 
> With this patch, we now get the following output:
> 
> $ task -eunhalted_core_cycles,baclears,baclears -e unhalted_core_cycles,baclears,baclears sleep 5
> 2492050 unhalted_core_cycles (71.75% scaling, ena=1093330, run=308841)
>   11243 baclears (71.75% scaling, ena=1093330, run=308841)
>   11243 baclears (71.75% scaling, ena=1093330, run=308841)
> 
> 1852746 unhalted_core_cycles (0.00% scaling, ena=784489, run=784489)
>    9253 baclears (0.00% scaling, ena=784489, run=784489)
>    9253 baclears (0.00% scaling, ena=784489, run=784489)
> 
> Note that the uneven timing between groups is a side effect of
> the process spending most of its time sleeping, i.e., not enough
> event rotations (but that's a separate issue).
> 
> Signed-off-by: Stephane Eranian <eranian@...gle.com>

Yes, makes sense.. I'm a bit hesitant to slap a -stable tag on it due to
its size,.. Ingo, Paulus?


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