Subject: Re: [PATCH] perf_events: improve x86 event scheduling (v6 
 incremental)
From: Peter Zijlstra <peterz@infradead.org>
To: Stephane Eranian <eranian@google.com>
Cc: eranian@gmail.com, linux-kernel@vger.kernel.org, mingo@elte.hu,
       paulus@samba.org, davem@davemloft.net, fweisbec@gmail.com,
       perfmon2-devel@lists.sf.net
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Date: Sat, 30 Jan 2010 09:56:39 +0100
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On Sat, 2010-01-30 at 00:08 +0100, Stephane Eranian wrote:
> I think there is a problem with this following code:
> 
> void hw_perf_enable(void)
>                 for (i = 0; i < cpuc->n_events; i++) {
> 
>                         event = cpuc->event_list[i];
>                         hwc = &event->hw;
> 
>                         if (hwc->idx == -1 || hwc->idx == cpuc->assign[i])
>                                 continue;
> 
> Here you are looking for events which are moving. I think the 2nd
> part of the if is not good enough. It is not because hwc->idx is
> identical to the assignment, that you can assume the event was
> already there. It may have been there in the past, then scheduled
> out and replaced at idx by another event. When it comes back,
> it gets its spot back, but it needs to be reprogrammed.
> 
> That is why in v6 incremental, I have added last_cpu, last_tag
> to have a stronger checks and match_prev_assignment().
> 
> Somehow it is missing in the series you've committed unless
> I am missing something.

Right, that went missing because I was assuming that was for the
optimization of reducing to one loop. And since I didn't see that one
loop version work I left that part out.

(The risk of doing more than one thing in one patch)

Still, shouldn't be hard to correct, I'll look at doing a patch for this
on monday, unless you beat me to it :-)

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