linux-kernel - Re: [PATCH] perf_events: fix event scheduling issues introduced by transactional API (take 2)

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Date:	Tue, 25 May 2010 15:39:48 +0200
From:	stephane eranian <eranian@...glemail.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	eranian@...gle.com, linux-kernel@...r.kernel.org, mingo@...e.hu,
	paulus@...ba.org, davem@...emloft.net, fweisbec@...il.com,
	acme@...radead.org, ming.m.lin@...el.com,
	perfmon2-devel@...ts.sf.net
Subject: Re: [PATCH] perf_events: fix event scheduling issues introduced by 
	transactional API (take 2)

On Tue, May 25, 2010 at 3:35 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, 2010-05-25 at 15:20 +0200, Stephane Eranian wrote:
>
>> With this patch, you can now overcommit the PMU even with pinned
>> system-wide events present and still get valid counts.
>
> Does this patch differ from the one you send earlier?
>
Yes, it does. It changes the way n_added is updated.

The first version was buggy (perf top would crash the machine).
You cannot delay updating n_added until commit, because there
are paths where you don't go through transactions. This version
instead keeps the number of events last added and speculatively
updates n_added (assuming success). If the transaction succeeds,
then no correction is done (subtracting 0), if it fails, then the number
of events just added to n_added is subtracted.
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