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

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Date:	Wed, 26 May 2010 14:34:18 +0800
From:	Lin Ming <ming.m.lin@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Stephane Eranian <eranian@...gle.com>,
	"eranian@...il.com" <eranian@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"paulus@...ba.org" <paulus@...ba.org>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"fweisbec@...il.com" <fweisbec@...il.com>,
	"acme@...radead.org" <acme@...radead.org>,
	"perfmon2-devel@...ts.sf.net" <perfmon2-devel@...ts.sf.net>
Subject: Re: [PATCH] perf_events: fix event scheduling issues introduced by
 transactional API (take 2)

On Tue, 2010-05-25 at 23:32 +0800, Peter Zijlstra wrote:
> On Tue, 2010-05-25 at 17:02 +0200, Stephane Eranian wrote:
> > Ok, the patch look good expect it needs:
> > 
> > static int x86_pmu_commit_txn(const struct pmu *pmu)
> > {
> >         ......
> >         /*
> >          * copy new assignment, now we know it is possible
> >          * will be used by hw_perf_enable()
> >          */
> >         memcpy(cpuc->assign, assign, n*sizeof(int));
> > 
> >         cpuc->n_txn = 0;
> > 
> >         return 0;
> > }
> > 
> > Because you always call cancel_txn() even when commit()
> > succeeds. I don't really understand why. I think it could be
> > avoided by clearing the group_flag in commit_txn() if it
> > succeeds. It would also make the logical flow more natural. Why
> > cancel something that has succeeded. You cancel when you fail/abort.
> 
> Gah, I forgot about that. I think I suggested to Lin to do that and then
> promptly forgot.

cancel_txn() clears the transaction flag, so it is needed after both
success and fail transaction, although the function name is a bit
misleading.

Peter's patch adds the clear of transaction flag into each
implementation of ->commit_txn.

So cancel_txn() is only called after fail transaction now.

Thanks,
Lin Ming

> 
> Let me add that and at least push this patch fwd, we can try and clean
> up that detail later on.

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