linux-kernel - Re: [PATCH 06/14] perf, core: always switch pmu specific data during context switch

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Date:	Wed, 5 Feb 2014 18:55:28 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	"Yan, Zheng" <zheng.z.yan@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH 06/14] perf, core: always switch pmu specific data during
 context switch

On Wed, Feb 05, 2014 at 06:19:27PM +0100, Stephane Eranian wrote:
> On Fri, Jan 3, 2014 at 6:48 AM, Yan, Zheng <zheng.z.yan@...el.com> wrote:
> > If two tasks were both forked from the same parent task, Events in their perf
> > task contexts can be the same. Perf core optimizes context switch oout in this
> > case.
> >
> > Previous patch inroduces pmu specific data. The data is task specific, so we
> > should switch the data even when context switch is optimized out.
> >
> Reviwed-by: Stephane Eranian <eranian@...gle.com>

You should look again.. that xchg() is an atomic op and a total waste of
time since the assignment back onto ctx->task_ctx_data is non-atomic.

Complete fail there.

> > Signed-off-by: Yan, Zheng <zheng.z.yan@...el.com>
> > ---
> >  kernel/events/core.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index b6650ab..d6d8dea 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -2319,6 +2319,8 @@ static void perf_event_context_sched_out(struct task_struct *task, int ctxn,
> >                         next->perf_event_ctxp[ctxn] = ctx;
> >                         ctx->task = next;
> >                         next_ctx->task = task;
> > +                       ctx->task_ctx_data = xchg(&next_ctx->task_ctx_data,
> > +                                                 ctx->task_ctx_data);
> >                         do_switch = 0;
> >
> >                         perf_event_sync_stat(ctx, next_ctx);
> > --
> > 1.8.4.2
> >
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