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

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Date:	Thu, 06 Feb 2014 10:08:30 +0800
From:	"Yan, Zheng" <zheng.z.yan@...el.com>
To:	Stephane Eranian <eranian@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>
CC:	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 02/06/2014 02:35 AM, Stephane Eranian wrote:
> On Wed, Feb 5, 2014 at 6:55 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>> 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.
>>
> I admit, it was not clear to me why the xchg().

Sorry. I forget why I used xhcg(), maybe save a few lines of code.

Regards
Yan, Zheng

> 
>>>> 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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