linux-kernel - Re: [RFC perf,x86] P4 PMU early draft

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Message-ID: <aa79d98a1002100323v3a8e8211k309b5edfc3491ecb@mail.gmail.com>
Date:	Wed, 10 Feb 2010 14:23:27 +0300
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Stephane Eranian <eranian@...gle.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Don Zickus <dzickus@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC perf,x86] P4 PMU early draft

On 2/10/10, Peter Zijlstra <peterz@...radead.org> wrote:
> On Wed, 2010-02-10 at 13:38 +0300, Cyrill Gorcunov wrote:
>> > I'd try BUG_ON(cpu != smp_processor_id()) and scrap passing that cpu
>> > thing around.
>> >
>>
>> no, i need cpu to find out if event has migrated from other thread and
>> then i switch
>> some thread dependant flags in hw::config (ie escr and cccr), or i
>> miss something and events in one cpu just can't migrate to another
>> cpu?
>
> Well, if we validate that cpu == smp_processor_id() (looking at
> kernel/perf_event.c that does indeed seem true for
> hw_perf_group_sched_in() -- which suggests we should simply remove that
> cpu argument), and that cpu will stay constant throughout the whole
> callchain (it does, its a local variable), we can remove it and
> substitute smp_processor_id(), right?
>

yeah, seems so!

> As to migration of the event, its tied to a task, we're now installing
> the event for a task it wouldn't make sense to allow that to be
> preemptible.
>
>

ok, thanks for explanation, Peter! I'll take a closer look as only get
back from office.
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