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

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Date:	Wed, 10 Feb 2010 11:52:55 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Cyrill Gorcunov <gorcunov@...il.com>
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 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?

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.

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