linux-kernel - Re: [RFC] x86,perf: Implement minimal P4 PMU driver v14

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Message-ID: <1268343480.5037.145.camel@laptop>
Date:	Thu, 11 Mar 2010 22:38:00 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Lin Ming <ming.m.lin@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Stephane Eranian <eranian@...gle.com>,
	Robert Richter <robert.richter@....com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] x86,perf: Implement minimal P4 PMU driver v14

On Fri, 2010-03-12 at 00:31 +0300, Cyrill Gorcunov wrote:
> On Thu, Mar 11, 2010 at 10:24:22PM +0100, Peter Zijlstra wrote:
> > On Fri, 2010-03-12 at 00:15 +0300, Cyrill Gorcunov wrote:
> > 
> > > Perhaps something like the patch below (tested with kvm)? With this patch
> > > we will actually waste ~4/8 bytes per PMU (intel,amd,p6) since this call
> > > hits on p4 only, so I think perhaps better to use one x86 scheduler hook
> > > instead of empty schedule_events() in PMU, hmm?
> > > ---
> > > 
> > > x86,perf: Fix NULL deref on not assigned x86_pmu
> > > 
> > > In case of not assigned x86_pmu and software events
> > > NULL dereference may being hit via x86_pmu::schedule_events
> > > method.
> > > 
> > > Fix it by calling x86_pmu::schedule_events only if we
> > > have one. Otherwise use general scheduler.
> > > 
> > > Also the former x86_schedule_events calls restored.
> > 
> > Hrm,.. not sure that makes sense, sure it might not crash anymore, but
> > its not making much sense to compute anything if we don't have an
> > initialized x86_pmu.
> > 
> > Doesn't adding something like:
> > 
> >   if (!x86_pmu_initialized())
> >    return;
> > 
> > to hw_perf_group_sched_in() make more sense? We seem to do that for all
> > these weak things except this one.
> > 
> 
> As far as I see it'll not update tstamp_running then (in x86_event_sched_in).
> Or I miss somethig?

Have it return 0 and it will fallback to defaults. Since there is no
initialized x86_pmu there's no point in doing anything x86 specific.

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