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

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Message-ID: <20100311214147.GF25162@lenovo>
Date:	Fri, 12 Mar 2010 00:41:47 +0300
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 Thu, Mar 11, 2010 at 10:38:00PM +0100, Peter Zijlstra wrote:
> 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.
> 

OK, thanks, I see what you mean. Will cook patch shortly.

	-- Cyrill
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