linux-kernel - Re: [tip:perf/core] perf: Ignore non-sampling overflows

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Message-ID: <20110701135738.GU4590@erda.amd.com>
Date:	Fri, 1 Jul 2011 15:57:38 +0200
From:	Robert Richter <robert.richter@....com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Francis Moreau <francis.moro@...il.com>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hpa@...or.com" <hpa@...or.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...e.hu" <mingo@...e.hu>
Subject: Re: [tip:perf/core] perf: Ignore non-sampling overflows

On 01.07.11 09:36:56, Peter Zijlstra wrote:
> On Thu, 2011-06-30 at 14:47 +0200, Robert Richter wrote:
> > 
> > It covers the case there the sampling period may be specified in the
> > raw config register (line 286). I know, this is not the preferred way,
> > but this way you can directly set the config value in perf without
> > bypassing some bits through event->attr. As IBS events are sampling
> > events I will change the code so that it writes back the specified
> > period to event->attr.sample_period/event->hw.sample_period. Something
> > like:
> > 
> >         if (event->hw.sample_period) {
> >                 ...
> >         } else {
> >                 max_cnt = event->attr.config & map->cnt_mask;
> >                 event->attr.sample_period = max_cnt << 4;
> >                 event->hw.sample_period = event->attr.sample_period;
> >         }
> > 
> > This will then proper identify IBS samples as samling event.
> 
> Ah, being a naught boy then ;-), yeah passing stuff through like that
> will confuse bits and the fixup you propose will indeed work around
> that.
> 
> But why complicate things like this? You just add extra code to the init
> path since you need to support attr.sample_period input as well.

For people who only want to set the raw register value as in the spec
it is much easier, they don't need to know perf internals. 3 lines of
code for this are okay.

-Robert

-- 
Advanced Micro Devices, Inc.
Operating System Research Center

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