linux-kernel - Re: [PATCH 1/1] Change IBS PMU to use perf_hw

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Message-ID: <20121217094403.GF1893@rric.localhost>
Date:	Mon, 17 Dec 2012 10:44:03 +0100
From:	Robert Richter <rric@...nel.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	suravee.suthikulpanit@....com, a.p.zijlstra@...llo.nl,
	paulus@...ba.org, mingo@...hat.com, acme@...stprotocols.net,
	tglx@...utronix.de, hpa@...or.com, x86@...nel.org,
	linux-kernel@...r.kernel.org, jacob.shin@....com
Subject: Re: [PATCH 1/1] Change IBS PMU to use perf_hw_context

On 16.12.12 10:04:10, Ingo Molnar wrote:
> 
> * suravee.suthikulpanit@....com <suravee.suthikulpanit@....com> wrote:
> 
> > From: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
> > 
> > Currently, the AMD IBS PMU initialize pmu.task_ctx_nr to 
> > perf_invalid_context which only allows IBS to be running only 
> > in system-wide mode (e.g. perf record -a). IBS hardware is 
> > available in each core and should be per-context.  This patch 
> > modifies the task_ctx_nr to use the perf_hw_context (default) 
> > instead.
> 
> I'm wondering how extensively was it tested/verified that it's 
> safe to enable IBS in per context mode as well, and that the 
> profiling results are precise and accurate?

>From the implementation's point of view this is very similar to hw
perf counters. I wouldn't expect any issues here. Since IBS can be
immediatly started/stopped and there is no caching, there won't be any
incomming sample that is not related to that context.

The only potential problem I see could be a security risk in a way
that an IBS sample might expose data related to other contexts such as
cache information. This is similar to uncore/northbridge events so I
don't think this is an issue, but we might want to evaluate this.

> We never used the IBS hardware in this fashion before, so some 
> extra care is prudent - and traces of that extra care should be 
> visible in the changelog as well.

Yeah, a comparison of numbers for IBS and hw counter (-e r076:p,r076
and -e r0C1:p,r0C1) in per-context mode would be useful here.

-Robert
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