linux-kernel - Re: [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown metrics

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Date:   Thu, 29 Aug 2019 11:17:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     kan.liang@...ux.intel.com, acme@...nel.org, mingo@...hat.com,
        linux-kernel@...r.kernel.org, tglx@...utronix.de, jolsa@...nel.org,
        eranian@...gle.com, alexander.shishkin@...ux.intel.com
Subject: Re: [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown
 metrics

On Wed, Aug 28, 2019 at 08:11:51PM -0700, Andi Kleen wrote:
> On Wed, Aug 28, 2019 at 06:28:57PM +0200, Peter Zijlstra wrote:
> > On Wed, Aug 28, 2019 at 09:17:54AM -0700, Andi Kleen wrote:
> > > > This really doesn't make sense to me; if you set FIXED_CTR3 := 0, you'll
> > > > never trigger the overflow there; this then seems to suggest the actual
> > > 
> > > The 48bit counter might overflow in a few hours.
> > 
> > Sure; the point is? Kan said it should not be too big; a full 48bit wrap
> > around must be too big or nothing is.
> 
> We expect users to avoid making it too big, but we cannot rule it out.
> 
> Actually for the common perf stat for a long time case you can make it fairly
> big because the percentages still work out over the complete run time.
> 
> The problem with letting it accumulate too much is mainly if you
> want to use a continuously running metrics counter to time smaller
> regions by taking deltas, for example using RDPMC. 
> 
> In this case the small regions would be too inaccurate after some time.
> 
> But to make the first case work absolutely need to handle the overflow
> case. Otherwise the metrics would just randomly stop at some
> point.

But then you need -max_period, not 0. By using half the period, there is
no ambiguity on overflow.