linux-kernel - Re: [PATCH v1 00/13] perf/x86/amd: Add AMD Fam19h Branch Sampling support

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Date:   Tue, 14 Sep 2021 22:55:12 -0700
From:   Stephane Eranian <eranian@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, acme@...hat.com, jolsa@...hat.com,
        kim.phillips@....com, namhyung@...nel.org, irogers@...gle.com
Subject: Re: [PATCH v1 00/13] perf/x86/amd: Add AMD Fam19h Branch Sampling support

On Thu, Sep 9, 2021 at 1:55 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Thu, Sep 09, 2021 at 12:56:47AM -0700, Stephane Eranian wrote:
> > This patch series adds support for the AMD Fam19h 16-deep branch sampling
> > feature as described in the AMD PPR Fam19h Model 01h Revision B1 section 2.1.13.
>
> Yay..
>
> > BRS interacts with the NMI interrupt as well. Because enabling BRS is expensive,
> > it is only activated after P event occurrences, where P is the desired sampling period.
> > At P occurrences of the event, the counter overflows, the CPU catches the NMI interrupt,
> > activates BRS for 16 branches until it saturates, and then delivers the NMI to the kernel.
>
> WTF... ?!? Srsly? You're joking right?
>

As I said, this is because of the cost of running BRS usually for
millions of branches to keep only the last 16.
Running branch sampling in general on any arch is  never totally free.

>
> Also, can you please fix you MUA to wrap at 78 chars like normal people?

Ok, I fixed that now.