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From:   Stephane Eranian <eranian@google.com>
Date:   Thu, 7 Jun 2018 09:01:30 -0700
Message-ID: <CABPqkBR88VBZB1mLttUx0UxnFgWTJ=ggb66PsMti--svJU70UQ@mail.gmail.com>
Subject: Re: [PATCH 09/10] perf/cputime: Don't stop idle tick if there's live
 cputime event
To:     Andi Kleen <andi@firstfloor.org>
Cc:     Jiri Olsa <jolsa@kernel.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        David Ahern <dsahern@gmail.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Milian Wolff <milian.wolff@kdab.com>, frederic@kernel.org
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Hi Jiri,

On Thu, Jun 7, 2018 at 8:45 AM Andi Kleen <andi@firstfloor.org> wrote:
>
> On Thu, Jun 07, 2018 at 12:15:12AM +0200, Jiri Olsa wrote:
> > Disable stopping of the idle tick when having live cputime
> > event. When the tick is disabled, the idle counts are out
> > of date until next tick/update and perf cputime PMU provides
> > misleading counts.
>
> I really don't like this. This can totally change performance
> (e.g. less Turbo due to less idle) and performance tools shouldn't
> change the performance profile drastically.
>
You do not want to change the behavior of the kernel just because you
are monitoring.
This may introduce side effects on other events which may not otherwise exist.