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Date:   Fri, 8 Jun 2018 02:12:03 +0200
From:   Jiri Olsa <jolsa@redhat.com>
To:     Stephane Eranian <eranian@google.com>
Cc:     Andi Kleen <andi@firstfloor.org>, 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
Subject: Re: [PATCH 09/10] perf/cputime: Don't stop idle tick if there's live
 cputime event
Message-ID: <20180608001203.GB12322@krava>
References: <20180606221513.11302-1-jolsa@kernel.org>
 <20180606221513.11302-10-jolsa@kernel.org>
 <20180607154522.hyjmxn3q6zgcvsnx@two.firstfloor.org>
 <CABPqkBR88VBZB1mLttUx0UxnFgWTJ=ggb66PsMti--svJU70UQ@mail.gmail.com>
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On Thu, Jun 07, 2018 at 09:01:30AM -0700, Stephane Eranian wrote:
> 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.

right.. I guess we can survive few seconds of misleading idle counts
and perhaps we could detect nohz is enabled and warn about this

thanks,
jirka