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Date: Mon, 30 Mar 2015 21:34:11 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: David Ahern <dsahern@gmail.com>
Cc: Stephane Eranian <eranian@google.com>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>, Jiri Olsa <jolsa@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        John Stultz <john.stultz@linaro.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH] perf, record: Add clockid parameter
Message-ID: <20150330193411.GG23123@twins.programming.kicks-ass.net>
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On Mon, Mar 30, 2015 at 11:33:42AM -0600, David Ahern wrote:
> On 3/28/15 1:55 AM, Peter Zijlstra wrote:
> >@@ -761,6 +762,11 @@ void perf_evsel__config(struct perf_evse
> >  		attr->disabled = 0;
> >  		attr->enable_on_exec = 0;
> >  	}
> >+
> >+	if (opts->clockid >= 0) {
> >+		attr->use_clockid = 1;
> >+		attr->clockid = opts->clockid;
> >+	}
> >  }
> 
> One more: you need to set attr->clockid to -1 if use_clockid is not set so
> that the analysis side knows whether attr->clockid was used. Otherwise it
> defaults to 0 == CLOCK_REALTIME which is misleading.
> 
> 
> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> index 1abf6919b8a2..27679ab38511 100644
> --- a/tools/perf/util/evsel.c
> +++ b/tools/perf/util/evsel.c
> @@ -766,7 +766,8 @@ void perf_evsel__config(struct perf_evsel *evsel, struct
> record_opts *opts)
>     if (opts->clockid >= 0) {
>         attr->use_clockid = 1;
>         attr->clockid = opts->clockid;
> -   }
> +   } else
> +       attr->clockid = -1;
>  }

No, we must not have a !0 value in ->clockid when we do not set
use_clockid. The kernel checks for nonzero tail values.