linux-kernel - Re: [PATCH 1/1] perf/x86: Fix time_shift in perf_event_mmap

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Message-ID: <20151019112126.GA32722@gmail.com>
Date:	Mon, 19 Oct 2015 13:21:26 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Adrian Hunter <adrian.hunter@...el.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org,
	Stephane Eranian <eranian@...gle.com>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 1/1] perf/x86: Fix time_shift in perf_event_mmap_page


* Adrian Hunter <adrian.hunter@...el.com> wrote:

> On 19/10/15 11:08, Ingo Molnar wrote:
> > 
> > * Adrian Hunter <adrian.hunter@...el.com> wrote:
> > 
> >> Commit b20112edeadf ("perf/x86: Improve accuracy of perf/sched clock")
> >> allowed the time_shift value in perf_event_mmap_page to be as much
> >> as 32.  Unfortunately the documented algorithms for using time_shift
> >> have it shifting an integer, whereas to work correctly with the value
> >> 32, the type must be u64.
> >>
> >> Fix by limiting the shift to 31 and adjusting the multiplier accordingly.
> >>
> >> Also update the documentation of perf_event_mmap_page so that new code
> >> based on it will be more future-proof.
> >>
> >> Fixes: b20112edeadf ("perf/x86: Improve accuracy of perf/sched clock")
> >> Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>
> > 
> > Would be nice to point out via what symptoms the code misbehaves and how users 
> > notice.
> 
> In the case of perf tools, Intel PT decodes correctly but the timestamps
> that are output (for example by perf script) have lost 32-bits of
> granularity so they look like they are not changing at all.

Sounds like a nice and very informative paragraph to add to the changelog! :-)

Thanks,

	Ingo
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