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

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Message-ID: <5624ADF8.3090505@intel.com>
Date:	Mon, 19 Oct 2015 11:46:48 +0300
From:	Adrian Hunter <adrian.hunter@...el.com>
To:	Ingo Molnar <mingo@...nel.org>
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

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.

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