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

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Message-ID: <20151019080852.GA24924@gmail.com>
Date:	Mon, 19 Oct 2015 10:08:52 +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:

> 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.

Thanks,

	Ingo
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