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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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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