From: Adrian Hunter <adrian.hunter@intel.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
Arnaldo Carvalho de Melo <acme@kernel.org>,
Andy Lutomirski <luto@amacapital.net>,
Thomas Gleixner <tglx@linutronix.de>,
linux-kernel@vger.kernel.org,
Stephane Eranian <eranian@google.com>,
Andi Kleen <ak@linux.intel.com>
Subject: Re: [PATCH 1/1] perf/x86: Fix time_shift in perf_event_mmap_page
Date: Mon, 19 Oct 2015 11:46:48 +0300 [thread overview]
Message-ID: <5624ADF8.3090505@intel.com> (raw)
In-Reply-To: <20151019080852.GA24924@gmail.com>
On 19/10/15 11:08, Ingo Molnar wrote:
>
> * Adrian Hunter <adrian.hunter@intel.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@intel.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.
next prev parent reply other threads:[~2015-10-19 8:49 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-10-16 13:24 [PATCH 0/1] perf/x86: Fix time_shift in perf_event_mmap_page Adrian Hunter
2015-10-16 13:24 ` [PATCH 1/1] " Adrian Hunter
2015-10-19 8:08 ` Ingo Molnar
2015-10-19 8:46 ` Adrian Hunter [this message]
2015-10-19 11:21 ` Ingo Molnar
2015-10-19 11:57 ` [PATCH V2 " Adrian Hunter
2015-10-20 9:35 ` [tip:perf/core] " tip-bot for Adrian Hunter
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