Messages in this thread | | | Date | Thu, 28 Sep 2017 12:29:15 +0100 | From | Will Deacon <> | Subject | Re: [PATCH] perf/aux: Only update aux_wakeup in non-overwrite mode |
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Hi Alexander,
On Wed, Sep 13, 2017 at 01:34:03PM +0300, Alexander Shishkin wrote: > Will Deacon <will.deacon@arm.com> writes: > > On Wed, Sep 06, 2017 at 07:08:11PM +0300, Alexander Shishkin wrote: > >> Commit d9a50b0256 ("perf/aux: Ensure aux_wakeup represents most recent > >> wakeup index") changed aux wakeup position calculation to rounddown(), > >> which causes a division-by-zero in AUX overwrite mode (aka "snapshot > >> mode"). > >> > >> The zero denominator results from the fact that perf record doesn't set > >> aux_watermark to anything, in which case the kernel will set it to half > >> the AUX buffer size, but only for non-overwrite mode. In the overwrite > >> mode aux_watermark stays zero. > >> > >> The good news is that, AUX overwrite mode, wakeups don't happen and > >> related bookkeeping is not relevant, so we can simply forego the whole > >> wakeup updates. > >> > >> Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com> > >> --- > >> kernel/events/ring_buffer.c | 20 +++++++++++++++----- > >> 1 file changed, 15 insertions(+), 5 deletions(-) > > > > Damn, sorry about that. How did you spot the problem? > > A normal perf record with -S should trigger it, I don't remember what > exactly I was doing at that moment. But no worries, we all missed it the > first time around. :) > > > Anyway, I think the code is much better with this factored out: > > > > Acked-by: Will Deacon <will.deacon@arm.com> > > Thanks!
What's the status on this patch? I don't see it in mainline or linux-next, but it would ideally get in as a fix for 4.14.
Will
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