Messages in this thread | | | From | Alexander Shishkin <> | Subject | Re: [PATCH] perf/aux: Only update aux_wakeup in non-overwrite mode | Date | Wed, 13 Sep 2017 13:34:03 +0300 |
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Will Deacon <will.deacon@arm.com> writes:
> Hi Alexander, > > 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!
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