Re: [PATCH] perf/aux: Only update aux_wakeup in non-overwrite mode

From: Alexander Shishkin
Date: Wed Sep 13 2017 - 06:40:34 EST

Will Deacon <will.deacon@xxxxxxx> 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@xxxxxxxxxxxxxxx>
>> ---
>> 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@xxxxxxx>