perf/aux: Only update ->aux_wakeup in non-overwrite mode
authorAlexander Shishkin <alexander.shishkin@linux.intel.com>
Wed, 6 Sep 2017 16:08:11 +0000 (19:08 +0300)
committerIngo Molnar <mingo@kernel.org>
Fri, 29 Sep 2017 08:06:45 +0000 (10:06 +0200)
commit441430eb54a00586f95f1aefc48e0801bbd6a923
tree9e9bfffb3625e73466e63120ad9014980c97be46
parent770b782f555d663d133fcd4dc1632023f79357b9
perf/aux: Only update ->aux_wakeup in non-overwrite mode

The following commit:

  d9a50b0256 ("perf/aux: Ensure aux_wakeup represents most recent wakeup index")

changed the 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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: will.deacon@arm.com
Link: http://lkml.kernel.org/r/20170906160811.16510-1-alexander.shishkin@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
kernel/events/ring_buffer.c