perf/x86: Fix full width counter, counter overflow
authorPeter Zijlstra (Intel) <peterz@infradead.org>
Tue, 29 Nov 2016 20:33:28 +0000 (20:33 +0000)
committerIngo Molnar <mingo@kernel.org>
Tue, 6 Dec 2016 08:44:28 +0000 (09:44 +0100)
commit7f612a7f0bc13a2361a152862435b7941156b6af
treedc5a2c959c6e1900bf90c726fb58730395bfd55e
parent1dba23b12f49d7cf3d4504171c62541122b55141
perf/x86: Fix full width counter, counter overflow

Lukasz reported that perf stat counters overflow handling is broken on KNL/SLM.

Both these parts have full_width_write set, and that does indeed have
a problem. In order to deal with counter wrap, we must sample the
counter at at least half the counter period (see also the sampling
theorem) such that we can unambiguously reconstruct the count.

However commit:

  069e0c3c4058 ("perf/x86/intel: Support full width counting")

sets the sampling interval to the full period, not half.

Fixing that exposes another issue, in that we must not sign extend the
delta value when we shift it right; the counter cannot have
decremented after all.

With both these issues fixed, counter overflow functions correctly
again.

Reported-by: Lukasz Odzioba <lukasz.odzioba@intel.com>
Tested-by: Liang, Kan <kan.liang@intel.com>
Tested-by: Odzioba, Lukasz <lukasz.odzioba@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Cc: stable@vger.kernel.org
Fixes: 069e0c3c4058 ("perf/x86/intel: Support full width counting")
Signed-off-by: Ingo Molnar <mingo@kernel.org>
arch/x86/events/core.c
arch/x86/events/intel/core.c