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perf events, x86: Work around the Nehalem AAJ80 erratum

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On Nehalem CPUs the retired branch-misses event can be completely bogus,
when there are no branch-misses occuring. When there are a lot of branch
misses then the count is pretty accurate. Still, this leaves us with an
event that over-counts a lot.

Detect this erratum and work it around by using BR_MISP_EXEC.ANY events.
These will also count speculated branches but still it's a lot more
precise in practice than the architectural event.

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Link: http://lkml.kernel.org/n/tip-yyfg0bxo9jsqxd6a0ovfny27@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Ingo Molnar 2011-04-27 11:51:41 +02:00
parent 18a073a3ac
commit ec75a71634
1 changed files with 14 additions and 2 deletions
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16
arch/x86/kernel/cpu/perf_event_intel.c
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@ -25,7 +25,7 @@ struct intel_percore {
/*
* Intel PerfMon, used on Core and later.
*/
static const u64 intel_perfmon_event_map[] =
static u64 intel_perfmon_event_map[PERF_COUNT_HW_MAX] __read_mostly =
{
[PERF_COUNT_HW_CPU_CYCLES] = 0x003c,
[PERF_COUNT_HW_INSTRUCTIONS] = 0x00c0,
@ -1308,7 +1308,7 @@ static void intel_clovertown_quirks(void)
* AJ106 could possibly be worked around by not allowing LBR
* usage from PEBS, including the fixup.
* AJ68 could possibly be worked around by always programming
* a pebs_event_reset[0] value and coping with the lost events.
* a pebs_event_reset[0] value and coping with the lost events.
*
* But taken together it might just make sense to not enable PEBS on
* these chips.
@ -1412,6 +1412,18 @@ static __init int intel_pmu_init(void)
x86_pmu.percore_constraints = intel_nehalem_percore_constraints;
x86_pmu.enable_all = intel_pmu_nhm_enable_all;
x86_pmu.extra_regs = intel_nehalem_extra_regs;
if (ebx & 0x40) {
/*
* Erratum AAJ80 detected, we work it around by using
* the BR_MISP_EXEC.ANY event. This will over-count
* branch-misses, but it's still much better than the
* architectural event which is often completely bogus:
*/
intel_perfmon_event_map[PERF_COUNT_HW_BRANCH_MISSES] = 0x7f89;
pr_cont("erratum AAJ80 worked around, ");
}
pr_cont("Nehalem events, ");
break;