diff options
| author | Stephane Eranian <eranian@google.com> | 2022-03-22 15:15:10 -0700 |
|---|---|---|
| committer | Peter Zijlstra <peterz@infradead.org> | 2022-04-05 10:24:37 +0200 |
| commit | ba2fe7500845a30fc845a72081999cf632051862 (patch) | |
| tree | b6479f3c32d2e10a25b6d951478bba6c8d46c037 /arch/x86/events/core.c | |
| parent | 8910075d61a37e5b0d82e6c83ed9a0a31fe9ea08 (diff) | |
perf/x86/amd: Add AMD branch sampling period adjustment
Add code to adjust the sampling event period when used with the Branch
Sampling feature (BRS). Given the depth of the BRS (16), the period is
reduced by that depth such that in the best case scenario, BRS saturates at
the desired sampling period. In practice, though, the processor may execute
more branches. Given a desired period P and a depth D, the kernel programs
the actual period at P - D. After P occurrences of the sampling event, the
counter overflows. It then may take X branches (skid) before the NMI is
caught and held by the hardware and BRS activates. Then, after D branches,
BRS saturates and the NMI is delivered. With no skid, the effective period
would be (P - D) + D = P. In practice, however, it will likely be (P - D) +
X + D. There is no way to eliminate X or predict X.
Signed-off-by: Stephane Eranian <eranian@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220322221517.2510440-7-eranian@google.com
Diffstat (limited to 'arch/x86/events/core.c')
| -rw-r--r-- | arch/x86/events/core.c | 7 |
1 files changed, 7 insertions, 0 deletions
diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c index 7ada9172b074..54f992e65252 100644 --- a/arch/x86/events/core.c +++ b/arch/x86/events/core.c @@ -1375,6 +1375,13 @@ int x86_perf_event_set_period(struct perf_event *event) return x86_pmu.set_topdown_event_period(event); /* + * decrease period by the depth of the BRS feature to get + * the last N taken branches and approximate the desired period + */ + if (has_branch_stack(event)) + period = amd_brs_adjust_period(period); + + /* * If we are way outside a reasonable range then just skip forward: */ if (unlikely(left <= -period)) { |