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perf/amd/ibs: Ceil sample_period to min_period

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The sample_period needs to be recalibrated after every sample to match
the desired sampling freq for a 'freq mode event'. Since the next
sample_period is calculated by generic kernel, PMU specific constraints
are not (explicitly) reckoned.

The sample_period value is programmed in a MaxCnt field of IBS PMUs, and
the MaxCnt field has following constraints:

1) MaxCnt must be multiple of 0x10.

  Kernel keeps track of residual / over-counted period into period_left,
  which should take care of this constraint by programming MaxCnt with
  (sample_period & ~0xF) and adding remaining period into the next sample.

2) MaxCnt must be >= 0x10 for IBS Fetch PMU and >= 0x90 for IBS Op PMU.

  Currently, IBS PMU driver allows sample_period below min_period, which
  is an undefined HW behavior. Reset sample_period to min_period whenever
  it's less than that.

Signed-off-by: Ravi Bangoria <ravi.bangoria@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250115054438.1021-9-ravi.bangoria@amd.com
This commit is contained in:
Ravi Bangoria
2025-01-15 05:44:37 +00:00
committed by Peter Zijlstra
parent 1afbdd970f
commit fa5d0a824e
1 changed files with 7 additions and 0 deletions
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7
arch/x86/events/amd/ibs.c
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@@ -457,6 +457,9 @@ static void perf_ibs_start(struct perf_event *event, int flags)
WARN_ON_ONCE(!(hwc->state & PERF_HES_UPTODATE));
hwc->state = 0;
if (event->attr.freq && hwc->sample_period < perf_ibs->min_period)
hwc->sample_period = perf_ibs->min_period;
perf_ibs_set_period(perf_ibs, hwc, &period);
if (perf_ibs == &perf_ibs_op && (ibs_caps & IBS_CAPS_OPCNTEXT)) {
config |= period & IBS_OP_MAX_CNT_EXT_MASK;
@@ -1191,6 +1194,10 @@ static int perf_ibs_handle_irq(struct perf_ibs *perf_ibs, struct pt_regs *iregs)
perf_sample_save_callchain(&data, event, iregs);
throttle = perf_event_overflow(event, &data, &regs);
if (event->attr.freq && hwc->sample_period < perf_ibs->min_period)
hwc->sample_period = perf_ibs->min_period;
out:
if (throttle) {
perf_ibs_stop(event, 0);