Commit fa5d0a82 authored by Ravi Bangoria's avatar Ravi Bangoria Committed by Peter Zijlstra
Browse files

perf/amd/ibs: Ceil sample_period to min_period 



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: default avatarRavi Bangoria <ravi.bangoria@amd.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250115054438.1021-9-ravi.bangoria@amd.com
parent 1afbdd97

arch/x86/events/amd/ibs.c

+7 −0
Original line number Diff line number Diff line
@@ -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);