The following commit has been merged into the perf/core branch of tip:
Commit-ID: fa5d0a824e3bbd1f793d962f9e012ab0a8ee11c5
Gitweb: https://git.kernel.org/tip/fa5d0a824e3bbd1f793d962f9e012ab0a8ee11c5
Author: Ravi Bangoria <ravi.bangoria@xxxxxxx>
AuthorDate: Wed, 15 Jan 2025 05:44:37
Committer: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Mon, 03 Feb 2025 11:46:06 +01:00
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: Ravi Bangoria <ravi.bangoria@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20250115054438.1021-9-ravi.bangoria@xxxxxxx
---
arch/x86/events/amd/ibs.c | 7 +++++++
1 file changed, 7 insertions(+)
diff --git a/arch/x86/events/amd/ibs.c b/arch/x86/events/amd/ibs.c
index aea893a..7978d79 100644
--- a/arch/x86/events/amd/ibs.c
+++ b/arch/x86/events/amd/ibs.c
@@ -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 @@ fail:
perf_sample_save_callchain(&data, event, iregs);
throttle = perf_event_overflow(event, &data, ®s);
+
+ 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);
|