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Message-ID: <20241107160721.1401614-3-deepak.surti@arm.com>
Date: Thu, 7 Nov 2024 16:07:19 +0000
From: Deepak Surti <deepak.surti@....com>
To: <peterz@...radead.org>, <mingo@...hat.com>, <acme@...nel.org>,
<namhyung@...nel.org>
CC: <deepak.surti@....com>, <mark.barnett@....com>, <ben.gainey@....com>,
<ak@...ux.intel.com>, <will@...nel.org>, <james.clark@....com>,
<mark.rutland@....com>, <alexander.shishkin@...ux.intel.com>,
<jolsa@...nel.org>, <irogers@...gle.com>, <adrian.hunter@...el.com>,
<linux-perf-users@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
<linux-arm-kernel@...ts.infradead.org>
Subject: [PATCH v1 2/4] perf: Allow adding fixed random jitter to the alternate sampling period
From: Ben Gainey <ben.gainey@....com>
This change modifies the core perf overflow handler, adding some small
random jitter to each sample period whenever an event switches between the
two alternate sample periods. A new flag is added to perf_event_attr to
opt into this behaviour.
This change follows the discussion in [1], where it is recognized that it
may be possible for certain patterns of execution to end up with biased
results.
[1] https://lore.kernel.org/linux-perf-users/Zc24eLqZycmIg3d2@tassilo/
Signed-off-by: Ben Gainey <ben.gainey@....com>
---
include/uapi/linux/perf_event.h | 10 +++++++++-
kernel/events/core.c | 10 +++++++++-
2 files changed, 18 insertions(+), 2 deletions(-)
diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index bedae424ba36..16dbeea5803e 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -461,7 +461,15 @@ struct perf_event_attr {
inherit_thread : 1, /* children only inherit if cloned with CLONE_THREAD */
remove_on_exec : 1, /* event is removed from task on exec */
sigtrap : 1, /* send synchronous SIGTRAP on event */
- __reserved_1 : 26;
+ /*
+ * jitter_alternate_period:
+ *
+ * add a limited amount of jitter on each
+ * alternate period, where the jitter is between
+ * [0, (2<<jitter_alternate_period) - 1]
+ */
+ jitter_alternate_period : 3,
+ __reserved_1 : 23;
union {
__u32 wakeup_events; /* wakeup every n events */
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 579b04af70a2..14dfac6f13dd 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -15,6 +15,7 @@
#include <linux/idr.h>
#include <linux/file.h>
#include <linux/poll.h>
+#include <linux/random.h>
#include <linux/slab.h>
#include <linux/hash.h>
#include <linux/tick.h>
@@ -9831,7 +9832,11 @@ static int __perf_event_overflow(struct perf_event *event,
if (event->attr.alternative_sample_period) {
bool using_alt = hwc->using_alternative_sample_period;
u64 sample_period = (using_alt ? event->attr.sample_period
- : event->attr.alternative_sample_period);
+ : event->attr.alternative_sample_period)
+ + (event->attr.jitter_alternate_period
+ ? get_random_u32_below(2 <<
+ event->attr.jitter_alternate_period)
+ : 0);
hwc->sample_period = sample_period;
hwc->using_alternative_sample_period = !using_alt;
@@ -12744,6 +12749,9 @@ SYSCALL_DEFINE5(perf_event_open,
}
}
+ if (attr.jitter_alternate_period && !attr.alternative_sample_period)
+ return -EINVAL;
+
/* Only privileged users can get physical addresses */
if ((attr.sample_type & PERF_SAMPLE_PHYS_ADDR)) {
err = perf_allow_kernel(&attr);
--
2.43.0
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