linux-kernel - [RFC PATCH v2 2/4] perf: Allow adding fixed random jitter to the alternate sampling period

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Date: Mon, 22 Apr 2024 11:49:27 +0100
From: Ben Gainey <ben.gainey@....com>
To: peterz@...radead.org,
	mingo@...hat.com,
	acme@...nel.org,
	namhyung@...nel.org
Cc: 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,
	Ben Gainey <ben.gainey@....com>
Subject: [RFC PATCH v2 2/4] perf: Allow adding fixed random jitter to the alternate sampling period

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 |  3 ++-
 kernel/events/core.c            | 11 ++++++++++-
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index 5c1701d091cf..dd3697a4b300 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -461,7 +461,8 @@ 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 : 1, /* add a limited amount of jitter on each alternate period */
+				__reserved_1   : 25;
 
 	union {
 		__u32		wakeup_events;	  /* wakeup every n events */
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 07f1f931e18e..079ae520e836 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>
@@ -9546,6 +9547,8 @@ static inline bool sample_is_allowed(struct perf_event *event, struct pt_regs *r
 	return true;
 }
 
+# define MAX_ALT_SAMPLE_PERIOD_JITTER 16
+
 /*
  * Generic event overflow handling, sampling.
  */
@@ -9573,7 +9576,10 @@ 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(MAX_ALT_SAMPLE_PERIOD_JITTER)
+					: 0);
 
 		hwc->sample_period = sample_period;
 		hwc->using_alternative_sample_period = !using_alt;
@@ -12503,6 +12509,9 @@ 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);
-- 
2.44.0