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

[Ben Gainey <ben.gainey@arm.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@arm.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