[PATCH v2 2/5] perf: Allow adding fixed random jitter to the alternate sampling period

[mark.barnett@arm.com]

From: 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>
Signed-off-by: Mark Barnett <mark.barnett@arm.com>
---
 include/uapi/linux/perf_event.h | 7 ++++++-
 kernel/events/core.c            | 9 ++++++++-
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index 499a8673df8e..c0076ce8f80a 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -461,7 +461,12 @@ 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;
+				/*
+				 * Add a limited amount of jitter on each alternate period, where
+				 * the jitter is between [0, (2<<jitter_alt_period) - 1]
+				 */
+				jitter_alt_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 7e339d12363a..c5480965df32 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>
@@ -9878,7 +9879,10 @@ static int __perf_event_overflow(struct perf_event *event,
 	if (event->attr.alt_sample_period) {
 		bool using_alt = hwc->using_alt_sample_period;
 		u64 sample_period = (using_alt ? event->attr.sample_period
-					       : event->attr.alt_sample_period);
+					       : event->attr.alt_sample_period)
+				  + (event->attr.jitter_alt_period
+					? get_random_u32_below(2 << event->attr.jitter_alt_period)
+					: 0);
 
 		hwc->sample_period = sample_period;
 		hwc->using_alt_sample_period = !using_alt;
@@ -12803,6 +12807,9 @@ SYSCALL_DEFINE5(perf_event_open,
 		}
 	}
 
+	if (attr.jitter_alt_period && !attr.alt_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