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

Ben Gainey
2024-04-22T10:50:06Z

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);