perf/core: Fix implicitly enable dynamic interrupt throttle

This change “perf/core: Fix implicitly enable dynamic interrupt throttle” in Linux kernel is authored by Kan Liang <kan.liang [at]> on Tue May 3 00:26:06 2016 -0700.

perf/core: Fix implicitly enable dynamic interrupt throttle

This patch fixes an issue which was introduced by commit:

  91a612eea9a3 ("perf/core: Fix dynamic interrupt throttle")

... which commit unconditionally sets the perf_sample_allowed_ns value
to !0. But that could trigger a bug in the following corner case:

The user can disable the dynamic interrupt throttle mechanism by setting
perf_cpu_time_max_percent to 0. Then they change perf_event_max_sample_rate.
For this case, the mechanism will be enabled implicitly, because
perf_sample_allowed_ns becomes !0 - which is not what we want.

This patch only updates perf_sample_allowed_ns when the dynamic
interrupt throttle mechanism is enabled.

Signed-off-by: Kan Liang <>
Signed-off-by: Peter Zijlstra (Intel) <>
Cc: Alexander Shishkin <>
Cc: Arnaldo Carvalho de Melo <>
Cc: Jiri Olsa <>
Cc: Linus Torvalds <>
Cc: Peter Zijlstra <>
Cc: Stephane Eranian <>
Cc: Thomas Gleixner <>
Cc: Vince Weaver <>
Signed-off-by: Ingo Molnar <>

This Linux change may have been applied to various maintained Linux releases and you can find Linux releases including commit ab7fdef.

There are 7 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 kernel/events/core.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index f54454e..f94f164 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -397,6 +397,13 @@ int perf_proc_update_handler(struct ctl_table *table, int write,
 	if (ret || !write)
 		return ret;
+	/*
+	 * If throttling is disabled don't allow the write:
+	 */
+	if (sysctl_perf_cpu_time_max_percent == 100 ||
+	    sysctl_perf_cpu_time_max_percent == 0)
+		return -EINVAL;
 	max_samples_per_tick = DIV_ROUND_UP(sysctl_perf_event_sample_rate, HZ);
 	perf_sample_period_ns = NSEC_PER_SEC / sysctl_perf_event_sample_rate;

The commit for this change in Linux stable tree is ab7fdef (patch).

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