[PATCH 1/1] perf/core: fix implicitly enable dynamic interrupt throttle

[kan . liang]

From: Kan Liang <kan.liang@xxxxxxxxx>

This patch fixes an issue which was introduced from 'commit 91a612eea9a3
("perf/core: Fix dynamic interrupt throttle")'
The old patch unconditionally sets the perf_sample_allowed_ns value to
!0. But that could trigger an issue 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 changes
perf_event_max_sample_rate.
For this case, the mechanism will be enabled implicitly, because
perf_sample_allowed_ns becomes !0.

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

Signed-off-by: Kan Liang <kan.liang@xxxxxxxxx>
---
 kernel/events/core.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 4e2ebf6..4042a3d 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -375,6 +375,14 @@ static void update_perf_cpu_limits(void)
 {
 	u64 tmp = perf_sample_period_ns;
 
+	/*
+	 * Don't update the perf_sample_allowed_ns,
+	 * if the dynamic interrupt throttle mechanism is disabled.
+	 */
+	if (sysctl_perf_cpu_time_max_percent == 100 ||
+	    sysctl_perf_cpu_time_max_percent == 0)
+		return;
+
 	tmp *= sysctl_perf_cpu_time_max_percent;
 	tmp = div_u64(tmp, 100);
 	if (!tmp)
-- 
2.5.0