[tip:perf/core] perf/core: Fix implicitly enable dynamic interrupt throttle

From: tip-bot for Kan Liang
Date: Fri Jun 03 2016 - 06:51:06 EST

Commit-ID: ab7fdefba68f66c8523571c3b3a940635d781824
Gitweb: http://git.kernel.org/tip/ab7fdefba68f66c8523571c3b3a940635d781824
Author: Kan Liang <kan.liang@xxxxxxxxx>
AuthorDate: Tue, 3 May 2016 00:26:06 -0700
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Fri, 3 Jun 2016 09:40:16 +0200

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 <kan.liang@xxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Stephane Eranian <eranian@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Vince Weaver <vincent.weaver@xxxxxxxxx>
Cc: acme@xxxxxxxxxx
Link: http://lkml.kernel.org/r/1462260366-3160-1-git-send-email-kan.liang@xxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
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;