linux-kernel - [PATCH -stable 4.1 2/4] perf/core: Fix implicitly enable dynamic interrupt throttle

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Date:   Mon, 6 Mar 2017 15:14:23 +0800
From:   Tan Xiaojun <tanxiaojun@...wei.com>
To:     <alexander.levin@...izon.com>, <peterz@...radead.org>,
        <wangnan0@...wei.com>, <guohanjun@...wei.com>
CC:     <linuxarm@...wei.com>, <linux-kernel@...r.kernel.org>
Subject: [PATCH -stable 4.1 2/4] perf/core: Fix implicitly enable dynamic interrupt throttle

From: Kan Liang <kan.liang@...el.com>

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@...el.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
Cc: Jiri Olsa <jolsa@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Stephane Eranian <eranian@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Vince Weaver <vincent.weaver@...ne.edu>
Cc: acme@...nel.org
Link: http://lkml.kernel.org/r/1462260366-3160-1-git-send-email-kan.liang@intel.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Tan Xiaojun <tanxiaojun@...wei.com>
---
 kernel/events/core.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 3089a004..f96ad76 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -216,6 +216,13 @@ 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;
 	update_perf_cpu_limits();
-- 
1.9.1