linux-kernel - Re: "perf: interrupt took too long" messages even with perf_cpu_time_max

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Message-ID: <20160402110017.GP3448@twins.programming.kicks-ass.net>
Date:	Sat, 2 Apr 2016 13:00:17 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Markus Trippelsdorf <markus@...ppelsdorf.de>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Subject: Re: "perf: interrupt took too long" messages even with
 perf_cpu_time_max_percent==0

On Sat, Apr 02, 2016 at 11:32:39AM +0200, Markus Trippelsdorf wrote:
> Current git kernel sometimes shows:
> 
>  perf: interrupt took too long (71 > 52), lowering kernel.perf_event_max_sample_rate to 300
>  perf: interrupt took too long (103 > 88), lowering kernel.perf_event_max_sample_rate to 300
>  perf: interrupt took too long (130 > 128), lowering kernel.perf_event_max_sample_rate to 300
>  perf: interrupt took too long (175 > 162), lowering kernel.perf_event_max_sample_rate to 300
>  perf: interrupt took too long (219 > 218), lowering kernel.perf_event_max_sample_rate to 300
> ...
> 
> when running e.g. "perf top" even when  
> /proc/sys/kernel/perf_cpu_time_max_percent is set to 0.


Ah, was 0 also meant to disable it?

Does the below help?

---
 kernel/events/core.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 8c3b35f2a269..21ba024c9ed1 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -412,7 +412,8 @@ int perf_cpu_time_max_percent_handler(struct ctl_table *table, int write,
 	if (ret || !write)
 		return ret;
 
-	if (sysctl_perf_cpu_time_max_percent == 100) {
+	if (sysctl_perf_cpu_time_max_percent == 100 ||
+	    sysctl_perf_cpu_time_max_percent == 0) {
 		printk(KERN_WARNING
 		       "perf: Dynamic interrupt throttling disabled, can hang your system!\n");
 		WRITE_ONCE(perf_sample_allowed_ns, 0);