linux-kernel - Re: [BUG] scheduler doesn't balance thread to idle cpu for 3 seconds

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Date:	Fri, 29 Jan 2016 05:33:45 -0500 (EST)
From:	Jan Stancek <jstancek@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	alex shi <alex.shi@...el.com>, guz fnst <guz.fnst@...fujitsu.com>,
	mingo@...hat.com, jolsa@...hat.com, riel@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [BUG] scheduler doesn't balance thread to idle cpu for 3
 seconds





----- Original Message -----
> From: "Peter Zijlstra" <peterz@...radead.org>
> To: "Jan Stancek" <jstancek@...hat.com>
> Cc: "alex shi" <alex.shi@...el.com>, "guz fnst" <guz.fnst@...fujitsu.com>, mingo@...hat.com, jolsa@...hat.com,
> riel@...hat.com, linux-kernel@...r.kernel.org
> Sent: Friday, 29 January, 2016 11:15:22 AM
> Subject: Re: [BUG] scheduler doesn't balance thread to idle cpu for 3 seconds
> 
> On Thu, Jan 28, 2016 at 01:43:13PM -0500, Jan Stancek wrote:
> > > How long should I have to wait for a fail?
> > 
> > It's about 1000-2000 iterations for me, which I think you covered
> > by now in those 2 hours.
> 
> So I've been running:
> 
>   while ! ./pthread_cond_wait_1 ; do sleep 1; done
> 
> overnight on the machine, and have yet to hit a wobbly -- that is, its
> still running.

I have seen similar result.

Then, instead of turning CPUs off, I spawned more low prio threads to scale
with number of CPUs on system:

@@ -213,10 +213,14 @@
                printf(ERROR_PREFIX "pthread_attr_setschedparam\n");
                exit(PTS_UNRESOLVED);
        }
-       rc = pthread_create(&low_id, &low_attr, low_priority_thread, NULL);
-       if (rc != 0) {
-               printf(ERROR_PREFIX "pthread_create\n");
-               exit(PTS_UNRESOLVED);
+
+       int i, ncpus = sysconf(_SC_NPROCESSORS_ONLN);
+       for (i = 0; i < ncpus - 1; i++) {
+               rc = pthread_create(&low_id, &low_attr, low_priority_thread, NULL);
+               if (rc != 0) {
+                       printf(ERROR_PREFIX "pthread_create\n");
+                       exit(PTS_UNRESOLVED);
+               }

and let this ran on 3 bare metal x86 systems over night (v4.5-rc1). It
failed on 2 systems (12 and 24 CPUs) with 1:1000 chance, it never failed
on 3rd one (4 CPUs).

> 
> Also note that I don't think failing this test is a bug per se.
> Undesirable maybe, but within spec, since SIGALRM is process wide, so it
> being delivered to the SCHED_OTHER task is accepted, and SCHED_OTHER has
> no timeliness guarantees.
> 
> That said; if I could reliably reproduce I'd have a go at fixing this, I
> suspect there's a 'fun' problem at the bottom of this.

Thanks for trying, I'll see if I can find some more reliable way.

Regards,
Jan