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

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Message-ID: <333246323.13611103.1454006593261.JavaMail.zimbra@redhat.com>
Date:	Thu, 28 Jan 2016 13:43:13 -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: Thursday, 28 January, 2016 6:49:03 PM
> Subject: Re: [BUG] scheduler doesn't balance thread to idle cpu for 3 seconds
> 
> On Thu, Jan 28, 2016 at 04:55:02PM +0100, Jan Stancek wrote:
> > On 01/27/2016 03:52 PM, Jan Stancek wrote:
> > > Hello,
> > > 
> > > pthread_cond_wait_1/2 [1] is rarely failing for me on 4.5.0-rc1,
> > > on x86_64 KVM guest with 2 CPUs.
> > > 
> > > This test [1]:
> > > - spawns 2 SCHED_RR threads
> > > - first thread with higher priority sets alarm for 2 seconds and blocks
> > > on condition
> > > - second thread with lower priority is busy looping for 5 seconds
> > > - after 2 seconds alarm signal arrives and handler signals condition
> > > - high priority thread should resume running
> > 
> > I have slightly modified testcase, so it will finish immediately when high
> > prio
> > thread is done. And also to allow it to compile outside of openposix
> > testsuite.
> 
> Yeah, I 'fixed' the testcase too.
> 
> So I've had it run for almost 2 hours without a single fail. I've
> hot-plugged my cpu count down to 2.

I can try that too. I'm mostly seeing this on s390 and x86_64 KVM guests,
both have 2 CPUs.

Have you noticed if iteration times vary or if they stay consitently
at ~2 seconds?

> 
> 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.

Regards,
Jan