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

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Message-ID: <20160129101522.GF6357@twins.programming.kicks-ass.net>
Date:	Fri, 29 Jan 2016 11:15:22 +0100
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
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.

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.