linux-kernel - Re: deadlock in scheduler enabling HRTICK feature

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Message-ID: <20130628090939.GD29209@dyad.programming.kicks-ass.net>
Date:	Fri, 28 Jun 2013 11:09:39 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	David Ahern <dsahern@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: deadlock in scheduler enabling HRTICK feature

On Thu, Jun 27, 2013 at 04:28:57PM -0600, David Ahern wrote:
> On 6/27/13 4:43 AM, Peter Zijlstra wrote:
> >On Wed, Jun 26, 2013 at 10:46:33AM -0600, David Ahern wrote:
> >>On 6/26/13 1:05 AM, Peter Zijlstra wrote:
> >>>>What is the expectation that the feature provides? not a whole lot of
> >>>>documentation on it. I walked down the path wondering if it solved an odd
> >>>>problem we are seeing with the CFS in 2.6.27 kernel.
> >>>
> >>>Its supposed to use hrtimers for slice expiry instead of the regular tick.
> >>
> >>So theoretically CPU bound tasks would get preempted sooner? That was my
> >>guess/hope anyways.
> >
> >Doth the below worketh?
> 
> It doth.
> 
> Usually make -j 8 for a kernel build in a VM would lock it up pretty
> quickly. With the patch I was able to run full builds multiple times.

Good!

> As for the solution you are avoiding the nesting by not waking up the
> softirq daemon.

Yah! :-) Obviously doing a wakeup while holding scheduler locks isn't going to
work out well. And the only reason we really need that pesky softirq nonsense
is when we accidentally schedule a timer that's already expired; in that case
we'll run it from sirq context.

We don't care about missing events like that; there's always the actual tick
for backup.

I suppose I'd better go write a Changelog and properly submit the patch :-)
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