linux-kernel - Re: [BUG] sched_setattr() SCHED

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Date:	Mon, 12 May 2014 14:30:32 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc:	Juri Lelli <juri.lelli@...il.com>,
	Dario Faggioli <raistlin@...ux.it>,
	Ingo Molnar <mingo@...e.hu>,
	lkml <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>
Subject: Re: [BUG] sched_setattr() SCHED_DEADLINE hangs system

On Mon, May 12, 2014 at 11:19:39AM +0200, Michael Kerrisk (man-pages) wrote:
> Hi Peter,
> 
> On Mon, May 12, 2014 at 10:47 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Mon, May 12, 2014 at 08:53:59AM +0200, Michael Kerrisk (man-pages) wrote:
> >> On 05/11/2014 04:54 PM, Michael Kerrisk (man-pages) wrote:
> >
> >> > $ time sudo ./t_sched_setattr d 18446744072 18446744072 18446744073
> >>
> >> I realize my speculation was completely off the mark. time(2) really
> >> is reporting the truth, and the sched_setattr() call returns immediately.
> >> But it looks like with these settings the deadline scheduler gets itself
> >> into a confused state. The process chews up a vast amount of CPU time
> >> for the few actions (including process teardown) that occur after
> >> the sched_setattr() call, and since the SCHED_DEADLINE process has
> >> priority over everything else, the system locks up.
> >
> > Yeah, its doing something weird alright.. let me see if I can get
> > something useful out.
> 
> Thanks!

So I think its because the way we check wrapping

  (s64)(a - b) < 0

This means that its impossible to tell if time went fwd or bwd with
64bit increments. I've not entirely pinpointed where this is wrecking
things, but it seems like a fair bet this is what's going wrong.

So I'm tempted to put a sanity check on all these values to make sure <=
2^63. That way the wrapping logic in the kernel keeps working.

And 2^63 [ns] should be plenty large enough for everyone (famous last
words of course).

> > Btw, you do know about EX_USAGE from sysexits.h ?
> 
> Yes, I'm peripherally aware of them, but have tended to avoid them
> because they're not in POSIX, and don't seem to be all that widely
> used.

Ah, so then its just something weird I've picked up along the way :-)
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