linux-kernel - Re: question on sched-rt group allocation cap: sched_rt_runtime

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Message-Id: <1252232289.29247.11.camel@marge.simson.net>
Date:	Sun, 06 Sep 2009 12:18:09 +0200
From:	Mike Galbraith <efault@....de>
To:	Ani <asinha@...gmasystems.com>
Cc:	Lucas De Marchi <lucas.de.marchi@...il.com>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: question on sched-rt group allocation cap: sched_rt_runtime_us

On Sun, 2009-09-06 at 08:32 +0200, Mike Galbraith wrote:
> On Sat, 2009-09-05 at 19:32 -0700, Ani wrote: 
> > On Sep 5, 3:50 pm, Lucas De Marchi <lucas.de.mar...@...il.com> wrote:
> > >
> > > Indeed. I've tested this same test program in a single core machine and it
> > > produces the expected behavior:
> > >
> > > rt_runtime_us / rt_period_us     % loops executed in SCHED_OTHER
> > > 95%                              4.48%
> > > 60%                              54.84%
> > > 50%                              86.03%
> > > 40%                              OTHER completed first
> > >
> > 
> > Hmm. This does seem to indicate that there is some kind of
> > relationship with SMP. So I wonder whether there is a way to turn this
> > 'RT bandwidth accumulation' heuristic off.
> 
> No there isn't, but maybe there should be, since this isn't the first
> time it's come up.  One pro argument is that pinned tasks are thoroughly
> screwed when an RT hog lands on their runqueue.  On the con side, the
> whole RT bandwidth restriction thing is intended (AFAIK) to allow an
> admin to regain control should RT app go insane, which the default 5%
> aggregate accomplishes just fine.
> 
> Dunno.  Fly or die little patchlet (toss). 

btw, a _kinda sorta_ pro is that it can prevent IO lockups like the
below.  Seems kjournald can end up depending on kblockd/3, which ain't
going anywhere with that 100% RT hog in the way, so the whole box is
fairly hosed.  (much better would be to wake some other kblockd)

top - 12:01:49 up 56 min, 20 users,  load average: 8.01, 4.96, 2.39
Tasks: 304 total,   4 running, 300 sleeping,   0 stopped,   0 zombie
Cpu(s): 25.8%us,  0.3%sy,  0.0%ni,  0.0%id, 73.7%wa,  0.3%hi,  0.0%si,  0.0%st

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  P COMMAND
13897 root      -2   0  7920  592  484 R  100  0.0   1:13.43 3 xx
12716 root      20   0  8868 1328  860 R    1  0.0   0:01.44 0 top
   14 root      15  -5     0    0    0 R    0  0.0   0:00.02 3 events/3
   94 root      15  -5     0    0    0 R    0  0.0   0:00.00 3 kblockd/3
 1212 root      15  -5     0    0    0 D    0  0.0   0:00.04 2 kjournald
14393 root      20   0  9848 2296  756 D    0  0.1   0:00.01 0 make
14404 root      20   0 38012  25m 5552 D    0  0.8   0:00.21 1 cc1
14405 root      20   0 20220 8852 2388 D    0  0.3   0:00.02 1 as
14437 root      20   0 24132  10m 2680 D    0  0.3   0:00.06 2 cc1
14448 root      20   0 18324 1724 1240 D    0  0.1   0:00.00 2 cc1
14452 root      20   0 12540  792  656 D    0  0.0   0:00.00 2 mv


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