linux-kernel - Re: SCHED_FIFO and SCHED

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Date:	Sat, 16 Aug 2008 23:29:50 +0200
From:	Stefani Seibold <stefani@...bold.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...hat.com
Subject: Re: SCHED_FIFO and SCHED_RR broken by cfs

After disabling kernel support for "Group CPU scheduler" and applying
'echo -1 > /proc/sys/kernel/sched_rt_runtime_us' the behaviour is as
expected.

chrt -f 99 ./a.out

 average:13 usec
 min. jitter:0 usec
 max. jitter:29 usec

chrt -o 0 ./a.out 
 average:153 usec
 min. jitter:0 usec
 max. jitter:37035 usec

So the problem is located first in the new sched_rt_runtime_us default
value and second in the "Group CPU scheduler".

A last question: I though that the kernel will never break user space.
Would it not better to make the old behaviour as the default?

Greetings from Munich/Germany
Stefani

Am Samstag, den 16.08.2008, 16:53 +0200 schrieb Peter Zijlstra:
> On Sat, 2008-08-16 at 11:55 +0200, Stefani Seibold wrote:
> > Hi kernel hackers,
> > 
> > it seems that the new completely fair scheduler breaks the SCHED_RR and
> > SCHED_FIFO realtime scheduler.
> > 
> > In my opinion a high priority real time user process with SCHED_FIFO
> > should be only interrupted by the kernel or a process with an higher
> > priority. So a user process running under SCHED_FIFO and priority 99
> > should never be interrupted by any other process.  This was true under
> > kernel 2.6.20. 
> > 
> > On my pentium/celeron III/400 MHz system with kernel 2.6.20 a busy loop
> > using the "time stamp counter" of the x86 cpu for delaying, this was
> > very accurate. The max. jitter of the delaying was about 5 microseconds.
> > 
> > With the new kernel 2.6.26 the jitter is about 51177 microseconds or in
> > other words 51 milliseconds or more the 10000 times greater than kernel
> > 2.6.20. This huge latency is far away from realtime.
> > 
> > Below are the results of the attached test program. Maybe somebody else
> > can confirm this results. All measurements was done with no other
> > process running, only the busybox 1.11.1 shell and the init process was
> > there.
> 
> Has nothing to do with CFS, but everything to do with the fact that we
> now have a 95% bandwidth control by default.
> 
> Does doing:
> 
> echo -1 > /proc/sys/kernel/sched_rt_runtime_us
> 
> fix it?
> 
> So, up to 95% cpu usage (per sched_rt_period_us) FIFO and RR behave like
> they always did, once they cross that line, they'll be throttled.
> 
> 95% seemed like a sane default in that it leaves a little room to
> recover from a run-away rt process (esp handy now that !root users can
> also use RT scheduling classes), and should be enough for most
> applications as they usually don't consume all that much time.
> 
> 
> 

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