Re: SCHED_FIFO and SCHED_RR broken by cfs

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.



--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

Relevant Pages

  • Re: Xterm Hangs - Possible scheduler defect?
    ... >> perogative to don steel toed shoes(set important kernel threads to a higher ... >> priority) before pulling the trigger. ... I specifically directed the scheduler to run my thread at a higher ... >higher priority than the kernel I would have written something in kernel ...
    (Linux-Kernel)
  • Re: Kernel newbie questions
    ... > loosely) suck so badly... ... vaguaries of the user-proc scheduler. ... Once you move it into the kernel, ... is setting it's priority such that other processes, ...
    (comp.os.linux.misc)
  • Re: help needed pls. scheduler(kernel 2.6) + hyperthreaded related questions?
    ... I saw frm the kernel docs tht realtime tasks hav priority 0 to 99. ... is there any way I can tell the scheduler (assuming I make the ...
    (Linux-Kernel)
  • SCHED_FIFO and SCHED_RR broken by cfs
    ... 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 ... static inline void getCPUfreq ...
    (Linux-Kernel)
  • Re: RFC for a new Scheduling policy/class in the Linux-kernel
    ... B is holding a lock. ... A grants its priority B until B releases the lock. ... We want to charge B's critical section to B, ... scheduler computes how much time A has used recently, ...
    (Linux-Kernel)