Messages in this thread | | | Subject | Re: preempt timing violations | From | Lee Revell <> | Date | Mon, 26 Jul 2004 05:09:14 -0400 |
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On Mon, 2004-07-26 at 04:50, Ingo Molnar wrote: > * Lee Revell <rlrevell@joe-job.com> wrote: > > > Latency with 2.6.8-rc2 + voluntary-preempt-I4 is the best so far. > > After extended testing there only seem to be a few hot spots. In > > several minutes I saw an 11ms violation, a 14ms violation, and several > > 2ms violations. > > > > get_user_pages() is much better in 2.6.8-rc1-mm1 than 2.6.8-rc2. Is > > there any chance of getting the fix into mainline? > > in -J3 i've added a cond_resched to the latency-generating point of > get_user_pages(). (The biggest latencies happen via > make_pages_present(), which gets triggered by mlockall()/MAP_LOCKED.) >
OK, great, this is the biggest remaing issue with -I4, because jackd does that a lot.
> > 2ms non-preemptible critical section violated 1 ms preempt threshold > > starting at unmap_vmas+0x1ff/0x210 and ending at > > unmap_vmas+0x1f5/0x210 > > this is the normal sys_exit()->exit_mmap()->unmap_vmas() path. It's > weird that it generated a 2ms latency. What are the values of > voluntary_preemption and kernel_preemption on your current kernel? With > a 2:0 setting we ought to have a reschedule point every 32 pages. Do > you know which application triggers this latency and is it easy to > reproduce? >
2 and 1. Now that I think about it, this could have happened during bootup, before my rc.local set these. I will try passing them on the kernel command line.
Not sure I understand the difference between 2:1 and 2:0. Would the latter make the kernel only preemptible at the voluntary preemption points?
> > 14ms non-preemptible critical section violated 1 ms preempt threshold > > starting at tty_write+0x1b6/0x290 and ending at schedule+0x2fd/0x5b0 > > does this one trigger when you are using the VGA console? (or fbcon)? > > it's not immediately obvious to me precisely where this latency comes > from, it would be nice to know how to reproduce it.
It think this one was caused by switching virtual consoles. At one point Andrew Morton suggested I remove the (un)lock_kernel calls from do_tty_write. This fixed the problem, with no detectable side effects. Maybe this could be incorporated into voluntary-preempt, it would be useful to have more than one person to test it.
Lee
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