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    SubjectRe: preempt timing violations
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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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