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    SubjectRe: 2.6.14-rt22 (and mainline) excessive latency
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    On Wed, 2005-12-21 at 05:36 -0800, Paul E. McKenney wrote:
    > On Tue, Dec 20, 2005 at 10:32:48PM -0500, Lee Revell wrote:
    > > On Tue, 2005-12-20 at 17:47 -0800, Paul E. McKenney wrote:
    > > > On Tue, Dec 20, 2005 at 05:24:42AM +0100, Ingo Molnar wrote:
    > > > >
    > > > > * Lee Revell <rlrevell@joe-job.com> wrote:
    > > > >
    > > > > > I captured this 3+ ms latency trace when killing a process with a few
    > > > > > thousand threads. Can a cond_resched be added to this code path?
    > > > >
    > > > > > bash-17992 0.n.1 29us : eligible_child (do_wait)
    > > > > >
    > > > > > [ 3000+ of these deleted ]
    > > > > >
    > > > > > bash-17992 0.n.1 3296us : eligible_child (do_wait)
    > > > >
    > > > > Atomicity of signal delivery is pretty much a must, so i'm not sure this
    > > > > particular latency can be fixed, short of running PREEMPT_RT. Paul E.
    > > > > McKenney is doing some excellent stuff by RCU-ifying the task lookup and
    > > > > signal code, but i'm not sure whether it could cover do_wait().
    > > >
    > > > Took a quick break from repeatedly shooting myself in the foot with
    > > > RCU read-side priority boosting (still have a few toes left) to take
    > > > a quick look at this. The TASK_TRACED and TASK_STOPPED cases seem
    > > > non-trivial, and I am concerned about races with exit.
    > > >
    > > > Any thoughts on whether the latency is due to contention on the
    > > > tasklist lock vs. the "goto repeat" in do_wait()?
    > >
    > > It's a UP system so I'd be surprised if there were any contention.
    >
    > Couldn't there be contention due to preemption of someone holding
    > the tasklist lock?

    But I'm running with PREEMPT_DESKTOP (specifically I configured a system
    to have the exact same preemption model as mainline - PREEMPT_DESKTOP
    with no soft/hardirq preemption) so holding a spinlock will disable
    preemption.

    Lee

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