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    SubjectRe: [patch 2/2] sched: fix nr_uninterruptible accounting of frozen tasks really
    On Fri, Jul 17, 2009 at 02:31:50PM +0200, Peter Zijlstra wrote:
    > On Fri, 2009-07-17 at 12:25 +0000, Thomas Gleixner wrote:
    > > plain text document attachment (freezer-fix-accounting-for-real.patch)
    > > commit e3c8ca8336 (sched: do not count frozen tasks toward load) broke
    > > the nr_uninterruptible accounting on freeze/thaw. On freeze the task
    > > is excluded from accounting with a check for (task->flags &
    > > PF_FROZEN), but that flag is cleared before the task is thawed. So
    > > while we prevent that the freezing task with state
    > > TASK_UNINTERRUPTIBLE is accounted to nr_uninterruptible we decrement
    > > nr_uninterruptible on thaw.
    > >
    > > Use a separate flag which is handled by the freezing task itself. Set
    > > it before calling the scheduler with TASK_UNINTERRUPTIBLE state and
    > > clear it after we return from frozen state.
    > Right, so I'm wondering why we don't fully revert e3c8ca8336 to begin
    > with.
    > The changelog reads:
    > ---
    > commit e3c8ca8336707062f3f7cb1cd7e6b3c753baccdd
    > Author: Nathan Lynch <>
    > Date: Wed Apr 8 19:45:12 2009 -0500
    > sched: do not count frozen tasks toward load
    > Freezing tasks via the cgroup freezer causes the load average to climb
    > because the freezer's current implementation puts frozen tasks in
    > uninterruptible sleep (D state).
    > Some applications which perform job-scheduling functions consult the
    > load average when making decisions. If a cgroup is frozen, the load
    > average does not provide a useful measure of the system's utilization
    > to such applications. This is especially inconvenient if the job
    > scheduler employs the cgroup freezer as a mechanism for preempting low
    > priority jobs. Contrast this with using SIGSTOP for the same purpose:
    > the stopped tasks do not count toward system load.
    > Change task_contributes_to_load() to return false if the task is
    > frozen. This results in /proc/loadavg behavior that better meets
    > users' expectations.
    > ---
    > It appears to me that a frozen cgroup is a transient state. Either you
    > would typically do something like:
    > freeze -> {snapshot, migrate} -> {thaw, destroy}
    > Therefore a short increase in load doesn't seem like too big a problem,
    > its going to be gone soon anyway.
    > Hmm?

    The job scheduler in question does not use FROZEN as a transient state and
    does not use checkpoint/restart at all since c/r is still a work in progress.
    Even when used for power management it seems wrong to count frozen tasks
    towards the loadavg since they aren't using CPU time or waiting for IO.

    -Matt Helsley

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