• A
    sched: Set an initial value of runnable avg for new forked task · a75cdaa9
    Alex Shi 提交于
    We need to initialize the se.avg.{decay_count, load_avg_contrib} for a
    new forked task. Otherwise random values of above variables cause a
    mess when a new task is enqueued:
    
        enqueue_task_fair
            enqueue_entity
                enqueue_entity_load_avg
    
    and make fork balancing imbalance due to incorrect load_avg_contrib.
    
    Further more, Morten Rasmussen notice some tasks were not launched at
    once after created. So Paul and Peter suggest giving a start value for
    new task runnable avg time same as sched_slice().
    
    PeterZ said:
    
    > So the 'problem' is that our running avg is a 'floating' average; ie. it
    > decays with time. Now we have to guess about the future of our newly
    > spawned task -- something that is nigh impossible seeing these CPU
    > vendors keep refusing to implement the crystal ball instruction.
    >
    > So there's two asymptotic cases we want to deal well with; 1) the case
    > where the newly spawned program will be 'nearly' idle for its lifetime;
    > and 2) the case where its cpu-bound.
    >
    > Since we have to guess, we'll go for worst case and assume its
    > cpu-bound; now we don't want to make the avg so heavy adjusting to the
    > near-idle case takes forever. We want to be able to quickly adjust and
    > lower our running avg.
    >
    > Now we also don't want to make our avg too light, such that it gets
    > decremented just for the new task not having had a chance to run yet --
    > even if when it would run, it would be more cpu-bound than not.
    >
    > So what we do is we make the initial avg of the same duration as that we
    > guess it takes to run each task on the system at least once -- aka
    > sched_slice().
    >
    > Of course we can defeat this with wakeup/fork bombs, but in the 'normal'
    > case it should be good enough.
    
    Paul also contributed most of the code comments in this commit.
    Signed-off-by: NAlex Shi <alex.shi@intel.com>
    Reviewed-by: NGu Zheng <guz.fnst@cn.fujitsu.com>
    Reviewed-by: NPaul Turner <pjt@google.com>
    [peterz; added explanation of sched_slice() usage]
    Signed-off-by: NPeter Zijlstra <peterz@infradead.org>
    Link: http://lkml.kernel.org/r/1371694737-29336-4-git-send-email-alex.shi@intel.comSigned-off-by: NIngo Molnar <mingo@kernel.org>
    a75cdaa9
sched.h 35.1 KB