• V
    sched/fair: Fix incorrect task group ->load_avg · b5a9b340
    Vincent Guittot 提交于
    A scheduler performance regression has been reported by Joseph Salisbury,
    which he bisected back to:
    
      3d30544f ("sched/fair: Apply more PELT fixes)
    
    The regression triggers when several levels of task groups are involved
    (read: SystemD) and cpu_possible_mask != cpu_present_mask.
    
    The root cause is that group entity's load (tg_child->se[i]->avg.load_avg)
    is initialized to scale_load_down(se->load.weight). During the creation of
    a child task group, its group entities on possible CPUs are attached to
    parent's cfs_rq (tg_parent) and their loads are added to the parent's load
    (tg_parent->load_avg) with update_tg_load_avg().
    
    But only the load on online CPUs will then be updated to reflect real load,
    whereas load on other CPUs will stay at the initial value.
    
    The result is a tg_parent->load_avg that is higher than the real load, the
    weight of group entities (tg_parent->se[i]->load.weight) on online CPUs is
    smaller than it should be, and the task group gets a less running time than
    what it could expect.
    
    ( This situation can be detected with /proc/sched_debug. The ".tg_load_avg"
      of the task group will be much higher than sum of ".tg_load_avg_contrib"
      of online cfs_rqs of the task group. )
    
    The load of group entities don't have to be intialized to something else
    than 0 because their load will increase when an entity is attached.
    Reported-by: NJoseph Salisbury <joseph.salisbury@canonical.com>
    Tested-by: NDietmar Eggemann <dietmar.eggemann@arm.com>
    Signed-off-by: NVincent Guittot <vincent.guittot@linaro.org>
    Acked-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: <stable@vger.kernel.org> # 4.8.x
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: joonwoop@codeaurora.org
    Fixes: 3d30544f ("sched/fair: Apply more PELT fixes)
    Link: http://lkml.kernel.org/r/1476881123-10159-1-git-send-email-vincent.guittot@linaro.orgSigned-off-by: NIngo Molnar <mingo@kernel.org>
    b5a9b340
fair.c 237.3 KB