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    SubjectRe: [PATCH] sched/fair: Fix the wrong throttled clock time for cfs_rq_clock_task()
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    On 2016/05/11 at 14:49, Peter Zijlstra wrote:
    > On Tue, May 10, 2016 at 11:19:44AM -0700, bsegall@google.com wrote:
    >> Xunlei Pang <xlpang@redhat.com> writes:
    >>
    >>> Two minor fixes for cfs_rq_clock_task().
    >>> 1) If cfs_rq is currently being throttled, we need to subtract the cfs
    >>> throttled clock time.
    >>>
    >>> 2) Make "throttled_clock_task_time" update SMP unrelated. Now UP cases
    >>> need it as well.
    >>>
    >>> Signed-off-by: Xunlei Pang <xlpang@redhat.com>
    >>> ---
    >>> kernel/sched/fair.c | 4 +---
    >>> 1 file changed, 1 insertion(+), 3 deletions(-)
    >>>
    >>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    >>> index 1708729e..fb80a12 100644
    >>> --- a/kernel/sched/fair.c
    >>> +++ b/kernel/sched/fair.c
    >>> @@ -3655,7 +3655,7 @@ static inline struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg)
    >>> static inline u64 cfs_rq_clock_task(struct cfs_rq *cfs_rq)
    >>> {
    >>> if (unlikely(cfs_rq->throttle_count))
    >>> - return cfs_rq->throttled_clock_task;
    >>> + return cfs_rq->throttled_clock_task - cfs_rq->throttled_clock_task_time;
    >>>
    >>> return rq_clock_task(rq_of(cfs_rq)) - cfs_rq->throttled_clock_task_time;
    >>> }
    > The alternative is obviously to do the subtraction in
    > tg_throttle_down(), were we set ->throttled_clock_task.

    It is possible, but throttled_clock_task is a timestamp, I think doing it here is semantically better.

    >
    >>> @@ -3793,13 +3793,11 @@ static int tg_unthrottle_up(struct task_group *tg, void *data)
    >>> struct cfs_rq *cfs_rq = tg->cfs_rq[cpu_of(rq)];
    >>>
    >>> cfs_rq->throttle_count--;
    >>> -#ifdef CONFIG_SMP
    >>> if (!cfs_rq->throttle_count) {
    >>> /* adjust cfs_rq_clock_task() */
    >>> cfs_rq->throttled_clock_task_time += rq_clock_task(rq) -
    >>> cfs_rq->throttled_clock_task;
    >>> }
    >>> -#endif
    >>>
    >>> return 0;
    >>> }
    >> [Cc: pjt@google.com]
    >>
    >> This looks reasonable to me (at least the first part; I'm not
    >> certain why the CONFIG_SMP ifdef was put in place).
    > 64660c864f46 ("sched: Prevent interactions with throttled entities")
    >
    > Introduced it, because at that time it was about updating shares, which
    > is only present on SMP. Then:
    >
    > f1b17280efbd ("sched: Maintain runnable averages across throttled periods")
    >
    > Added the clock thing inside it, and:
    >
    > 82958366cfea ("sched: Replace update_shares weight distribution with per-entity computation")
    >
    > took out the shares update and left the clock update, resulting in the
    > current code.
    >
    >

    Thanks,
    Xunlei

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