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    SubjectRe: [RFC patch 1/2] sched: dynamically adapt granularity with nr_running
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    On Mon, 2010-09-13 at 14:53 +0200, Peter Zijlstra wrote:
    > On Sun, 2010-09-12 at 16:37 -0400, Mathieu Desnoyers wrote:
    > > The whole point of my patch is not to have to do this latency vs performance
    > > tradeoff for low number of running threads. With your approach, lowering the
    > > granularity even when there are few threads running will very likely hurt
    > > performance, no ?
    >
    > But you presented it as a latency patch, not a throughput patch. And I'm
    > not sure it will matter enough to offset the computational cost it
    > introduces.


    ---
    On Mon, 2010-09-13 at 14:53 +0200, Peter Zijlstra wrote:
    On Sun, 2010-09-12 at 16:37 -0400, Mathieu Desnoyers wrote:
    > > The whole point of my patch is not to have to do this latency vs performance
    > > tradeoff for low number of running threads. With your approach, lowering the
    > > granularity even when there are few threads running will very likely hurt
    > > performance, no ?
    >
    > But you presented it as a latency patch, not a throughput patch. And I'm
    > not sure it will matter enough to offset the computational cost it
    > introduces.
    >

    One option is to simply get rid of that stuff in check_preempt_tick()
    and instead do a wakeup-preempt check on the leftmost task instead.

    The code as it stands today does that delta_exec < min_gran check to
    ensure current gets some runtime before doing that second preemption
    check, which compares vruntime with a wall-time measure.

    Making that gran more complex doesn't really buy us much because for a
    system with different weights in the gran and slice lengths don't match
    up anyway.

    ---
    Subject: sched: Simplify tick preemption
    From: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Date: Mon Jul 05 13:56:30 CEST 2010

    Check the current slice, if not expired, see if the leftmost task
    would otherwise have preempted current.

    Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    ---
    kernel/sched_fair.c | 43 +++++++++++++++----------------------------
    1 file changed, 15 insertions(+), 28 deletions(-)

    Index: linux-2.6/kernel/sched_fair.c
    ===================================================================
    --- linux-2.6.orig/kernel/sched_fair.c
    +++ linux-2.6/kernel/sched_fair.c
    @@ -838,44 +838,34 @@ dequeue_entity(struct cfs_rq *cfs_rq, st
    se->vruntime -= cfs_rq->min_vruntime;
    }

    +static int
    +wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se);
    +
    /*
    * Preempt the current task with a newly woken task if needed:
    */
    static void
    check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
    {
    - unsigned long ideal_runtime, delta_exec;
    + unsigned long slice = sched_slice(cfs_rq, curr);
    +
    + if (curr->sum_exec_runtime - curr->prev_sum_exec_runtime < slice) {
    + struct sched_entity *pse = __pick_next_entity(cfs_rq);
    +
    + if (pse && wakeup_preempt_entity(curr, pse) == 1)
    + goto preempt;

    - ideal_runtime = sched_slice(cfs_rq, curr);
    - delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
    - if (delta_exec > ideal_runtime) {
    - resched_task(rq_of(cfs_rq)->curr);
    - /*
    - * The current task ran long enough, ensure it doesn't get
    - * re-elected due to buddy favours.
    - */
    - clear_buddies(cfs_rq, curr);
    return;
    }

    /*
    - * Ensure that a task that missed wakeup preemption by a
    - * narrow margin doesn't have to wait for a full slice.
    - * This also mitigates buddy induced latencies under load.
    + * The current task ran long enough, ensure it doesn't get
    + * re-elected due to buddy favours.
    */
    - if (!sched_feat(WAKEUP_PREEMPT))
    - return;
    -
    - if (delta_exec < sysctl_sched_min_granularity)
    - return;
    + clear_buddies(cfs_rq, curr);

    - if (cfs_rq->nr_running > 1) {
    - struct sched_entity *se = __pick_next_entity(cfs_rq);
    - s64 delta = curr->vruntime - se->vruntime;
    -
    - if (delta > ideal_runtime)
    - resched_task(rq_of(cfs_rq)->curr);
    - }
    +preempt:
    + resched_task(rq_of(cfs_rq)->curr);
    }

    static void
    @@ -908,9 +898,6 @@ set_next_entity(struct cfs_rq *cfs_rq, s
    se->prev_sum_exec_runtime = se->sum_exec_runtime;
    }

    -static int
    -wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se);
    -
    static struct sched_entity *pick_next_entity(struct cfs_rq *cfs_rq)
    {
    struct sched_entity *se = __pick_next_entity(cfs_rq);


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