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SubjectRe: [PATCH] sched: next buddy hint on sleep and preempt path - v1
On Mon, Mar 7, 2011 at 4:59 PM, Venkatesh Pallipadi <venki@google.com> wrote:
> When a task in a taskgroup sleeps, pick_next_task starts all the way back at
> the root and picks the task/taskgroup with the min vruntime across all
> runnable tasks. But, when there are many frequently sleeping tasks
> across different taskgroups, it makes better sense to stay with same taskgroup
> for its slice period (or until all tasks in the taskgroup sleeps) instead of
> switching cross taskgroup on each sleep after a short runtime.
> This helps specifically where taskgroups corresponds to a process with
> multiple threads. The change reduces the number of CR3 switches in this case.
>
> Example:
> Two taskgroups with 2 threads each which are running for 2ms and
> sleeping for 1ms. Looking at sched:sched_switch shows -
>
> BEFORE: taskgroup_1 threads [5004, 5005], taskgroup_2 threads [5016, 5017]
>      cpu-soaker-5004  [003]  3683.391089
>      cpu-soaker-5016  [003]  3683.393106
>      cpu-soaker-5005  [003]  3683.395119
>      cpu-soaker-5017  [003]  3683.397130
>      cpu-soaker-5004  [003]  3683.399143
>      cpu-soaker-5016  [003]  3683.401155
>      cpu-soaker-5005  [003]  3683.403168
>      cpu-soaker-5017  [003]  3683.405170
>
> AFTER: taskgroup_1 threads [21890, 21891], taskgroup_2 threads [21934, 21935]
>      cpu-soaker-21890 [003]   865.895494
>      cpu-soaker-21935 [003]   865.897506
>      cpu-soaker-21934 [003]   865.899520
>      cpu-soaker-21935 [003]   865.901532
>      cpu-soaker-21934 [003]   865.903543
>      cpu-soaker-21935 [003]   865.905546
>      cpu-soaker-21891 [003]   865.907548
>      cpu-soaker-21890 [003]   865.909560
>      cpu-soaker-21891 [003]   865.911571
>      cpu-soaker-21890 [003]   865.913582
>      cpu-soaker-21891 [003]   865.915594
>      cpu-soaker-21934 [003]   865.917606
>
> Similar problem is there when there are multiple taskgroups and say a task A
> preempts currently running task B of taskgroup_1. On schedule, pick_next_task
> can pick an unrelated task on taskgroup_2. Here it would be better to give some
> preference to task B on pick_next_task.
>
> A simple (may be extreme case) benchmark I tried was tbench with 2 tbench
> client processes with 2 threads each running on a single CPU. Avg throughput
> across 5 50 sec runs was -
> BEFORE: 105.84 MB/sec
> AFTER: 112.42 MB/sec
>
> Changes from v0:
> * Always pass task se to set_next_buddy
> * Avoid repeated set_next_buddy in check_preempt_wakeup
> * Minor flag cleanup in dequeue_task_fair
>
> Signed-off-by: Venkatesh Pallipadi <venki@google.com>
> ---
>  kernel/sched_fair.c |   41 ++++++++++++++++++++++++++++++++++++++---
>  1 files changed, 38 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> index 3a88dee..cbe442e 100644
> --- a/kernel/sched_fair.c
> +++ b/kernel/sched_fair.c
> @@ -1339,6 +1339,20 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>        hrtick_update(rq);
>  }
>
> +static struct sched_entity *pick_next_taskse_on_cfsrq(struct cfs_rq *cfs_rq)
> +{
> +       struct sched_entity *se;
> +
> +       do {
> +               se = pick_next_entity(cfs_rq);
> +               cfs_rq = group_cfs_rq(se);
> +       } while (cfs_rq);
> +
> +       return se;
> +}
> +

I think the original approach was much cleaner; the notion of a
SCHED_IDLE task is only relative versus siblings in group scheduling

Generalizing the buddies to work on entities, e.g.:

@@ -2137,10 +2180,11 @@ static void set_last_buddy(struct sched_entity *se)

static void set_next_buddy(struct sched_entity *se)
{
- if (likely(task_of(se)->policy != SCHED_IDLE)) {
- for_each_sched_entity(se)
- cfs_rq_of(se)->next = se;
- }
+ if (entity_is_task(se) && unlikely(task_of(se)->policy == SCHED_IDLE))
+ return;
+
+ for_each_sched_entity(se)
+ cfs_rq_of(se)->next = se;
}

Avoids all the picking descent and gets us back there.

> +static void set_next_buddy(struct sched_entity *se);
> +
>  /*
>  * The dequeue_task method is called before nr_running is
>  * decreased. We remove the task from the rbtree and
> @@ -1348,14 +1362,25 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>  {
>        struct cfs_rq *cfs_rq;
>        struct sched_entity *se = &p->se;
> +       int task_sleep = flags & DEQUEUE_SLEEP;
>
>        for_each_sched_entity(se) {
>                cfs_rq = cfs_rq_of(se);
>                dequeue_entity(cfs_rq, se, flags);
>
>                /* Don't dequeue parent if it has other entities besides us */
> -               if (cfs_rq->load.weight)
> +               if (cfs_rq->load.weight) {
> +                       /*
> +                        * Bias pick_next to pick a task from this cfs_rq, as
> +                        * p is sleeping when it is within its sched_slice.
> +                        */
> +                       if (task_sleep) {
> +                               struct sched_entity *next_se;
> +                               next_se = pick_next_taskse_on_cfsrq(cfs_rq);
> +                               set_next_buddy(next_se);
> +                       }
>                        break;
> +               }
>                flags |= DEQUEUE_SLEEP;
>        }
>
> @@ -1856,12 +1881,15 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
>        struct sched_entity *se = &curr->se, *pse = &p->se;
>        struct cfs_rq *cfs_rq = task_cfs_rq(curr);
>        int scale = cfs_rq->nr_running >= sched_nr_latency;
> +       int next_buddy_marked = 0;
>
>        if (unlikely(se == pse))
>                return;
>
> -       if (sched_feat(NEXT_BUDDY) && scale && !(wake_flags & WF_FORK))
> +       if (sched_feat(NEXT_BUDDY) && scale && !(wake_flags & WF_FORK)) {
>                set_next_buddy(pse);
> +               next_buddy_marked = 1;
> +       }
>
>        /*
>         * We can come here with TIF_NEED_RESCHED already set from new task
> @@ -1887,8 +1915,15 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
>        update_curr(cfs_rq);
>        find_matching_se(&se, &pse);
>        BUG_ON(!pse);
> -       if (wakeup_preempt_entity(se, pse) == 1)
> +       if (wakeup_preempt_entity(se, pse) == 1) {

Can't this just be:

if ((wakeup_preempt_entity(se, pse) == 1) || (scale && !fork))

Or even:

( wakeup || scale ) && !fork

Storing the state seems messy just for the
prempt-with-resched-already-set case (effective behavioral difference.
With this the other case can be deleted.

> +               /*
> +                * Bias pick_next to pick the task that is
> +                * triggering this preemption.
> +                */
> +               if (!next_buddy_marked)
> +                       set_next_buddy(&p->se);
>                goto preempt;
> +       }
>
>        return;
>
> --
> 1.7.3.1
>
>
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