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Message-ID: <AANLkTi=dSdnn0p8973C5-URTLbTkJ2XROV9XePrQezJ3@mail.gmail.com>
Date: Mon, 7 Mar 2011 17:29:04 -0800
From: Paul Turner <pjt@...gle.com>
To: Venkatesh Pallipadi <venki@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
linux-kernel@...r.kernel.org, Mike Galbraith <efault@....de>,
Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH] sched: next buddy hint on sleep and preempt path - v1
On Mon, Mar 7, 2011 at 4:59 PM, Venkatesh Pallipadi <venki@...gle.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@...gle.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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