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authorVenkatesh Pallipadi <venki@google.com>2011-04-14 10:30:53 -0700
committerIngo Molnar <mingo@elte.hu>2011-04-19 10:08:38 +0200
commit2f36825b176f67e5c5228aa33d828bc39718811f (patch)
tree065130f8eb74a419d8ddf1856ccc64ee7b195843 /kernel/sched_fair.c
parent69c80f3e9d3c569f8a3cee94ba1a324b5a7fa6b9 (diff)
downloadlinux-linaro-stable-2f36825b176f67e5c5228aa33d828bc39718811f.tar.gz
sched: Next buddy hint on sleep and preempt path
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 Signed-off-by: Venkatesh Pallipadi <venki@google.com> Acked-by: Rik van Riel <riel@redhat.com> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Link: http://lkml.kernel.org/r/1302802253-25760-1-git-send-email-venki@google.com Signed-off-by: Ingo Molnar <mingo@elte.hu>
Diffstat (limited to 'kernel/sched_fair.c')
-rw-r--r--kernel/sched_fair.c26
1 files changed, 23 insertions, 3 deletions
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 501ab637b94b..5280272cce3e 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1344,6 +1344,8 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
hrtick_update(rq);
}
+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
@@ -1353,14 +1355,22 @@ 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 && parent_entity(se))
+ set_next_buddy(parent_entity(se));
break;
+ }
flags |= DEQUEUE_SLEEP;
}
@@ -1877,12 +1887,15 @@ 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
@@ -1910,8 +1923,15 @@ 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) {
+ /*
+ * Bias pick_next to pick the sched entity that is
+ * triggering this preemption.
+ */
+ if (!next_buddy_marked)
+ set_next_buddy(pse);
goto preempt;
+ }
return;