Messages in this thread Patch in this message | | | From | Paul Turner <> | Date | Mon, 7 Mar 2011 17:29:04 -0800 | 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@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 > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |