On Wed, May 29, 2019 at 08:36:44PM +0000, Vineeth Remanan Pillai wrote: > From: Peter Zijlstra <pet...@infradead.org> > > Avoid the RETRY_TASK case in the pick_next_task() slow path. > > By doing the put_prev_task() early, we get the rt/deadline pull done, > and by testing rq->nr_running we know if we need newidle_balance(). > > This then gives a stable state to pick a task from. > > Since the fast-path is fair only; it means the other classes will > always have pick_next_task(.prev=NULL, .rf=NULL) and we can simplify. > > Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> > --- > kernel/sched/core.c | 19 ++++++++++++------- > kernel/sched/deadline.c | 30 ++---------------------------- > kernel/sched/fair.c | 9 ++++++--- > kernel/sched/idle.c | 4 +++- > kernel/sched/rt.c | 29 +---------------------------- > kernel/sched/sched.h | 13 ++++++++----- > kernel/sched/stop_task.c | 3 ++- > 7 files changed, 34 insertions(+), 73 deletions(-) > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 9dfa0c53deb3..b883c70674ba 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -3363,7 +3363,7 @@ pick_next_task(struct rq *rq, struct task_struct *prev, > struct rq_flags *rf) > > p = fair_sched_class.pick_next_task(rq, prev, rf); > if (unlikely(p == RETRY_TASK)) > - goto again; > + goto restart; > > /* Assumes fair_sched_class->next == idle_sched_class */ > if (unlikely(!p)) > @@ -3372,14 +3372,19 @@ pick_next_task(struct rq *rq, struct task_struct > *prev, struct rq_flags *rf) > return p; > } > > -again: > +restart: > + /* > + * Ensure that we put DL/RT tasks before the pick loop, such that they > + * can PULL higher prio tasks when we lower the RQ 'priority'. > + */ > + prev->sched_class->put_prev_task(rq, prev, rf); > + if (!rq->nr_running) > + newidle_balance(rq, rf); > + > for_each_class(class) { > - p = class->pick_next_task(rq, prev, rf); > - if (p) { > - if (unlikely(p == RETRY_TASK)) > - goto again; > + p = class->pick_next_task(rq, NULL, NULL); > + if (p) > return p; > - } > } > > /* The idle class should always have a runnable task: */ > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c > index 45425f971eec..d3904168857a 100644 > --- a/kernel/sched/deadline.c > +++ b/kernel/sched/deadline.c > @@ -1729,39 +1729,13 @@ pick_next_task_dl(struct rq *rq, struct task_struct > *prev, struct rq_flags *rf) > struct task_struct *p; > struct dl_rq *dl_rq; > > - dl_rq = &rq->dl; > - > - if (need_pull_dl_task(rq, prev)) { > - /* > - * This is OK, because current is on_cpu, which avoids it being > - * picked for load-balance and preemption/IRQs are still > - * disabled avoiding further scheduler activity on it and we're > - * being very careful to re-start the picking loop. > - */ > - rq_unpin_lock(rq, rf); > - pull_dl_task(rq); > - rq_repin_lock(rq, rf); > - /* > - * pull_dl_task() can drop (and re-acquire) rq->lock; this > - * means a stop task can slip in, in which case we need to > - * re-start task selection. > - */ > - if (rq->stop && task_on_rq_queued(rq->stop)) > - return RETRY_TASK; > - } > + WARN_ON_ONCE(prev || rf); should there be a helpful message to go with this warning?
> > - /* > - * When prev is DL, we may throttle it in put_prev_task(). > - * So, we update time before we check for dl_nr_running. > - */ > - if (prev->sched_class == &dl_sched_class) > - update_curr_dl(rq); > + dl_rq = &rq->dl; > > if (unlikely(!dl_rq->dl_nr_running)) > return NULL; > > - put_prev_task(rq, prev); > - > dl_se = pick_next_dl_entity(rq, dl_rq); > BUG_ON(!dl_se); > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 8e3eb243fd9f..e65f2dfda77a 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -6979,7 +6979,7 @@ pick_next_task_fair(struct rq *rq, struct task_struct > *prev, struct rq_flags *rf > goto idle; > > #ifdef CONFIG_FAIR_GROUP_SCHED > - if (prev->sched_class != &fair_sched_class) > + if (!prev || prev->sched_class != &fair_sched_class) > goto simple; > > /* > @@ -7056,8 +7056,8 @@ pick_next_task_fair(struct rq *rq, struct task_struct > *prev, struct rq_flags *rf > goto done; > simple: > #endif > - > - put_prev_task(rq, prev); > + if (prev) > + put_prev_task(rq, prev); > > do { > se = pick_next_entity(cfs_rq, NULL); > @@ -7085,6 +7085,9 @@ done: __maybe_unused; > return p; > > idle: > + if (!rf) > + return NULL; > + > new_tasks = newidle_balance(rq, rf); > > /* > diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c > index 1b65a4c3683e..7ece8e820b5d 100644 > --- a/kernel/sched/idle.c > +++ b/kernel/sched/idle.c > @@ -388,7 +388,9 @@ pick_next_task_idle(struct rq *rq, struct task_struct > *prev, struct rq_flags *rf > { > struct task_struct *next = rq->idle; > > - put_prev_task(rq, prev); > + if (prev) > + put_prev_task(rq, prev); > + > set_next_task_idle(rq, next); > > return next; > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c > index 51ee87c5a28a..79f2e60516ef 100644 > --- a/kernel/sched/rt.c > +++ b/kernel/sched/rt.c > @@ -1554,38 +1554,11 @@ pick_next_task_rt(struct rq *rq, struct task_struct > *prev, struct rq_flags *rf) > struct task_struct *p; > struct rt_rq *rt_rq = &rq->rt; > > - if (need_pull_rt_task(rq, prev)) { > - /* > - * This is OK, because current is on_cpu, which avoids it being > - * picked for load-balance and preemption/IRQs are still > - * disabled avoiding further scheduler activity on it and we're > - * being very careful to re-start the picking loop. > - */ > - rq_unpin_lock(rq, rf); > - pull_rt_task(rq); > - rq_repin_lock(rq, rf); > - /* > - * pull_rt_task() can drop (and re-acquire) rq->lock; this > - * means a dl or stop task can slip in, in which case we need > - * to re-start task selection. > - */ > - if (unlikely((rq->stop && task_on_rq_queued(rq->stop)) || > - rq->dl.dl_nr_running)) > - return RETRY_TASK; > - } > - > - /* > - * We may dequeue prev's rt_rq in put_prev_task(). > - * So, we update time before rt_queued check. > - */ > - if (prev->sched_class == &rt_sched_class) > - update_curr_rt(rq); > + WARN_ON_ONCE(prev || rf); > > if (!rt_rq->rt_queued) > return NULL; > > - put_prev_task(rq, prev); > - > p = _pick_next_task_rt(rq); > > set_next_task_rt(rq, p); > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > index 4cbe2bef92e4..460dd04e76af 100644 > --- a/kernel/sched/sched.h > +++ b/kernel/sched/sched.h > @@ -1665,12 +1665,15 @@ struct sched_class { > void (*check_preempt_curr)(struct rq *rq, struct task_struct *p, int > flags); > > /* > - * It is the responsibility of the pick_next_task() method that will > - * return the next task to call put_prev_task() on the @prev task or > - * something equivalent. > + * Both @prev and @rf are optional and may be NULL, in which case the > + * caller must already have invoked put_prev_task(rq, prev, rf). > * > - * May return RETRY_TASK when it finds a higher prio class has runnable > - * tasks. > + * Otherwise it is the responsibility of the pick_next_task() to call > + * put_prev_task() on the @prev task or something equivalent, IFF it > + * returns a next task. > + * > + * In that case (@rf != NULL) it may return RETRY_TASK when it finds a > + * higher prio class has runnable tasks. > */ > struct task_struct * (*pick_next_task)(struct rq *rq, > struct task_struct *prev, > diff --git a/kernel/sched/stop_task.c b/kernel/sched/stop_task.c > index 8f414018d5e0..7e1cee4e65b2 100644 > --- a/kernel/sched/stop_task.c > +++ b/kernel/sched/stop_task.c > @@ -33,10 +33,11 @@ pick_next_task_stop(struct rq *rq, struct task_struct > *prev, struct rq_flags *rf > { > struct task_struct *stop = rq->stop; > > + WARN_ON_ONCE(prev || rf); should there be a helpful message to go with this warning? --mark > + > if (!stop || !task_on_rq_queued(stop)) > return NULL; > > - put_prev_task(rq, prev); > set_next_task_stop(rq, stop); > > return stop; > -- > 2.17.1 >