Messages in this thread Patch in this message |  | | Date | Fri, 14 Feb 2014 13:38:45 +0100 | From | Peter Zijlstra <> | Subject | Re: [RFC PATCH] sched: make sure sched-priority after invoke idle_balance() |
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On Fri, Feb 14, 2014 at 12:54:37PM +0800, Michael wang wrote: > Since idle_balance() will release rq-lock for a while, there is a chance that > RT/DL tasks will be enqueued and ask for the resched, the func used to be > invoked ahead of pick_next_task(), which will make sure we drop into the > bottom-half inside pick_next_task(). > > Now since idle_balance() was done inside pick_next_task_fair(), pick_next_task() > can no longer make sure the priority, the worst case is that we will going to > pick the pulled fair task while there is RT/DL on rq which actually should be > picked up. > > This patch will prevent this happen by some rechecking after idle_balance(), it > utilize the resched-flag for the case when RT/DL task was enqueued but don't ask > for resched (will that ever happened?).
I'm not sure this is actually working right; the problem is that while we do retry on need_resched() in the main schedule() loop, that last need_resched() is on @next (then current). So clearing/resetting @prev's need_resched() is not going to trigger that loop.
Not to mention we explicitly clear @prev's need_resched right after pick_next_task().
So how about something like this?
I don't particularly like adding that condition to pick_next_task(); but the alternative is recursively calling pick_next_task() and while recursion is strictly limited to the number of sched_classes, it does feel kinda icky.
Anybody got any preferences?
--- Subject: sched: Guarantee task priority in pick_next_task() From: Peter Zijlstra <peterz@infradead.org> Date: Fri Feb 14 12:25:08 CET 2014
Michael spotted that the idle_balance() push down created a task priority problem.
Previously, when we called idle_balance() before pick_next_task() it wasn't a problem when -- because of the rq->lock droppage -- an rt/dl task slipped in.
Similarly for pre_schedule(), rt pre-schedule could have a dl task slip in.
But by pulling it into the pick_next_task() loop, we'll not try a higher task priority again.
Cure this by creating a re-start condition in pick_next_task(); and triggering this from pick_next_task_{rt,fair}().
Fixes: 38033c37faab ("sched: Push down pre_schedule() and idle_balance()") Cc: Juri Lelli <juri.lelli@gmail.com> Cc: Ingo Molnar <mingo@kernel.org> Cc: Steven Rostedt <rostedt@goodmis.org> Reported-by: Michael Wang <wangyun@linux.vnet.ibm.com> Signed-off-by: Peter Zijlstra <peterz@infradead.org> Link: http://lkml.kernel.org/n/tip-jrdk7auga87duk4lkpo8xusk@git.kernel.org --- kernel/sched/core.c | 19 +++++++++++++++---- kernel/sched/fair.c | 16 +++++++++++++++- kernel/sched/rt.c | 10 +++++++++- 3 files changed, 39 insertions(+), 6 deletions(-)
--- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2574,27 +2574,38 @@ static inline void schedule_debug(struct static inline struct task_struct * pick_next_task(struct rq *rq, struct task_struct *prev) { - const struct sched_class *class; + const struct sched_class *class = &fair_sched_class; struct task_struct *p; /* * Optimization: we know that if all tasks are in * the fair class we can call that function directly: */ - if (likely(prev->sched_class == &fair_sched_class && + if (likely(prev->sched_class == class && rq->nr_running == rq->cfs.h_nr_running)) { p = fair_sched_class.pick_next_task(rq, prev); if (likely(p)) - return p; + goto got_task; } +again: for_each_class(class) { p = class->pick_next_task(rq, prev); if (p) - return p; + goto got_task; } BUG(); /* the idle class will always have a runnable task */ + +got_task: + /* + * See pick_next_task_{fair,rt}(); they return rq->idle in case + * they want to re-start the task selection. + */ + if (unlikely(p->sched_class != class)) + goto again; + + return p; } /* --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -4684,6 +4684,7 @@ pick_next_task_fair(struct rq *rq, struc struct cfs_rq *cfs_rq = &rq->cfs; struct sched_entity *se; struct task_struct *p; + int new_tasks; again: #ifdef CONFIG_FAIR_GROUP_SCHED @@ -4782,7 +4783,20 @@ pick_next_task_fair(struct rq *rq, struc return p; idle: - if (idle_balance(rq)) /* drops rq->lock */ + /* + * Because idle_balance() releases (and re-acquires) rq->lock, it is + * possible for any higher priority task to appear. In that case we + * must re-start the pick_next_entity() loop. + */ + new_tasks = idle_balance(rq); + + /* + * See pick_next_task(); we return rq->idle to restart task selection. + */ + if (rq->nr_running != rq->cfs.h_nr_running) + return rq->idle; + + if (new_tasks) goto again; return NULL; --- a/kernel/sched/rt.c +++ b/kernel/sched/rt.c @@ -1360,8 +1360,16 @@ pick_next_task_rt(struct rq *rq, struct struct task_struct *p; struct rt_rq *rt_rq = &rq->rt; - if (need_pull_rt_task(rq, prev)) + if (need_pull_rt_task(rq, prev)) { pull_rt_task(rq); + /* + * pull_rt_task() can drop (and re-acquire) rq->lock; this + * means a dl task can slip in, in which case we need to + * re-start task selection. + */ + if (unlikely(rq->dl.dl_nr_running)) + return rq->idle; + } if (!rt_rq->rt_nr_running) return NULL;
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