Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752066AbaBNMi5 (ORCPT ); Fri, 14 Feb 2014 07:38:57 -0500 Received: from merlin.infradead.org ([205.233.59.134]:43606 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751753AbaBNMi4 (ORCPT ); Fri, 14 Feb 2014 07:38:56 -0500 Date: Fri, 14 Feb 2014 13:38:45 +0100 From: Peter Zijlstra To: Michael wang Cc: Ingo Molnar , LKML , Steven Rostedt , Juri Lelli Subject: Re: [RFC PATCH] sched: make sure sched-priority after invoke idle_balance() Message-ID: <20140214123845.GI27965@twins.programming.kicks-ass.net> References: <52FDA18D.40100@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <52FDA18D.40100@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 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 Cc: Ingo Molnar Cc: Steven Rostedt Reported-by: Michael Wang Signed-off-by: Peter Zijlstra 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; -- 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/