From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754477AbbE1RHH (ORCPT ); Thu, 28 May 2015 13:07:07 -0400 Received: from forward15m.cmail.yandex.net ([5.255.216.146]:44057 "EHLO forward15m.cmail.yandex.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754144AbbE1RG6 (ORCPT ); Thu, 28 May 2015 13:06:58 -0400 X-Greylist: delayed 453 seconds by postgrey-1.27 at vger.kernel.org; Thu, 28 May 2015 13:06:58 EDT From: Kirill Tkhai To: Peter Zijlstra , Mike Galbraith Cc: LKML , Ingo Molnar , "ktkhai@parallels.com" In-Reply-To: <20150528135355.GK3644@twins.programming.kicks-ass.net> References: <1432799032.3237.119.camel@gmail.com> <20150528135355.GK3644@twins.programming.kicks-ass.net> Subject: Re: sched_setscheduler() vs idle_balance() race MIME-Version: 1.0 Message-Id: <1258701432832360@web10m.yandex.ru> X-Mailer: Yamail [ http://yandex.ru ] 5.0 Date: Thu, 28 May 2015 19:59:20 +0300 Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=koi8-r Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org В Чт, 28/05/2015 в 15:53 +0200, Peter Zijlstra пишет: On Thu, May 28, 2015 at 09:43:52AM +0200, Mike Galbraith wrote: > > Hi Peter, > > > > I'm not seeing what prevents pull_task() from yanking a task out from > > under __sched_setscheduler(). A box sprinkling smoldering 3.0 kernel > > wreckage all over my bugzilla mbox isn't seeing it either ;-) > > Say, how easy can that thing be reproduced? > > The below is compile tested only, but it might just work if I didn't > miss anything :-) > > > --- > kernel/sched/core.c | 137 +++++++++++++++++++++++++++++----------------------- > 1 file changed, 77 insertions(+), 60 deletions(-) > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 4eec60757b16..28f1ddc0bef2 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -1000,22 +1000,6 @@ inline int task_curr(const struct task_struct *p) > return cpu_curr(task_cpu(p)) == p; > } > > -/* > - * Can drop rq->lock because from sched_class::switched_from() methods drop it. > - */ > -static inline void check_class_changed(struct rq *rq, struct task_struct *p, > - const struct sched_class *prev_class, > - int oldprio) > -{ > - if (prev_class != p->sched_class) { > - if (prev_class->switched_from) > - prev_class->switched_from(rq, p); > - /* Possble rq->lock 'hole'. */ > - p->sched_class->switched_to(rq, p); > - } else if (oldprio != p->prio || dl_task(p)) > - p->sched_class->prio_changed(rq, p, oldprio); > -} > - > void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags) > { > const struct sched_class *class; > @@ -3075,12 +3059,38 @@ void rt_mutex_setprio(struct task_struct *p, int prio) > > p->prio = prio; > > + if (prev_class != p->sched_class) { > + prev_class->switched_from(rq, p); > switched_from_XXX() doesn't depend on on_YYY_rq state, so, yeah, this should be safe. + /* > + * switched_from() is allowed to drop @rq->lock; which opens a > + * race against load-balancing, however since @p is not > + * currently enqueued it is invisible to the load-balancer. > + * > + * double check @p is still where we thought it was. > + */ > + WARN_ON_ONCE(task_rq(p) != rq); > + } > + > if (running) > p->sched_class->set_curr_task(rq); > if (queued) > enqueue_task(rq, p, enqueue_flag); > > - check_class_changed(rq, p, prev_class, oldprio); > + /* > + * Both switched_to() and prio_changed() are allowed to drop @rq->lock; > + * which opens a race against load-balancing, and since @p is now > + * enqueued it can indeed be subject to this. > + * > + * This means that any balancing done by these functions must double > + * check a task's rq. > + */ > + if (prev_class != p->sched_class) > + p->sched_class->switched_to(rq, p); > + else if (oldprio != p->prio || dl_task(p)) > + p->sched_class->prio_changed(rq, p, oldprio); > + /* > + * It further means we should not rely on @p's rq from here on. > + */ > out_unlock: > __task_rq_unlock(rq); > } > @@ -3420,7 +3430,7 @@ static bool dl_param_changed(struct task_struct *p, > > static int __sched_setscheduler(struct task_struct *p, > const struct sched_attr *attr, > - bool user) > + bool user, bool pi) > { > int newprio = dl_policy(attr->sched_policy) ? MAX_DL_PRIO - 1 : > MAX_RT_PRIO - 1 - attr->sched_priority; > @@ -3606,18 +3616,20 @@ static int __sched_setscheduler(struct task_struct *p, > p->sched_reset_on_fork = reset_on_fork; > oldprio = p->prio; > > - /* > - * Take priority boosted tasks into account. If the new > - * effective priority is unchanged, we just store the new > - * normal parameters and do not touch the scheduler class and > - * the runqueue. This will be done when the task deboost > - * itself. > - */ > - new_effective_prio = rt_mutex_get_effective_prio(p, newprio); > - if (new_effective_prio == oldprio) { > - __setscheduler_params(p, attr); > - task_rq_unlock(rq, p, &flags); > - return 0; > + if (pi) { > + /* > + * Take priority boosted tasks into account. If the new > + * effective priority is unchanged, we just store the new > + * normal parameters and do not touch the scheduler class and > + * the runqueue. This will be done when the task deboost > + * itself. > + */ > + new_effective_prio = rt_mutex_get_effective_prio(p, newprio); > + if (new_effective_prio == oldprio) { > + __setscheduler_params(p, attr); > + task_rq_unlock(rq, p, &flags); > + return 0; > + } > } > > queued = task_on_rq_queued(p); > @@ -3628,7 +3640,19 @@ static int __sched_setscheduler(struct task_struct *p, > put_prev_task(rq, p); > > prev_class = p->sched_class; > - __setscheduler(rq, p, attr, true); > + __setscheduler(rq, p, attr, pi); > + > + if (prev_class != p->sched_class) { > + prev_class->switched_from(rq, p); > + /* > + * switched_from() is allowed to drop @rq->lock; which opens a > + * race against load-balancing, however since @p is not > + * currently enqueued it is invisible to the load-balancer. > + * > + * double check @p is still where we thought it was. > + */ > + WARN_ON_ONCE(task_rq(p) != rq); > + } > > if (running) > p->sched_class->set_curr_task(rq); > @@ -3640,10 +3664,25 @@ static int __sched_setscheduler(struct task_struct *p, > enqueue_task(rq, p, oldprio <= p->prio ? ENQUEUE_HEAD : 0); > } > > - check_class_changed(rq, p, prev_class, oldprio); > + /* > + * Both switched_to() and prio_changed() are allowed to drop @rq->lock; > + * which opens a race against load-balancing, and since @p is now > + * enqueued it can indeed be subject to this. > + * > + * This means that any balancing done by these functions must double > + * check a task's rq. > + */ > + if (prev_class != p->sched_class) > + p->sched_class->switched_to(rq, p); > + else if (oldprio != p->prio || dl_task(p)) > + p->sched_class->prio_changed(rq, p, oldprio); > + /* > + * It further means we should not rely on @p's rq from here on. > + */ > task_rq_unlock(rq, p, &flags); > > - rt_mutex_adjust_pi(p); > + if (pi) > + rt_mutex_adjust_pi(p); > > return 0; > } > @@ -3664,7 +3703,7 @@ static int _sched_setscheduler(struct task_struct *p, int policy, > attr.sched_policy = policy; > } > > - return __sched_setscheduler(p, &attr, check); > + return __sched_setscheduler(p, &attr, check, true); > } > /** > * sched_setscheduler - change the scheduling policy and/or RT priority of a thread. > @@ -3685,7 +3724,7 @@ EXPORT_SYMBOL_GPL(sched_setscheduler); > > int sched_setattr(struct task_struct *p, const struct sched_attr *attr) > { > - return __sched_setscheduler(p, attr, true); > + return __sched_setscheduler(p, attr, true, true); > } > EXPORT_SYMBOL_GPL(sched_setattr); > > @@ -7346,32 +7385,12 @@ EXPORT_SYMBOL(___might_sleep); > #endif > > #ifdef CONFIG_MAGIC_SYSRQ > -static void normalize_task(struct rq *rq, struct task_struct *p) > +void normalize_rt_tasks(void) > { > - const struct sched_class *prev_class = p->sched_class; > + struct task_struct *g, *p; > struct sched_attr attr = { > .sched_policy = SCHED_NORMAL, > }; > - int old_prio = p->prio; > - int queued; > - > - queued = task_on_rq_queued(p); > - if (queued) > - dequeue_task(rq, p, 0); > - __setscheduler(rq, p, &attr, false); > - if (queued) { > - enqueue_task(rq, p, 0); > - resched_curr(rq); > - } > - > - check_class_changed(rq, p, prev_class, old_prio); > -} > - > -void normalize_rt_tasks(void) > -{ > - struct task_struct *g, *p; > - unsigned long flags; > - struct rq *rq; > > read_lock(&tasklist_lock); > for_each_process_thread(g, p) { > @@ -7398,9 +7417,7 @@ void normalize_rt_tasks(void) > continue; > } > > - rq = task_rq_lock(p, &flags); > - normalize_task(rq, p); > - task_rq_unlock(rq, p, &flags); > + __sched_setscheduler(p, &attr, false, false); > } > read_unlock(&tasklist_lock); > } >