Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758101Ab0LTQEU (ORCPT ); Mon, 20 Dec 2010 11:04:20 -0500 Received: from cantor.suse.de ([195.135.220.2]:43994 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757900Ab0LTQET (ORCPT ); Mon, 20 Dec 2010 11:04:19 -0500 Subject: Re: [RFC -v2 PATCH 2/3] sched: add yield_to function From: Mike Galbraith To: Rik van Riel Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Avi Kiviti , Srivatsa Vaddagiri , Peter Zijlstra , Chris Wright In-Reply-To: <4D0F78DF.9010908@redhat.com> References: <20101213224434.7495edb2@annuminas.surriel.com> <20101213224657.7e141746@annuminas.surriel.com> <1292306896.7448.157.camel@marge.simson.net> <4D0A6D34.6070806@redhat.com> <1292569018.7772.75.camel@marge.simson.net> <1292570143.7772.84.camel@marge.simson.net> <4D0F78DF.9010908@redhat.com> Content-Type: text/plain; charset="UTF-8" Date: Mon, 20 Dec 2010 17:04:19 +0100 Message-ID: <1292861059.15207.7.camel@marge.simson.net> Mime-Version: 1.0 X-Mailer: Evolution 2.30.1.2 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5655 Lines: 184 On Mon, 2010-12-20 at 10:40 -0500, Rik van Riel wrote: > On 12/17/2010 02:15 AM, Mike Galbraith wrote: > > > BTW, with this vruntime donation thingy, what prevents a task from > > forking off accomplices who do nothing but wait for a wakeup and > > yield_to(exploit)? > > > > Even swapping vruntimes in the same cfs_rq is dangerous as hell, because > > one party is going backward. > > I just realized the answer to this question. > > We only give cpu time to tasks that are runnable, but not > currently running. That ensures one task cannot block others > from running by having time yielded to it constantly. Hm. Don't think that will 100% sure prevent clock stoppage, because the running task doesn't necessarily advance min_vruntime. What about something like the below instead? It compiles, but may eat your first born child. sched: implement fair class yield_to(task) using cfs_rq->next as a selection hint. Not-signed-off-by: Mike Galbraith --- include/linux/sched.h | 1 kernel/sched.c | 47 +++++++++++++++++++++++++++++++++++++++++ kernel/sched_fair.c | 56 ++++++++++++++++++++++++++++++++++++++++++++++++++ 3 files changed, 104 insertions(+) Index: linux-2.6/include/linux/sched.h =================================================================== --- linux-2.6.orig/include/linux/sched.h +++ linux-2.6/include/linux/sched.h @@ -1056,6 +1056,7 @@ struct sched_class { void (*enqueue_task) (struct rq *rq, struct task_struct *p, int flags); void (*dequeue_task) (struct rq *rq, struct task_struct *p, int flags); void (*yield_task) (struct rq *rq); + int (*yield_to_task) (struct task_struct *p, int preempt); void (*check_preempt_curr) (struct rq *rq, struct task_struct *p, int flags); Index: linux-2.6/kernel/sched.c =================================================================== --- linux-2.6.orig/kernel/sched.c +++ linux-2.6/kernel/sched.c @@ -5325,6 +5325,53 @@ void __sched yield(void) } EXPORT_SYMBOL(yield); +/** + * yield_to - yield the current processor to another thread in + * your thread group, or accelerate that thread toward the + * processor it's on. + * + * It's the caller's job to ensure that the target task struct + * can't go away on us before we can do any checks. + */ +void __sched yield_to(struct task_struct *p, int preempt) +{ + struct task_struct *curr = current; + struct rq *rq, *p_rq; + unsigned long flags; + int yield = 0; + + local_irq_save(flags); + rq = this_rq(); + +again: + p_rq = task_rq(p); + double_rq_lock(rq, p_rq); + while (task_rq(p) != p_rq) { + double_rq_unlock(rq, p_rq); + goto again; + } + + if (task_running(p_rq, p) || p->state || !p->se.on_rq || + !same_thread_group(p, curr) || + !curr->sched_class->yield_to_task || + curr->sched_class != p->sched_class) { + goto out; + } + + yield = curr->sched_class->yield_to_task(p, preempt); + +out: + double_rq_unlock(rq, p_rq); + local_irq_restore(flags); + + if (yield) { + set_current_state(TASK_RUNNING); + schedule(); + } +} +EXPORT_SYMBOL(yield_to); + + /* * This task is about to go to sleep on IO. Increment rq->nr_iowait so * that process accounting knows that this is a task in IO wait state. Index: linux-2.6/kernel/sched_fair.c =================================================================== --- linux-2.6.orig/kernel/sched_fair.c +++ linux-2.6/kernel/sched_fair.c @@ -1320,6 +1320,61 @@ static void yield_task_fair(struct rq *r } #ifdef CONFIG_SMP +static void pull_task(struct rq *src_rq, struct task_struct *p, + struct rq *this_rq, int this_cpu); +#endif + +static int yield_to_task_fair(struct task_struct *p, int preempt) +{ + struct sched_entity *se = ¤t->se; + struct sched_entity *pse = &p->se; + struct sched_entity *curr = &(task_rq(p)->curr)->se; + struct cfs_rq *cfs_rq = cfs_rq_of(se); + struct cfs_rq *p_cfs_rq = cfs_rq_of(pse); + int yield = this_rq() == task_rq(p); + int want_preempt = preempt; + +#ifdef CONFIG_FAIR_GROUP_SCHED + if (cfs_rq->tg != p_cfs_rq->tg) + return 0; + + /* Preemption only allowed within the same task group. */ + if (preempt && cfs_rq->tg != cfs_rq_of(curr)->tg) + preempt = 0; +#endif + /* Preemption only allowed within the same thread group. */ + if (preempt && !same_thread_group(current, task_of(p_cfs_rq->curr))) + preempt = 0; + +#ifdef CONFIG_SMP + /* + * If this yield is important enough to want to preempt instead + * of only dropping a ->next hint, we're alone, and the target + * is not alone, pull the target to this cpu. + */ + if (want_preempt && !yield && cfs_rq->nr_running == 1 && + cpumask_test_cpu(smp_processor_id(), &p->cpus_allowed)) { + pull_task(task_rq(p), p, this_rq(), smp_processor_id()); + p_cfs_rq = cfs_rq_of(pse); + yield = 1; + } +#endif + + if (yield) + clear_buddies(cfs_rq, se); + else if (preempt) + clear_buddies(p_cfs_rq, curr); + + /* Tell the scheduler that we'd really like pse to run next. */ + p_cfs_rq->next = pse; + + if (!yield && preempt) + resched_task(task_of(p_cfs_rq->curr)); + + return yield; +} + +#ifdef CONFIG_SMP static void task_waking_fair(struct rq *rq, struct task_struct *p) { @@ -4126,6 +4181,7 @@ static const struct sched_class fair_sch .enqueue_task = enqueue_task_fair, .dequeue_task = dequeue_task_fair, .yield_task = yield_task_fair, + .yield_to_task = yield_to_task_fair, .check_preempt_curr = check_preempt_wakeup, -- 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/