Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751381Ab1BZFpB (ORCPT ); Sat, 26 Feb 2011 00:45:01 -0500 Received: from mx1.redhat.com ([209.132.183.28]:54527 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751143Ab1BZFpA (ORCPT ); Sat, 26 Feb 2011 00:45:00 -0500 Message-ID: <4D689346.1090002@redhat.com> Date: Sat, 26 Feb 2011 00:44:38 -0500 From: Rik van Riel User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.13) Gecko/20101209 Fedora/3.1.7-0.35.b3pre.fc13 Lightning/1.0b3pre Thunderbird/3.1.7 MIME-Version: 1.0 To: Venkatesh Pallipadi CC: mingo@redhat.com, hpa@zytor.com, linux-kernel@vger.kernel.org, a.p.zijlstra@chello.nl, mtosatti@redhat.com, efault@gmx.de, tglx@linutronix.de, mingo@elte.hu Subject: Re: [tip:sched/core] sched: Add yield_to(task, preempt) functionality References: <20110201095051.4ddb7738@annuminas.surriel.com>

In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4382 Lines: 132 On 02/25/2011 07:43 PM, Venkatesh Pallipadi wrote: > On Thu, Feb 3, 2011 at 6:12 AM, tip-bot for Mike Galbraith > wrote: >> Commit-ID: d95f412200652694e63e64bfd49f0ae274a54479 >> Gitweb: http://git.kernel.org/tip/d95f412200652694e63e64bfd49f0ae274a54479 >> Author: Mike Galbraith >> AuthorDate: Tue, 1 Feb 2011 09:50:51 -0500 >> Committer: Ingo Molnar >> CommitDate: Thu, 3 Feb 2011 14:20:33 +0100 >> >> sched: Add yield_to(task, preempt) functionality > > I was looking at this patch, while trying to figure out how best to > use next buddy to solve another unrelated to this cgroup context > switching problem. While going through this change I see something > that I don't really understand (inlined below). Not sure whether what > I am looking at is a bug or I am missing something very obvious. > > Thanks in advance for clarification. > >> >> Currently only implemented for fair class tasks. >> >> Add a yield_to_task method() to the fair scheduling class. allowing the >> caller of yield_to() to accelerate another thread in it's thread group, >> task group. >> > > > >> >> static void calc_load_account_idle(struct rq *this_rq); >> @@ -5448,6 +5481,58 @@ 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. >> + * >> + * Returns true if we indeed boosted the target task. >> + */ >> +bool __sched yield_to(struct task_struct *p, bool preempt) >> +{ >> + struct task_struct *curr = current; >> + struct rq *rq, *p_rq; >> + unsigned long flags; >> + bool yielded = 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 (!curr->sched_class->yield_to_task) >> + goto out; >> + >> + if (curr->sched_class != p->sched_class) >> + goto out; >> + >> + if (task_running(p_rq, p) || p->state) >> + goto out; >> + >> + yielded = curr->sched_class->yield_to_task(rq, p, preempt); >> + if (yielded) >> + schedstat_inc(rq, yld_count); >> + >> +out: >> + double_rq_unlock(rq, p_rq); >> + local_irq_restore(flags); >> + >> + if (yielded) >> + schedule(); >> + >> + return yielded; >> +} >> +EXPORT_SYMBOL_GPL(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. >> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c >> index c0fbeb9..0270246 100644 >> --- a/kernel/sched_fair.c >> +++ b/kernel/sched_fair.c >> @@ -1975,6 +1975,25 @@ static void yield_task_fair(struct rq *rq) >> set_skip_buddy(se); >> } >> >> +static bool yield_to_task_fair(struct rq *rq, struct task_struct *p, bool preempt) >> +{ >> + struct sched_entity *se =&p->se; >> + >> + if (!se->on_rq) >> + return false; >> + >> + /* Tell the scheduler that we'd really like pse to run next. */ >> + set_next_buddy(se); > > The below comment says about rescheduling p's CPU. But the rq variable > we have here is the curr_rq and not p_rq. So, should this be done in > yield_to() with p_rq. I did try to see the discussion on other > versions of this patch. v3 and before had - > "resched_task(task_of(p_cfs_rq->curr));" which seems to be correct... You are correct. We are calling resched_task on the wrong task, we should call it on p's runqueue's current task... >> + >> + /* Make p's CPU reschedule; pick_next_entity takes care of fairness. */ >> + if (preempt) >> + resched_task(rq->curr); -- All rights reversed -- 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/