linux-kernel - Re: [tip:sched/core] sched: Add yield

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Date:	Sat, 26 Feb 2011 00:44:38 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Venkatesh Pallipadi <venki@...gle.com>
CC:	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	a.p.zijlstra@...llo.nl, mtosatti@...hat.com, efault@....de,
	tglx@...utronix.de, mingo@...e.hu
Subject: Re: [tip:sched/core] sched: Add yield_to(task, preempt) functionality

On 02/25/2011 07:43 PM, Venkatesh Pallipadi wrote:
> On Thu, Feb 3, 2011 at 6:12 AM, tip-bot for Mike Galbraith
> <efault@....de>  wrote:
>> Commit-ID:  d95f412200652694e63e64bfd49f0ae274a54479
>> Gitweb:     http://git.kernel.org/tip/d95f412200652694e63e64bfd49f0ae274a54479
>> Author:     Mike Galbraith<efault@....de>
>> AuthorDate: Tue, 1 Feb 2011 09:50:51 -0500
>> Committer:  Ingo Molnar<mingo@...e.hu>
>> 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.
>>
>
> <snip>
>
>>
>>   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);



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