Re: [tip:sched/core] sched: Add yield_to(task, preempt) functionality

Rik van Riel
2011-02-26T05:45:01Z

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@gmx.de> wrote:
>> Commit-ID: d95f412200652694e63e64bfd49f0ae274a54479
>> Gitweb: http://git.kernel.org/tip/d95f412200652694e63e64bfd49f0ae274a54479
>> Author: Mike Galbraith<efault@gmx.de>
>> AuthorDate: Tue, 1 Feb 2011 09:50:51 -0500
>> Committer: Ingo Molnar<mingo@elte.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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