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SubjectRe: [PATCH] Prevent immediate process rescheduling
On 09/18/2009 11:03 PM, Peter Zijlstra wrote:
> On Fri, 2009-09-18 at 21:54 +0200, Ingo Molnar wrote:
>
>
>>> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
>>> index 652e8bd..4fad08f 100644
>>> --- a/kernel/sched_fair.c
>>> +++ b/kernel/sched_fair.c
>>> @@ -353,11 +353,25 @@ static void __dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
>>> static struct sched_entity *__pick_next_entity(struct cfs_rq *cfs_rq)
>>> {
>>> struct rb_node *left = cfs_rq->rb_leftmost;
>>> + struct sched_entity *se, *curr;
>>>
>>> if (!left)
>>> return NULL;
>>>
>>> - return rb_entry(left, struct sched_entity, run_node);
>>> + se = rb_entry(left, struct sched_entity, run_node);
>>> + curr =&current->se;
>>> +
>>> + /*
>>> + * Don't select the entity who just tried to schedule away
>>> + * if there's another entity available.
>>> + */
>>> + if (unlikely(se == curr&& cfs_rq->nr_running> 1)) {
>>> + struct rb_node *next_node = rb_next(&curr->run_node);
>>> + if (next_node)
>>> + se = rb_entry(next_node, struct sched_entity, run_node);
>>> + }
>>> +
>>> + return se;
>>> }
>>>
> Really hate this change though,. doesn't seem right to not pick the same
> task again if its runnable. Bad for cache footprint.
>
> The scenario is quite common for stuff like:
>
> CPU0 CPU1
>
> set_task_state(TASK_INTERRUPTIBLE)
>
> if (cond)
> goto out;
> <--- ttwu()
> schedule();
>
>

I agree, yielding should be explicitly requested.

Also, on a heavily overcommitted box an undirected yield might take
quite a long time to find the thread that's holding the lock. I think a
yield_to() will be a lot better:

- we can pick one of the vcpus belonging to the same guest to improve
the probability that the lock actually get released
- we avoid an issue when the other vcpus are on different runqueues (in
which case the current patch does nothing)
- we can fix the accounting by donating vruntime from current to the
yielded-to vcpu


--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.



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