Re: [PATCH][RFC]: mutex: adaptive spin

Lai Jiangshan
2009-01-07T07:38:22Z

Peter Zijlstra wrote:
> On Wed, 2009-01-07 at 11:57 +0800, Lai Jiangshan wrote:
>> Peter Zijlstra wrote:
>>> +void mutex_spin_or_schedule(struct mutex_waiter *waiter, long state, unsigned long *flags)
>>> +{
>>> + struct mutex *lock = waiter->lock;
>>> + struct task_struct *task = waiter->task;
>>> + struct task_struct *owner = lock->owner;
>>> + struct rq *rq;
>>> +
>>> + if (!owner)
>>> + goto do_schedule;
>>> +
>>> + rq = task_rq(owner);
>>> +
>>> + if (rq->curr != owner) {
>>> +do_schedule:
>>> + __set_task_state(task, state);
>>> + spin_unlock_mutex(&lock->wait_lock, *flags);
>>> + schedule();
>>> + } else {
>>> + spin_unlock_mutex(&lock->wait_lock, *flags);
>>> + for (;;) {
>>> + /* Stop spinning when there's a pending signal. */
>>> + if (signal_pending_state(state, task))
>>> + break;
>>> +
>>> + /* Owner changed, bail to revalidate state */
>>> + if (lock->owner != owner)
>>> + break;
>>> +
>>> + /* Owner stopped running, bail to revalidate state */
>>> + if (rq->curr != owner)
>>> + break;
>>> +
>> 2 questions from my immature thought:
>>
>> 1) Do we need keep gcc from optimizing when we access lock->owner
>> and rq->curr in the loop?
>
> cpu_relax() is a compiler barrier iirc.
>
>> 2) "if (rq->curr != owner)" need become smarter.
>> schedule()
>> {
>> select_next
>> rq->curr = next;
>> contex_swith
>> }
>> we also spin when owner is select_next-ing in schedule().
>> but select_next is not fast enough.
>
> I'm not sure what you're saying here..
>
>

I means when mutex owner calls schedule(), current task is also spinning
until rq->curr is changed.

I think such spin is not necessary, it is doing nothing but wasting time.
And this spin period is not short, and when this spin period ended,
rq->curr is changed too, current task has to sleep.

So I think current task should sleep earlier when it detects that
mutex owner start schedule().