Re: [PATCH] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock_page fastpath v4

[Peter Zijlstra]

On Thu, May 15, 2014 at 05:34:24PM +0200, Oleg Nesterov wrote:
> On 05/15, Peter Zijlstra wrote:
> >
> > So I suppose I'm failing to see the problem with something like:
> 
> Yeeees, I was thinking about something like this too ;)
> 
> > static inline void lock_page(struct page *page)
> > {
> > 	if (!trylock_page(page))
> > 		__lock_page(page);
> > }
> >
> > static inline void unlock_page(struct page *page)
> > {
> > 	clear_bit_unlock(&page->flags, PG_locked);
> > 	if (PageWaiters(page))
> > 		__unlock_page();
> > }
> 
> but in this case we need mb() before PageWaiters(), I guess.

Ah indeed so, or rather, this is a good reason to use smp_mb__after_atomic().

> > void __lock_page(struct page *page)
> > {
> > 	struct wait_queue_head_t *wqh = page_waitqueue(page);
> > 	DEFINE_WAIT_BIT(wait, &page->flags, PG_locked);
> >
> > 	spin_lock_irq(&wqh->lock);
> > 	if (!PageWaiters(page))
> > 		SetPageWaiters(page);
> >
> > 	wait.flags |= WQ_FLAG_EXCLUSIVE;
> > 	preempt_disable();
> 
> why?
> 
> > 	do {
> > 		if (list_empty(&wait->task_list))
> > 			__add_wait_queue_tail(wqh, &wait);
> >
> > 		set_current_state(TASK_UNINTERRUPTIBLE);
> >
> > 		if (test_bit(wait.key.bit_nr, wait.key.flags)) {
> > 			spin_unlock_irq(&wqh->lock);
> > 			schedule_preempt_disabled();
> > 			spin_lock_irq(&wqh->lock);
> 
> OK, probably to avoid the preemption before schedule().

Indeed.

> Still can't  undestand why this makes sense,

Because calling schedule twice in a row is like a bit of wasted effort.
Its just annoying there isn't a more convenient way to express this,
because its a fairly common thing in wait loops.

> but in this case it would be better
> to do disable/enable under "if (test_bit())" ?

Ah yes.. that code grew and the preempt_disable came about before that
test_bit() block.. :-)

> Of course, this needs more work for lock_page_killable(), but this
> should be simple.

Yeah, I just wanted to illustrate the point, and cobbling one together
from various wait loops was plenty I thought ;-)

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