Messages in this thread | | | Date | Thu, 15 May 2014 17:45:06 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock_page fastpath v4 |
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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 ;-) [unhandled content-type:application/pgp-signature] | |