On Wed, May 09, 2007 at 12:41:24AM +0200, Nick Piggin wrote: > On Wed, May 09, 2007 at 07:30:27AM +1000, Benjamin Herrenschmidt wrote: > > On Tue, 2007-05-08 at 13:40 +0200, Nick Piggin wrote: > > > This patch trades a page flag for a significant improvement in the > > > unlock_page > > > fastpath. Various problems in the previous version were spotted by Hugh > > > and > > > Ben (and fixed in this one). > > > > > > Comments? > > > > > > -- > > > > > > Speed up unlock_page by introducing a new page flag to signal that there > > > are > > > page waitqueue waiters for PG_locked. This means a memory barrier and a > > > random > > > waitqueue hash cacheline load can be avoided in the fastpath when there > > > is no > > > contention. > > > > I'm not 100% familiar with the exclusive vs. non exclusive wait thingy > > but wake_up_page() does __wake_up_bit() which calls __wake_up() with > > nr_exclusive set to 1. Doesn't that mean that only one waiter will be > > woken up ? > > > > If that's the case, then we lose because we'll have clear PG_waiters but > > only wake up one of them. > > > > Waking them all would fix it but at the risk of causing other > > problems... Maybe PG_waiters need to actually be a counter but if that > > is the case, then it complicates things even more. > > > > Any smart idea ? > > It will wake up 1 exclusive waiter, but no limit on non exclusive waiters. > Hmm, but it won't wake up waiters behind the exclusive guy... maybe the > wake up code can check whether the waitqueue is still active after the > wakeup, and set PG_waiters again in that case?
Hm, I don't know if we can do that without a race either... OTOH, waking all non exclusive waiters may not be a really bad idea. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
