Messages in this thread | | | Date | Mon, 05 Nov 2012 15:01:05 +0100 | From | Maarten Lankhorst <> | Subject | Re: [PATCH 4/4] drm/ttm: Optimize reservation slightly |
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Hey,
Op 05-11-12 14:31, Thomas Hellstrom schreef: > Reservation locking currently always takes place under the LRU spinlock. > Hence, strictly there is no need for an atomic_cmpxchg call; we can use > atomic_read followed by atomic_write since nobody else will ever reserve > without the lru spinlock held. > At least on Intel this should remove a locked bus cycle on successful > reserve. > > Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com> > Is that really a good thing to submit when I am busy killing lru lock around reserve? :-)
- while (unlikely(atomic_cmpxchg(&bo->reserved, 0, 1) != 0)) { + while (unlikely(atomic_xchg(&bo->reserved, 1) != 0)) {
Works without lru lock too!
In fact mutexes are done in a similar way[1], except with some more magic, and unlocked state is 1, not 0. However I do think that to get that right (saves a irq disable in unlock path, and less wakeups in contended case), I should really just post the mutex extension patches for reservations and ride the flames. It's getting too close to real mutexes so I really want it to be a mutex in that case. So lets convert it.. Soon! :-)
~Maarten
[1] See linux/include/asm-generic/mutex-xchg.h and linux/include/asm-generic/mutex-dec.h for how archs generally implement mutex fastpaths.
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