| Message ID | 1352122278-12896-5-git-send-email-thellstrom@vmware.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
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.
On 11/05/2012 03:01 PM, Maarten Lankhorst wrote: > 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? :-) > If your patch series makes it into the same kernel, let's kill this patch. Otherwise it may live at least for a kernel release. It's not a big thing to rebase against, and I won't complain if your patch adds another atomic read-modify-write op here. :) /Thomas
diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c index bf6e4b5..46008ea 100644 --- a/drivers/gpu/drm/ttm/ttm_bo.c +++ b/drivers/gpu/drm/ttm/ttm_bo.c @@ -220,7 +220,7 @@ int ttm_bo_reserve_locked(struct ttm_buffer_object *bo, struct ttm_bo_global *glob = bo->glob; int ret; - while (unlikely(atomic_cmpxchg(&bo->reserved, 0, 1) != 0)) { + while (unlikely(atomic_read(&bo->reserved) != 0)) { /** * Deadlock avoidance for multi-bo reserving. */ @@ -249,6 +249,7 @@ int ttm_bo_reserve_locked(struct ttm_buffer_object *bo, return ret; } + atomic_set(&bo->reserved, 1); if (use_sequence) { /** * Wake up waiters that may need to recheck for deadlock,
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> --- drivers/gpu/drm/ttm/ttm_bo.c | 3 ++- 1 files changed, 2 insertions(+), 1 deletions(-)