Messages in this thread | | | Date | Wed, 17 Jun 2015 21:30:10 -0400 | From | Waiman Long <> | Subject | Re: [PATCH v3 1/2] locking/qrwlock: Better optimization for interrupt context readers |
| |
On 06/16/2015 08:17 AM, Will Deacon wrote: > Hi Waiman, > > On Mon, Jun 15, 2015 at 11:24:02PM +0100, Waiman Long wrote: >> The qrwlock is fair in the process context, but becoming unfair when >> in the interrupt context to support use cases like the tasklist_lock. >> >> The current code isn't that well-documented on what happens when >> in the interrupt context. The rspin_until_writer_unlock() will only >> spin if the writer has gotten the lock. If the writer is still in the >> waiting state, the increment in the reader count will cause the writer >> to remain in the waiting state and the new interrupt context reader >> will get the lock and return immediately. The current code, however, >> do an additional read of the lock value which is not necessary as the >> information have already been there in the fast path. This may sometime >> cause an additional cacheline load when the lock is highly contended. >> >> This patch passes the lock value information gotten in the fast path >> to the slow path to eliminate the additional read. It also clarify the >> action for the interrupt context readers more explicitly. >> >> Signed-off-by: Waiman Long<Waiman.Long@hp.com> >> --- >> include/asm-generic/qrwlock.h | 4 ++-- >> kernel/locking/qrwlock.c | 14 ++++++++------ >> 2 files changed, 10 insertions(+), 8 deletions(-) > [...] > >> diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c >> index 00c12bb..d7d7557 100644 >> --- a/kernel/locking/qrwlock.c >> +++ b/kernel/locking/qrwlock.c >> @@ -43,22 +43,24 @@ rspin_until_writer_unlock(struct qrwlock *lock, u32 cnts) >> * queue_read_lock_slowpath - acquire read lock of a queue rwlock >> * @lock: Pointer to queue rwlock structure >> */ >> -void queue_read_lock_slowpath(struct qrwlock *lock) >> +void queue_read_lock_slowpath(struct qrwlock *lock, u32 cnts) >> { >> - u32 cnts; >> - >> /* >> * Readers come here when they cannot get the lock without waiting >> */ >> if (unlikely(in_interrupt())) { >> /* >> - * Readers in interrupt context will spin until the lock is >> - * available without waiting in the queue. >> + * Readers in interrupt context will get the lock immediately >> + * if the writer is just waiting (not holding the lock yet) >> + * or they will spin until the lock is available without >> + * waiting in the queue. >> */ >> - cnts = smp_load_acquire((u32 *)&lock->cnts); >> + if ((cnts& _QW_WMASK) != _QW_LOCKED) >> + return; > I really doubt the check here is gaining you any performance, given > rspin_until_write_unlock does the same check immediately and should be > inlined. Just dropping the acquire and passing cnts through should be > sufficient.
Yes, you are right. I can just pass the cnt to rspin_until_write_unlock() and be done with it.
Cheers, Longman
| |