| Message ID | 20171005135618.yufhaklq5cefaiyn@hirez.programming.kicks-ass.net (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On 10/05/2017 09:56 AM, Peter Zijlstra wrote: > On Thu, Oct 05, 2017 at 01:54:56PM +0100, Will Deacon wrote: >> When a prospective writer takes the qrwlock locking slowpath due to the >> lock being held, it attempts to cmpxchg the wmode field from 0 to >> _QW_WAITING so that concurrent lockers also take the slowpath and queue >> on the spinlock accordingly, allowing the lockers to drain. >> >> Unfortunately, this isn't fair, because a fastpath writer that comes in >> after the lock is made available but before the _QW_WAITING flag is set >> can effectively jump the queue. If there is a steady stream of prospective >> writers, then the waiter will be held off indefinitely. >> >> This patch restores fairness by separating _QW_WAITING and _QW_LOCKED >> into two bits in the wmode byte and having the waiter set _QW_WAITING >> unconditionally. This then forces the slow-path for concurrent lockers, >> but requires that a writer unlock operation performs an >> atomic_sub_release instead of a store_release so that the waiting status >> is preserved. >> diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h >> index 02c0a768e6b0..8b7edef500e5 100644 >> --- a/include/asm-generic/qrwlock.h >> +++ b/include/asm-generic/qrwlock.h >> @@ -41,7 +41,7 @@ >> * +----+----+----+----+ >> */ >> #define _QW_WAITING 1 /* A writer is waiting */ >> -#define _QW_LOCKED 0xff /* A writer holds the lock */ >> +#define _QW_LOCKED 2 /* A writer holds the lock */ >> #define _QW_WMASK 0xff /* Writer mask */ >> #define _QR_SHIFT 8 /* Reader count shift */ >> #define _QR_BIAS (1U << _QR_SHIFT) >> @@ -134,7 +134,7 @@ static inline void queued_read_unlock(struct qrwlock *lock) >> */ >> static inline void queued_write_unlock(struct qrwlock *lock) >> { >> - smp_store_release(&lock->wmode, 0); >> + (void)atomic_sub_return_release(_QW_LOCKED, &lock->cnts); >> } > That is a fairly painful hit on x86. Changes a regular store into an > "LOCK XADD" +20 cycles right there. > > Can't we steal one of the reader bits for waiting? > > diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h > index 7d026bf27713..5524801a02a5 100644 > --- a/include/asm-generic/qrwlock.h > +++ b/include/asm-generic/qrwlock.h > @@ -40,10 +40,10 @@ > * | rd | wr | > * +----+----+----+----+ > */ > -#define _QW_WAITING 1 /* A writer is waiting */ > -#define _QW_LOCKED 0xff /* A writer holds the lock */ > -#define _QW_WMASK 0xff /* Writer mask */ > -#define _QR_SHIFT 8 /* Reader count shift */ > +#define _QW_WAITING 0x100 /* A writer is waiting */ > +#define _QW_LOCKED 0x0ff /* A writer holds the lock */ > +#define _QW_WMASK 0x1ff /* Writer mask */ > +#define _QR_SHIFT 9 /* Reader count shift */ > #define _QR_BIAS (1U << _QR_SHIFT) > > /* > diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c > index 2655f26ec882..5f75caea97e0 100644 > --- a/kernel/locking/qrwlock.c > +++ b/kernel/locking/qrwlock.c > @@ -54,7 +54,7 @@ struct __qrwlock { > static __always_inline void > rspin_until_writer_unlock(struct qrwlock *lock, u32 cnts) > { > - while ((cnts & _QW_WMASK) == _QW_LOCKED) { > + while ((cnts & _QW_LOCKED)) { > cpu_relax(); > cnts = atomic_read_acquire(&lock->cnts); > } > @@ -120,21 +120,10 @@ void queued_write_lock_slowpath(struct qrwlock *lock) > (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0)) > goto unlock; > > - /* > - * Set the waiting flag to notify readers that a writer is pending, > - * or wait for a previous writer to go away. > - */ > - for (;;) { > - struct __qrwlock *l = (struct __qrwlock *)lock; > - > - if (!READ_ONCE(l->wmode) && > - (cmpxchg_relaxed(&l->wmode, 0, _QW_WAITING) == 0)) > - break; > - > - cpu_relax(); > - } > + /* Set the waiting flag to notify readers that a writer is pending */ > + atomic_add(_QW_WAITING, &lock->cnts); > > - /* When no more readers, set the locked flag */ > + /* When no more readers or writers, set the locked flag */ > for (;;) { > cnts = atomic_read(&lock->cnts); > if ((cnts == _QW_WAITING) && I think this is the better of dealing with the fairness problem. Acked-by: Waiman Long <longman@redhat.com>
HI Peter, Thanks for having a look. On Thu, Oct 05, 2017 at 03:56:18PM +0200, Peter Zijlstra wrote: > On Thu, Oct 05, 2017 at 01:54:56PM +0100, Will Deacon wrote: > > When a prospective writer takes the qrwlock locking slowpath due to the > > lock being held, it attempts to cmpxchg the wmode field from 0 to > > _QW_WAITING so that concurrent lockers also take the slowpath and queue > > on the spinlock accordingly, allowing the lockers to drain. > > > > Unfortunately, this isn't fair, because a fastpath writer that comes in > > after the lock is made available but before the _QW_WAITING flag is set > > can effectively jump the queue. If there is a steady stream of prospective > > writers, then the waiter will be held off indefinitely. > > > > This patch restores fairness by separating _QW_WAITING and _QW_LOCKED > > into two bits in the wmode byte and having the waiter set _QW_WAITING > > unconditionally. This then forces the slow-path for concurrent lockers, > > but requires that a writer unlock operation performs an > > atomic_sub_release instead of a store_release so that the waiting status > > is preserved. > > > diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h > > index 02c0a768e6b0..8b7edef500e5 100644 > > --- a/include/asm-generic/qrwlock.h > > +++ b/include/asm-generic/qrwlock.h > > @@ -41,7 +41,7 @@ > > * +----+----+----+----+ > > */ > > #define _QW_WAITING 1 /* A writer is waiting */ > > -#define _QW_LOCKED 0xff /* A writer holds the lock */ > > +#define _QW_LOCKED 2 /* A writer holds the lock */ > > #define _QW_WMASK 0xff /* Writer mask */ > > #define _QR_SHIFT 8 /* Reader count shift */ > > #define _QR_BIAS (1U << _QR_SHIFT) > > @@ -134,7 +134,7 @@ static inline void queued_read_unlock(struct qrwlock *lock) > > */ > > static inline void queued_write_unlock(struct qrwlock *lock) > > { > > - smp_store_release(&lock->wmode, 0); > > + (void)atomic_sub_return_release(_QW_LOCKED, &lock->cnts); > > } > > That is a fairly painful hit on x86. Changes a regular store into an > "LOCK XADD" +20 cycles right there. Yeah, I mentioned that in the cover letter which is also why it's at the end of the series ;) However, it's worth noting that this is the same as the reader unlock path and, as it stands, there's a real risk of writer starvation with the current code which isn't great for a queued lock. > Can't we steal one of the reader bits for waiting? I considered this at LPC and somehow convinced myself it didn't work, but actually all it's really doing is making the _QW_LOCKED bit a byte, so it should work fine. I'll work that into v2. Will
diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h index 7d026bf27713..5524801a02a5 100644 --- a/include/asm-generic/qrwlock.h +++ b/include/asm-generic/qrwlock.h @@ -40,10 +40,10 @@ * | rd | wr | * +----+----+----+----+ */ -#define _QW_WAITING 1 /* A writer is waiting */ -#define _QW_LOCKED 0xff /* A writer holds the lock */ -#define _QW_WMASK 0xff /* Writer mask */ -#define _QR_SHIFT 8 /* Reader count shift */ +#define _QW_WAITING 0x100 /* A writer is waiting */ +#define _QW_LOCKED 0x0ff /* A writer holds the lock */ +#define _QW_WMASK 0x1ff /* Writer mask */ +#define _QR_SHIFT 9 /* Reader count shift */ #define _QR_BIAS (1U << _QR_SHIFT) /* diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c index 2655f26ec882..5f75caea97e0 100644 --- a/kernel/locking/qrwlock.c +++ b/kernel/locking/qrwlock.c @@ -54,7 +54,7 @@ struct __qrwlock { static __always_inline void rspin_until_writer_unlock(struct qrwlock *lock, u32 cnts) { - while ((cnts & _QW_WMASK) == _QW_LOCKED) { + while ((cnts & _QW_LOCKED)) { cpu_relax(); cnts = atomic_read_acquire(&lock->cnts); } @@ -120,21 +120,10 @@ void queued_write_lock_slowpath(struct qrwlock *lock) (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0)) goto unlock; - /* - * Set the waiting flag to notify readers that a writer is pending, - * or wait for a previous writer to go away. - */ - for (;;) { - struct __qrwlock *l = (struct __qrwlock *)lock; - - if (!READ_ONCE(l->wmode) && - (cmpxchg_relaxed(&l->wmode, 0, _QW_WAITING) == 0)) - break; - - cpu_relax(); - } + /* Set the waiting flag to notify readers that a writer is pending */ + atomic_add(_QW_WAITING, &lock->cnts); - /* When no more readers, set the locked flag */ + /* When no more readers or writers, set the locked flag */ for (;;) { cnts = atomic_read(&lock->cnts); if ((cnts == _QW_WAITING) &&