Hi Waiman, On Mon, Jul 06, 2015 at 04:43:04PM +0100, Waiman Long wrote: > Currently, a reader will check first to make sure that the writer mode > byte is cleared before incrementing the reader count. That waiting is > not really necessary. It increases the latency in the reader/writer > to reader transition and reduces readers performance. > > This patch eliminates that waiting. It also has the side effect > of reducing the chance of writer lock stealing and improving the > fairness of the lock. Using a locking microbenchmark, a 10-threads 5M > locking loop of mostly readers (RW ratio = 10,000:1) has the following > performance numbers in a Haswell-EX box: > > Kernel Locking Rate (Kops/s) > ------ --------------------- > 4.1.1 15,063,081 > Patched 4.1.1 17,241,552 > > Signed-off-by: Waiman Long <Waiman.Long@xxxxxx> I've just finished rebasing my arm64 qrwlock stuff, but I think it will conflict with these patches. Do you mind if I post them for review anyway, so we can at least co-ordinate our efforts? > --- > kernel/locking/qrwlock.c | 12 ++++-------- > 1 files changed, 4 insertions(+), 8 deletions(-) > > diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c > index 81bae99..ecd2d19 100644 > --- a/kernel/locking/qrwlock.c > +++ b/kernel/locking/qrwlock.c > @@ -88,15 +88,11 @@ void queue_read_lock_slowpath(struct qrwlock *lock, u32 cnts) > arch_spin_lock(&lock->lock); > > /* > - * At the head of the wait queue now, wait until the writer state > - * goes to 0 and then try to increment the reader count and get > - * the lock. It is possible that an incoming writer may steal the > - * lock in the interim, so it is necessary to check the writer byte > - * to make sure that the write lock isn't taken. > + * At the head of the wait queue now, increment the reader count > + * and wait until the writer, if it has the lock, has gone away. > + * At ths stage, it is not possible for a writer to remain in the > + * waiting state (_QW_WAITING). So there won't be any deadlock. > */ > - while (atomic_read(&lock->cnts) & _QW_WMASK) > - cpu_relax_lowlatency(); Thinking about it, can we kill _QW_WAITING altogether and set (cmpxchg from 0) wmode to _QW_LOCKED in the write_lock slowpath, polling (acquire) rmode until it hits zero? Will -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
