On 07/24/2018 03:10 PM, Waiman Long wrote: > It was discovered that a constant stream of readers with occassional > writers pounding on a rwsem may cause many of the readers to enter the > slowpath unnecessarily thus increasing latency and lowering performance. > > In the current code, a reader entering the slowpath critical section > will unconditionally set the WAITING_BIAS, if not set yet, and clear > its active count even if no one is in the wait queue and no writer > is present. This causes some incoming readers to observe the presence > of waiters in the wait queue and hence have to go into the slowpath > themselves. > > With sufficient numbers of readers and a relatively short lock hold time, > the WAITING_BIAS may be repeatedly turned on and off and a substantial > portion of the readers will go into the slowpath sustaining a rather > long queue in the wait queue spinlock and repeated WAITING_BIAS on/off > cycle until the logjam is broken opportunistically. > > To avoid this situation from happening, an additional check is added to > detect the special case that the reader in the critical section is the > only one in the wait queue and no writer is present. When that happens, > it can just exit the slowpath and return immediately as its active count > has already been set in the lock. Other incoming readers won't observe > the presence of waiters and so will not be forced into the slowpath. > > The issue was found in a customer site where they had an application > that pounded on the pread64 syscalls heavily on an XFS filesystem. The > application was run in a recent 4-socket boxes with a lot of CPUs. They > saw significant spinlock contention in the rwsem_down_read_failed() call. > With this patch applied, the system CPU usage went down from 85% to 57%, > and the spinlock contention in the pread64 syscalls was gone. > > v3: Revise the commit log and comment again. > v2: Add customer testing results and remove wording that may cause > confusion. > > Signed-off-by: Waiman Long <[email protected]> > --- > kernel/locking/rwsem-xadd.c | 13 ++++++++++++- > 1 file changed, 12 insertions(+), 1 deletion(-) > > diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c > index 3064c50..01fcb80 100644 > --- a/kernel/locking/rwsem-xadd.c > +++ b/kernel/locking/rwsem-xadd.c > @@ -233,8 +233,19 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem, > waiter.type = RWSEM_WAITING_FOR_READ; > > raw_spin_lock_irq(&sem->wait_lock); > - if (list_empty(&sem->wait_list)) > + if (list_empty(&sem->wait_list)) { > + /* > + * In case the wait queue is empty and the lock isn't owned > + * by a writer, this reader can exit the slowpath and return > + * immediately as its RWSEM_ACTIVE_READ_BIAS has already > + * been set in the count. > + */ > + if (atomic_long_read(&sem->count) >= 0) { > + raw_spin_unlock_irq(&sem->wait_lock); > + return sem; > + } > adjustment += RWSEM_WAITING_BIAS; > + } > list_add_tail(&waiter.list, &sem->wait_list); > > /* we're now waiting on the lock, but no longer actively locking */
Will this patch be eligible to go into 4.19 or 4.20? Thanks, Longman
