Re: [PATCH v3] locking/rwsem: Exit read lock slowpath if queue empty & no writer

[Waiman Long <longman@redhat.com>]

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 <longman@redhat.com>
> ---
>  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