Re: [PATCH v2] locking/rwsem: Take read lock immediate if queue empty with no writer

[Peter Zijlstra]

On Fri, Jul 13, 2018 at 02:30:53PM -0400, Waiman Long wrote:
> It was discovered that a constant stream of readers might cause the
> count to go negative most of the time after an initial trigger by a
> writer even if no writer was present afterward. As a result, most of the
> readers would have to go through the slowpath reducing their performance.

I'm slightly confused, what happens to trigger this?

(also, I'm forever confused by the whole BIAS scheme)

> To avoid that 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 have the lock and return immediately without further action.
> Other incoming readers won't see a waiter is present and be forced
> into the slowpath.
> 
> The additional code is in the slowpath and so should not have an impact
> on rwsem performance. However, in the special case listed above, it may
> greatly improve performance.


> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> index 3064c50..bf0570e 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,

your diff function thingy is busted, this is in fact
__rwsem_down_read_failed_common you're patching.

>  	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 the unlikely event that the task is the only one in
> +		 * the wait queue and a writer isn't present, it can have
> +		 * the lock and return immediately without going through
> +		 * the remaining slowpath code.
> +		 */
> +		if (unlikely(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);

So without this, we would add ourselves to the list and then immediately
call __rwsem_mark_wake() on ourselves and fall through the wait-loop, ow
what?