Re: [PATCH-tip v2 06/12] locking/rwsem: Wake up almost all readers in wait queue

[Davidlohr Bueso]

On Fri, 05 Apr 2019, Waiman Long wrote:

> When the front of the wait queue is a reader, other readers
> immediately following the first reader will also be woken up at the
> same time. However, if there is a writer in between. Those readers
> behind the writer will not be woken up.
>
> Because of optimistic spinning, the lock acquisition order is not FIFO
> anyway. The lock handoff mechanism will ensure that lock starvation
> will not happen.
>
> Assuming that the lock hold times of the other readers still in the
> queue will be about the same as the readers that are being woken up,
> there is really not much additional cost other than the additional
> latency due to the wakeup of additional tasks by the waker. Therefore
> all the readers up to a maximum of 256 in the queue are woken up when
> the first waiter is a reader to improve reader throughput.

Before we stopped waking readers when a writer was encountered but
would otherwise wakeup _all_ readers. I don't understand why you want
to limit this to MAX_READERS_WAKEUP, otherwise I agree it's nice to
skip the writer and continue waking readers in the queue (with the handoff
guarantees obviously).

Would it not be better to do the MAX_READERS_WAKEUP limit only when
a writer is found?

Thanks,
Davidlohr