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

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Date:   Wed, 10 Apr 2019 09:50:13 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [PATCH-tip v2 06/12] locking/rwsem: Wake up almost all readers
 in wait queue

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