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 13:08:28 -0400
From:   Waiman Long <longman@...hat.com>
To:     Davidlohr Bueso <dave@...olabs.net>
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 04/10/2019 12:50 PM, Davidlohr Bueso wrote:
> 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

There are 2 major reasons why there is a limit.

1) It will be unfair to the task that needs to spend so much of its own
CPU time to wake up too many readers.
2) I want to avoid the extreme case that there are more than 32k readers
in the wait queue and make the count overflow.

Cheers,
Longman