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Subject: Re: [PATCH-tip v2 06/12] locking/rwsem: Wake up almost all readers in
 wait queue
To:     Davidlohr Bueso <dave@stgolabs.net>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Will Deacon <will.deacon@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>
References: <20190405192115.17416-1-longman@redhat.com>
 <20190405192115.17416-7-longman@redhat.com>
 <20190410165013.njy5bg32pxq6syyr@linux-r8p5>
From:   Waiman Long <longman@redhat.com>
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Message-ID: <114e3bc3-3b66-4a50-d422-6a477d85fa72@redhat.com>
Date:   Wed, 10 Apr 2019 13:08:28 -0400
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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