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From: Davidlohr Bueso <dave@stgolabs.net>
To: Waiman Long <longman@redhat.com>
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>
Subject: Re: [PATCH-tip v2 06/12] locking/rwsem: Wake up almost all readers in wait queue
Date: Wed, 10 Apr 2019 09:50:13 -0700	[thread overview]
Message-ID: <20190410165013.njy5bg32pxq6syyr@linux-r8p5> (raw)
In-Reply-To: <20190405192115.17416-7-longman@redhat.com>

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

  reply	other threads:[~2019-04-10 16:50 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-05 19:21 [PATCH-tip v2 00/12] locking/rwsem: Rwsem rearchitecture part 2 Waiman Long
2019-04-05 19:21 ` [PATCH-tip v2 01/12] locking/rwsem: Implement a new locking scheme Waiman Long
2019-04-05 19:21 ` [PATCH-tip v2 02/12] locking/rwsem: Implement lock handoff to prevent lock starvation Waiman Long
2019-04-10 15:07   ` Peter Zijlstra
2019-04-10 15:28     ` Waiman Long
2019-04-10 15:10   ` Peter Zijlstra
2019-04-10 15:29     ` Waiman Long
2019-04-10 18:44   ` Peter Zijlstra
2019-04-11  2:25     ` Waiman Long
2019-04-11  7:16       ` Peter Zijlstra
2019-04-05 19:21 ` [PATCH-tip v2 03/12] locking/rwsem: Remove rwsem_wake() wakeup optimization Waiman Long
2019-04-10 18:38   ` Davidlohr Bueso
2019-04-05 19:21 ` [PATCH-tip v2 04/12] locking/rwsem: Make rwsem_spin_on_owner() return owner state Waiman Long
2019-04-05 19:21 ` [PATCH-tip v2 05/12] locking/rwsem: Ensure an RT task will not spin on reader Waiman Long
2019-04-05 19:21 ` [PATCH-tip v2 06/12] locking/rwsem: Wake up almost all readers in wait queue Waiman Long
2019-04-10 16:50   ` Davidlohr Bueso [this message]
2019-04-10 17:08     ` Waiman Long
2019-04-10 17:22       ` Davidlohr Bueso
2019-04-10 17:31         ` Davidlohr Bueso
2019-04-10 17:54           ` Waiman Long
2019-04-10 17:53         ` Waiman Long
2019-04-05 19:21 ` [PATCH-tip v2 07/12] locking/rwsem: Enable readers spinning on writer Waiman Long
2019-04-05 19:21 ` [PATCH-tip v2 08/12] locking/rwsem: Enable time-based spinning on reader-owned rwsem Waiman Long
2019-04-05 19:21 ` [PATCH-tip v2 09/12] locking/rwsem: Add more rwsem owner access helpers Waiman Long
2019-04-05 19:21 ` [PATCH-tip v2 10/12] locking/rwsem: Guard against making count negative Waiman Long
2019-04-05 19:21 ` [PATCH-tip v2 11/12] locking/rwsem: Merge owner into count on x86-64 Waiman Long
2019-04-05 19:21 ` [PATCH-tip v2 12/12] locking/rwsem: Remove redundant computation of writer lock word Waiman Long
2019-04-05 23:27 ` [PATCH-tip v2 00/12] locking/rwsem: Rwsem rearchitecture part 2 Linus Torvalds
2019-04-10 10:00 ` Ingo Molnar
2019-04-10 12:38   ` Waiman Long

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