linux-kernel - Re: [PATCH-tip v2 02/12] locking/rwsem: Implement lock handoff to prevent lock starvation

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Date:   Wed, 10 Apr 2019 17:07:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Davidlohr Bueso <dave@...olabs.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [PATCH-tip v2 02/12] locking/rwsem: Implement lock handoff to
 prevent lock starvation

On Fri, Apr 05, 2019 at 03:21:05PM -0400, Waiman Long wrote:
> Because of writer lock stealing, it is possible that a constant
> stream of incoming writers will cause a waiting writer or reader to
> wait indefinitely leading to lock starvation.
> 
> The mutex code has a lock handoff mechanism to prevent lock starvation.
> This patch implements a similar lock handoff mechanism to disable
> lock stealing and force lock handoff to the first waiter in the queue
> after at least a 5ms waiting period. The waiting period is used to
> avoid discouraging lock stealing too much to affect performance.

So the mutex code doesn't have that timeout, it foces the handoff if the
top waiter fails to acquire.

I don't find the above sufficiently justifies the additional complexity.
What were the numbers with the simple scheme vs this etc..