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Date:   Wed, 10 Apr 2019 17:07:14 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Waiman Long <longman@redhat.com>
Cc:     Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        Davidlohr Bueso <dave@stgolabs.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>
Subject: Re: [PATCH-tip v2 02/12] locking/rwsem: Implement lock handoff to
 prevent lock starvation
Message-ID: <20190410150714.GK7905@worktop.programming.kicks-ass.net>
References: <20190405192115.17416-1-longman@redhat.com>
 <20190405192115.17416-3-longman@redhat.com>
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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..