linux-kernel - Re: [PATCH v4 07/16] locking/rwsem: Implement lock handoff to prevent lock starvation

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Date:   Tue, 16 Apr 2019 16:12:05 +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>,
        huang ying <huang.ying.caritas@...il.com>
Subject: Re: [PATCH v4 07/16] locking/rwsem: Implement lock handoff to
 prevent lock starvation

On Sat, Apr 13, 2019 at 01:22:50PM -0400, Waiman Long wrote:
> +/*
> + * The typical HZ value is either 250 or 1000. So set the minimum waiting
> + * time to 4ms in the wait queue before initiating the handoff protocol.
> + */
> +#define RWSEM_WAIT_TIMEOUT	(HZ/250)

That seems equally unfortunate. For HZ=100 that results in 0ms, and for
HZ=300 that results in 3 1/3-rd ms.

(and this is not considering Alpha,ARM and MIPS, who all have various
other 'creative' HZ values)

In general aiming for sub 10ms timing using jiffies seems 'optimistic'.