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

From: Peter Zijlstra
Date: Tue Apr 16 2019 - 10:12:15 EST

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'.