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Date:   Tue, 16 Apr 2019 16:12:05 +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>,
        huang ying <huang.ying.caritas@gmail.com>
Subject: Re: [PATCH v4 07/16] locking/rwsem: Implement lock handoff to
 prevent lock starvation
Message-ID: <20190416141205.GK12232@hirez.programming.kicks-ass.net>
References: <20190413172259.2740-1-longman@redhat.com>
 <20190413172259.2740-8-longman@redhat.com>
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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'.