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

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Message-ID: <4b36b0a5-8507-f90c-d4a7-3f821ee8da6d@redhat.com>
Date:   Tue, 16 Apr 2019 16:26:26 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 04/16/2019 10:12 AM, Peter Zijlstra wrote:
> 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'.

I see your point. I will change it to use sched_clock() instead.

Thanks,
Longman