linux-kernel - Re: [PATCH v4 12/16] locking/rwsem: Enable time-based spinning on reader-owned rwsem

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Date:   Fri, 19 Apr 2019 10:33:14 -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 12/16] locking/rwsem: Enable time-based spinning on
 reader-owned rwsem

On 04/19/2019 03:56 AM, Peter Zijlstra wrote:
> On Thu, Apr 18, 2019 at 11:15:33AM -0400, Waiman Long wrote:
>> On 04/18/2019 09:06 AM, Peter Zijlstra wrote:
>>>> +			/*
>>>> +			 * Check time threshold every 16 iterations to
>>>> +			 * avoid calling sched_clock() too frequently.
>>>> +			 * This will make the actual spinning time a
>>>> +			 * bit more than that specified in the threshold.
>>>> +			 */
>>>> +			else if (!(++loop & 0xf) &&
>>>> +				 (sched_clock() > rspin_threshold)) {
>>> Why is calling sched_clock() lots a problem?
>> Actually I am more concern about the latency introduced by the
>> sched_clock() call. BTW, I haven't done any measurement myself. Do you
>> know how much cost the sched_clock() call is?
>>
>> If the cost is relatively high, the average latency period after the
>> lock is free and the spinner is ready to do a trylock will increase.
> Totally depends on the arch or course :/ For 'sane' x86 it is: RDTSC,
> MUL; SHRD; SHR; ADD, which is plenty fast.
>
> I know we have poll loops with sched_clock/local_clock in them, I just
> can't seem to find any atm.

Thanks, I will do some time measurement myself. If it is fast enough, I
can change the code to do sched_clock on every iteration.

Cheers,
Longman