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 11:36:44 -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 10:33 AM, Waiman Long wrote:
> 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
>
I had measured the time of doing 10 sched_clock() calls. On a 2.1GHz
skylake system, it was 83ns (~18 cycles per call). On a 2.5GHz Thunder
X2 arm system, it was 860ns (~215 cycles per call). On a 2.2GHz AMD EPYC
system, it was 200ns (~44 cycles per call). Intel is fastest, followed
by AMD and then the ARM64 chip.

Cheers,
Longman