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

[Peter Zijlstra]

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.