Re: [PATCH v3 2/2] arm64: add micro test

[Christoffer Dall <christoffer.dall@linaro.org>]

Hi Shih-Wei,

On Fri, Jan 19, 2018 at 04:57:55PM -0500, Shih-Wei Li wrote:
> Thanks for the feedback about the mistakes in math and some issues in
> naming, print msg, and coding style. I'll be careful and try to avoid
> the same problems the next patch set. Sorry for all of the confusion.
> 
> So we now skip the test when "sample == 0" happens over 1000 times.
> This is only due to the case that "cost is < 1/cntfrq" since it's not
> possible for the tick to overflow for that many times. Did I miss
> something here? I do agree that we should output better msgs to tell
> users that the cost of a certain test is constantly smaller than a
> tick.
> 

I think for things like vmexit counts, it's very likely that all the
samples will result in 0 ticks on many systems (fast CPU and slow arch
counter; the architecture doesn't give us any guarantees here).  For
example, a system with a 2 GHz CPU and a 1 MHz counter will give you a
granularity of 2000 cycles for each counter tick, which is not that
useful for low-level tuning of KVM.

So what I thought we were going to do was:

main_test_function()
{
	long ntimes = NTIMES;
	long cost, total_cost;

	cnt1 = read_cnt();
	do {
		run_test();
	} while(ntimes--);
	cnt2 = read_cnt();

	if (verify_sane_counter(cnt1, cnt2))
		return;

	total_cost = to_nanoseconds(cnt2 - cnt1);
	cost = total_cost / NTIMES;
	printf("testname: %l (%l)\n", cost, total_cost);
}

And in that way amortize the potential lack of precision over all the
iterations.  Did I miss some prior discussion about why that was a bad
idea?

It would also be possible to have two functions, one that does the above
and one that does a per-run measurement, in case the user wants to know
min/max/stddev and is running on a system with sufficient precision.
The method could be chosen via an argument.

Thanks,
-Christoffer