Re: [PATCH 3/3] KVM: selftests: Randomize page access order

[Colton Lewis <coltonlewis@google.com>]

On Wed, Aug 10, 2022 at 04:49:23PM -0700, David Matlack wrote:
> On Wed, Aug 10, 2022 at 05:58:30PM +0000, Colton Lewis wrote:
> > diff --git a/tools/testing/selftests/kvm/lib/perf_test_util.c b/tools/testing/selftests/kvm/lib/perf_test_util.c
> > index 3c7b93349fef..9838d1ad9166 100644
> > --- a/tools/testing/selftests/kvm/lib/perf_test_util.c
> > +++ b/tools/testing/selftests/kvm/lib/perf_test_util.c
> > @@ -52,6 +52,9 @@ void perf_test_guest_code(uint32_t vcpu_idx)
> >  	struct perf_test_vcpu_args *vcpu_args = &pta->vcpu_args[vcpu_idx];
> >  	uint64_t gva;
> >  	uint64_t pages;
> > +	uint64_t addr;
> > +	bool random_access = pta->random_access;
> > +	bool populated = false;
> >  	int i;
> >  
> >  	gva = vcpu_args->gva;
> > @@ -62,7 +65,11 @@ void perf_test_guest_code(uint32_t vcpu_idx)
> >  
> >  	while (true) {
> >  		for (i = 0; i < pages; i++) {
> > -			uint64_t addr = gva + (i * pta->guest_page_size);
> > +			if (populated && random_access)
> 
> Skipping the populate phase makes sense to ensure everything is
> populated I guess. What was your rational?

That's it. Wanted to ensure everything was populated. Random
population won't hit every page, but those unpopulated pages might be
hit on subsequent iterations. I originally let population be random
too and suspect this was driving an odd behavior I noticed early in
testing where later iterations would be much faster than earlier ones.

> Either way I think this policy should be driven by the test, rather than
> harde-coded in perf_test_guest_code(). i.e. Move the call
> perf_test_set_random_access() in dirty_log_perf_test.c to just after the
> population phase.

That makes sense. Will do.