[PATCH v2 2/2] KVM: selftests: Print summary stats of memory latency distribution

[Colton Lewis <coltonlewis@google.com>]

Print summary stats of the memory latency distribution in nanoseconds
for dirty_log_perf_test. For every iteration, this prints the minimum,
the maximum, and the 50th, 90th, and 99th percentiles.

Stats are calculated by sorting the samples taken from all vcpus and
picking from the index corresponding with each percentile.

Because keeping all samples is impractical due to the space required
in some cases (pooled memory w/ 64 vcpus would be 64 GB/vcpu * 64
vcpus * 250,000 samples/GB * 8 bytes/sample ~ 8 GB extra memory just
for samples), reservoir sampling [1] is used to only keep a small
number of samples per vcpu (settable via command argument). There is a
tradeoff between stat accuracy and memory usage. More samples means
more accuracy but also more memory use.

Because other selftests use memstress.c, those tests were hardcoded to
take 0 samples at this time.

[1] https://en.wikipedia.org/wiki/Reservoir_sampling#Simple:_Algorithm_R

Signed-off-by: Colton Lewis <coltonlewis@google.com>
---
 .../selftests/kvm/access_tracking_perf_test.c |  3 +-
 .../selftests/kvm/demand_paging_test.c        |  2 +-
 .../selftests/kvm/dirty_log_perf_test.c       | 10 ++-
 .../testing/selftests/kvm/include/memstress.h | 10 ++-
 tools/testing/selftests/kvm/lib/memstress.c   | 68 ++++++++++++++++---
 .../kvm/memslot_modification_stress_test.c    |  2 +-
 6 files changed, 81 insertions(+), 14 deletions(-)

diff --git a/tools/testing/selftests/kvm/access_tracking_perf_test.c b/tools/testing/selftests/kvm/access_tracking_perf_test.c
index 3c7defd34f56..89947aa4ff3a 100644
--- a/tools/testing/selftests/kvm/access_tracking_perf_test.c
+++ b/tools/testing/selftests/kvm/access_tracking_perf_test.c
@@ -306,7 +306,8 @@ static void run_test(enum vm_guest_mode mode, void *arg)
 	struct kvm_vm *vm;
 	int nr_vcpus = params->nr_vcpus;

-	vm = memstress_create_vm(mode, nr_vcpus, params->vcpu_memory_bytes, 1,
+	vm = memstress_create_vm(mode, nr_vcpus, params->vcpu_memory_bytes,
+				 0, 1,
 				 params->backing_src, !overlap_memory_access);

 	memstress_start_vcpu_threads(nr_vcpus, vcpu_thread_main);
diff --git a/tools/testing/selftests/kvm/demand_paging_test.c b/tools/testing/selftests/kvm/demand_paging_test.c
index b0e1fc4de9e2..bc5c171eca9e 100644
--- a/tools/testing/selftests/kvm/demand_paging_test.c
+++ b/tools/testing/selftests/kvm/demand_paging_test.c
@@ -135,7 +135,7 @@ static void run_test(enum vm_guest_mode mode, void *arg)
 	struct kvm_vm *vm;
 	int i;

-	vm = memstress_create_vm(mode, nr_vcpus, guest_percpu_mem_size, 1,
+	vm = memstress_create_vm(mode, nr_vcpus, guest_percpu_mem_size, 0, 1,
 				 p->src_type, p->partition_vcpu_memory_access);

 	demand_paging_size = get_backing_src_pagesz(p->src_type);
diff --git a/tools/testing/selftests/kvm/dirty_log_perf_test.c b/tools/testing/selftests/kvm/dirty_log_perf_test.c
index e9d6d1aecf89..40058fe2b58f 100644
--- a/tools/testing/selftests/kvm/dirty_log_perf_test.c
+++ b/tools/testing/selftests/kvm/dirty_log_perf_test.c
@@ -133,6 +133,7 @@ struct test_params {
 	int slots;
 	uint32_t write_percent;
 	uint32_t random_seed;
+	uint64_t samples_per_vcpu;
 	bool random_access;
 };

@@ -224,6 +225,7 @@ static void run_test(enum vm_guest_mode mode, void *arg)
 	int i;

 	vm = memstress_create_vm(mode, nr_vcpus, guest_percpu_mem_size,
+				 p->samples_per_vcpu,
 				 p->slots, p->backing_src,
 				 p->partition_vcpu_memory_access);

@@ -274,6 +276,7 @@ static void run_test(enum vm_guest_mode mode, void *arg)
 	ts_diff = timespec_elapsed(start);
 	pr_info("Populate memory time: %ld.%.9lds\n",
 		ts_diff.tv_sec, ts_diff.tv_nsec);
+	memstress_print_percentiles(vm, nr_vcpus);

 	/* Enable dirty logging */
 	clock_gettime(CLOCK_MONOTONIC, &start);
@@ -304,6 +307,7 @@ static void run_test(enum vm_guest_mode mode, void *arg)
 		vcpu_dirty_total = timespec_add(vcpu_dirty_total, ts_diff);
 		pr_info("Iteration %d dirty memory time: %ld.%.9lds\n",
 			iteration, ts_diff.tv_sec, ts_diff.tv_nsec);
+		memstress_print_percentiles(vm, nr_vcpus);

 		clock_gettime(CLOCK_MONOTONIC, &start);
 		get_dirty_log(vm, bitmaps, p->slots);
@@ -428,6 +432,7 @@ int main(int argc, char *argv[])
 		.slots = 1,
 		.random_seed = 1,
 		.write_percent = 100,
+		.samples_per_vcpu = 1000,
 	};
 	int opt;

@@ -438,7 +443,7 @@ int main(int argc, char *argv[])

 	guest_modes_append_default();

-	while ((opt = getopt(argc, argv, "ab:c:eghi:m:nop:r:s:v:x:w:")) != -1) {
+	while ((opt = getopt(argc, argv, "ab:c:eghi:m:nop:r:s:S:v:x:w:")) != -1) {
 		switch (opt) {
 		case 'a':
 			p.random_access = true;
@@ -480,6 +485,9 @@ int main(int argc, char *argv[])
 		case 's':
 			p.backing_src = parse_backing_src_type(optarg);
 			break;
+		case 'S':
+			p.samples_per_vcpu = atoi_positive("Number of samples/vcpu", optarg);
+			break;
 		case 'v':
 			nr_vcpus = atoi_positive("Number of vCPUs", optarg);
 			TEST_ASSERT(nr_vcpus <= max_vcpus,
diff --git a/tools/testing/selftests/kvm/include/memstress.h b/tools/testing/selftests/kvm/include/memstress.h
index 72e3e358ef7b..25a3b5e308e9 100644
--- a/tools/testing/selftests/kvm/include/memstress.h
+++ b/tools/testing/selftests/kvm/include/memstress.h
@@ -37,6 +37,12 @@ struct memstress_args {
 	uint64_t guest_page_size;
 	uint32_t random_seed;
 	uint32_t write_percent;
+	uint64_t samples_per_vcpu;
+	/* Store all samples in a flat array so they can be easily
+	 * sorted later.
+	 */
+	vm_vaddr_t latency_samples;
+

 	/* Run vCPUs in L2 instead of L1, if the architecture supports it. */
 	bool nested;
@@ -56,7 +62,8 @@ struct memstress_args {
 extern struct memstress_args memstress_args;

 struct kvm_vm *memstress_create_vm(enum vm_guest_mode mode, int nr_vcpus,
-				   uint64_t vcpu_memory_bytes, int slots,
+				   uint64_t vcpu_memory_bytes,
+				   uint64_t samples_per_vcpu, int slots,
 				   enum vm_mem_backing_src_type backing_src,
 				   bool partition_vcpu_memory_access);
 void memstress_destroy_vm(struct kvm_vm *vm);
@@ -72,4 +79,5 @@ void memstress_guest_code(uint32_t vcpu_id);
 uint64_t memstress_nested_pages(int nr_vcpus);
 void memstress_setup_nested(struct kvm_vm *vm, int nr_vcpus, struct kvm_vcpu *vcpus[]);

+void memstress_print_percentiles(struct kvm_vm *vm, int nr_vcpus);
 #endif /* SELFTEST_KVM_MEMSTRESS_H */
diff --git a/tools/testing/selftests/kvm/lib/memstress.c b/tools/testing/selftests/kvm/lib/memstress.c
index 5f1d3173c238..c275471ad6d8 100644
--- a/tools/testing/selftests/kvm/lib/memstress.c
+++ b/tools/testing/selftests/kvm/lib/memstress.c
@@ -48,14 +48,17 @@ void memstress_guest_code(uint32_t vcpu_idx)
 {
 	struct memstress_args *args = &memstress_args;
 	struct memstress_vcpu_args *vcpu_args = &args->vcpu_args[vcpu_idx];
-	struct guest_random_state rand_state;
 	uint64_t gva;
 	uint64_t pages;
 	uint64_t addr;
 	uint64_t page;
 	int i;
-
-	rand_state = new_guest_random_state(args->random_seed + vcpu_idx);
+	struct guest_random_state rand_state =
+		new_guest_random_state(args->random_seed + vcpu_idx);
+	uint64_t *vcpu_samples = (uint64_t *)args->latency_samples
+		+ args->samples_per_vcpu * vcpu_idx;
+	uint64_t cycles;
+	uint32_t maybe_sample;

 	gva = vcpu_args->gva;
 	pages = vcpu_args->pages;
@@ -73,15 +76,46 @@ void memstress_guest_code(uint32_t vcpu_idx)
 			addr = gva + (page * args->guest_page_size);

 			if (guest_random_u32(&rand_state) % 100 < args->write_percent)
-				*(uint64_t *)addr = 0x0123456789ABCDEF;
+				cycles = MEASURE_CYCLES(*(uint64_t *)addr = 0x0123456789ABCDEF);
 			else
-				READ_ONCE(*(uint64_t *)addr);
+				cycles = MEASURE_CYCLES(READ_ONCE(*(uint64_t *)addr));
+
+			if (i < args->samples_per_vcpu) {
+				vcpu_samples[i] = cycles;
+				continue;
+			}
+
+			maybe_sample = guest_random_u32(&rand_state) % (i + 1);
+
+			if (maybe_sample < args->samples_per_vcpu)
+				vcpu_samples[maybe_sample] = cycles;
 		}

 		GUEST_SYNC(1);
 	}
 }

+/* compare function for qsort */
+static int memstress_qcmp(const void *a, const void *b)
+{
+	return *(int *)a - *(int *)b;
+}
+
+void memstress_print_percentiles(struct kvm_vm *vm, int nr_vcpus)
+{
+	uint64_t n_samples = nr_vcpus * memstress_args.samples_per_vcpu;
+	uint64_t *host_latency_samples = addr_gva2hva(vm, memstress_args.latency_samples);
+
+	qsort(host_latency_samples, n_samples, sizeof(uint64_t), &memstress_qcmp);
+
+	pr_info("Latency distribution (ns) = min:%6.0lf, 50th:%6.0lf, 90th:%6.0lf, 99th:%6.0lf, max:%6.0lf\n",
+		cycles_to_ns(vcpus[0], (double)host_latency_samples[0]),
+		cycles_to_ns(vcpus[0], (double)host_latency_samples[n_samples / 2]),
+		cycles_to_ns(vcpus[0], (double)host_latency_samples[n_samples * 9 / 10]),
+		cycles_to_ns(vcpus[0], (double)host_latency_samples[n_samples * 99 / 100]),
+		cycles_to_ns(vcpus[0], (double)host_latency_samples[n_samples - 1]));
+}
+
 void memstress_setup_vcpus(struct kvm_vm *vm, int nr_vcpus,
 			   struct kvm_vcpu *vcpus[],
 			   uint64_t vcpu_memory_bytes,
@@ -119,21 +153,26 @@ void memstress_setup_vcpus(struct kvm_vm *vm, int nr_vcpus,
 }

 struct kvm_vm *memstress_create_vm(enum vm_guest_mode mode, int nr_vcpus,
-				   uint64_t vcpu_memory_bytes, int slots,
+				   uint64_t vcpu_memory_bytes,
+				   uint64_t samples_per_vcpu, int slots,
 				   enum vm_mem_backing_src_type backing_src,
 				   bool partition_vcpu_memory_access)
 {
 	struct memstress_args *args = &memstress_args;
 	struct kvm_vm *vm;
-	uint64_t guest_num_pages, slot0_pages = 0;
+	uint64_t guest_num_pages, sample_pages, slot0_pages = 0;
 	uint64_t backing_src_pagesz = get_backing_src_pagesz(backing_src);
 	uint64_t region_end_gfn;
+	uint64_t sample_size;
+	uint64_t sample_slot;
+	uint64_t sample_start;
 	int i;

 	pr_info("Testing guest mode: %s\n", vm_guest_mode_string(mode));

 	/* By default vCPUs will write to memory. */
 	args->write_percent = 100;
+	args->samples_per_vcpu = samples_per_vcpu;

 	/*
 	 * Snapshot the non-huge page size.  This is used by the guest code to
@@ -159,12 +198,17 @@ struct kvm_vm *memstress_create_vm(enum vm_guest_mode mode, int nr_vcpus,
 	if (args->nested)
 		slot0_pages += memstress_nested_pages(nr_vcpus);

+	sample_size = samples_per_vcpu * nr_vcpus * sizeof(uint64_t);
+	sample_pages = vm_adjust_num_guest_pages(
+		mode, 1 + sample_size / args->guest_page_size);
+
 	/*
 	 * Pass guest_num_pages to populate the page tables for test memory.
 	 * The memory is also added to memslot 0, but that's a benign side
 	 * effect as KVM allows aliasing HVAs in meslots.
 	 */
-	vm = __vm_create_with_vcpus(mode, nr_vcpus, slot0_pages + guest_num_pages,
+	vm = __vm_create_with_vcpus(mode, nr_vcpus,
+				    slot0_pages + guest_num_pages + sample_pages,
 				    memstress_guest_code, vcpus);

 	args->vm = vm;
@@ -190,7 +234,7 @@ struct kvm_vm *memstress_create_vm(enum vm_guest_mode mode, int nr_vcpus,
 		    " nr_vcpus: %d wss: %" PRIx64 "]\n",
 		    guest_num_pages, region_end_gfn - 1, nr_vcpus, vcpu_memory_bytes);

-	args->gpa = (region_end_gfn - guest_num_pages - 1) * args->guest_page_size;
+	args->gpa = (region_end_gfn - guest_num_pages - sample_pages - 1) * args->guest_page_size;
 	args->gpa = align_down(args->gpa, backing_src_pagesz);
 #ifdef __s390x__
 	/* Align to 1M (segment size) */
@@ -213,6 +257,12 @@ struct kvm_vm *memstress_create_vm(enum vm_guest_mode mode, int nr_vcpus,
 	/* Do mapping for the demand paging memory slot */
 	virt_map(vm, guest_test_virt_mem, args->gpa, guest_num_pages);

+	memstress_args.latency_samples = guest_test_virt_mem + args->size;
+	sample_start = args->gpa + args->size;
+	sample_slot = MEMSTRESS_MEM_SLOT_INDEX + slots;
+	vm_userspace_mem_region_add(vm, backing_src, sample_start, sample_slot, sample_pages, 0);
+	virt_map(vm, memstress_args.latency_samples, sample_start, sample_pages);
+
 	memstress_setup_vcpus(vm, nr_vcpus, vcpus, vcpu_memory_bytes,
 			      partition_vcpu_memory_access);

diff --git a/tools/testing/selftests/kvm/memslot_modification_stress_test.c b/tools/testing/selftests/kvm/memslot_modification_stress_test.c
index 9855c41ca811..0a14f7d16e0c 100644
--- a/tools/testing/selftests/kvm/memslot_modification_stress_test.c
+++ b/tools/testing/selftests/kvm/memslot_modification_stress_test.c
@@ -95,7 +95,7 @@ static void run_test(enum vm_guest_mode mode, void *arg)
 	struct test_params *p = arg;
 	struct kvm_vm *vm;

-	vm = memstress_create_vm(mode, nr_vcpus, guest_percpu_mem_size, 1,
+	vm = memstress_create_vm(mode, nr_vcpus, guest_percpu_mem_size, 0, 1,
 				 VM_MEM_SRC_ANONYMOUS,
 				 p->partition_vcpu_memory_access);

--
2.40.0.rc1.284.g88254d51c5-goog