KVM: selftests: Introduce access_tracking_perf_test

This test measures the performance effects of KVM's access tracking.
Access tracking is driven by the MMU notifiers test_young, clear_young,
and clear_flush_young. These notifiers do not have a direct userspace
API, however the clear_young notifier can be triggered by marking a
pages as idle in /sys/kernel/mm/page_idle/bitmap. This test leverages
that mechanism to enable access tracking on guest memory.

To measure performance this test runs a VM with a configurable number of
vCPUs that each touch every page in disjoint regions of memory.
Performance is measured in the time it takes all vCPUs to finish
touching their predefined region.

Example invocation:

  $ ./access_tracking_perf_test -v 8
  Testing guest mode: PA-bits:ANY, VA-bits:48,  4K pages
  guest physical test memory offset: 0xffdfffff000

  Populating memory             : 1.337752570s
  Writing to populated memory   : 0.010177640s
  Reading from populated memory : 0.009548239s
  Mark memory idle              : 23.973131748s
  Writing to idle memory        : 0.063584496s
  Mark memory idle              : 24.924652964s
  Reading from idle memory      : 0.062042814s

Breaking down the results:

 * "Populating memory": The time it takes for all vCPUs to perform the
   first write to every page in their region.

 * "Writing to populated memory" / "Reading from populated memory": The
   time it takes for all vCPUs to write and read to every page in their
   region after it has been populated. This serves as a control for the
   later results.

 * "Mark memory idle": The time it takes for every vCPU to mark every
   page in their region as idle through page_idle.

 * "Writing to idle memory" / "Reading from idle memory": The time it
   takes for all vCPUs to write and read to every page in their region
   after it has been marked idle.

This test should be portable across architectures but it is only enabled
for x86_64 since that's all I have tested.

Reviewed-by: Ben Gardon <bgardon@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20210713220957.3493520-7-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

3 files changed, 431 insertions, 0 deletions

diff --git a/tools/testing/selftests/kvm/.gitignore b/tools/testing/selftests/kvm/.gitignore
index 06a351b4f93b..0709af0144c8 100644
--- a/tools/testing/selftests/kvm/.gitignore
+++ b/tools/testing/selftests/kvm/.gitignore
@@ -38,6 +38,7 @@
 /x86_64/xen_vmcall_test
 /x86_64/xss_msr_test
 /x86_64/vmx_pmu_msrs_test
+/access_tracking_perf_test
 /demand_paging_test
 /dirty_log_test
 /dirty_log_perf_test
diff --git a/tools/testing/selftests/kvm/Makefile b/tools/testing/selftests/kvm/Makefile
index b853be2ae3c6..5832f510a16c 100644
--- a/tools/testing/selftests/kvm/Makefile
+++ b/tools/testing/selftests/kvm/Makefile
@@ -71,6 +71,7 @@ TEST_GEN_PROGS_x86_64 += x86_64/tsc_msrs_test
 TEST_GEN_PROGS_x86_64 += x86_64/vmx_pmu_msrs_test
 TEST_GEN_PROGS_x86_64 += x86_64/xen_shinfo_test
 TEST_GEN_PROGS_x86_64 += x86_64/xen_vmcall_test
+TEST_GEN_PROGS_x86_64 += access_tracking_perf_test
 TEST_GEN_PROGS_x86_64 += demand_paging_test
 TEST_GEN_PROGS_x86_64 += dirty_log_test
 TEST_GEN_PROGS_x86_64 += dirty_log_perf_test
diff --git a/tools/testing/selftests/kvm/access_tracking_perf_test.c b/tools/testing/selftests/kvm/access_tracking_perf_test.c
new file mode 100644
index 000000000000..e2baa187a21e
--- /dev/null
+++ b/tools/testing/selftests/kvm/access_tracking_perf_test.c
@@ -0,0 +1,429 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * access_tracking_perf_test
+ *
+ * Copyright (C) 2021, Google, Inc.
+ *
+ * This test measures the performance effects of KVM's access tracking.
+ * Access tracking is driven by the MMU notifiers test_young, clear_young, and
+ * clear_flush_young. These notifiers do not have a direct userspace API,
+ * however the clear_young notifier can be triggered by marking a pages as idle
+ * in /sys/kernel/mm/page_idle/bitmap. This test leverages that mechanism to
+ * enable access tracking on guest memory.
+ *
+ * To measure performance this test runs a VM with a configurable number of
+ * vCPUs that each touch every page in disjoint regions of memory. Performance
+ * is measured in the time it takes all vCPUs to finish touching their
+ * predefined region.
+ *
+ * Note that a deterministic correctness test of access tracking is not possible
+ * by using page_idle as it exists today. This is for a few reasons:
+ *
+ * 1. page_idle only issues clear_young notifiers, which lack a TLB flush. This
+ *    means subsequent guest accesses are not guaranteed to see page table
+ *    updates made by KVM until some time in the future.
+ *
+ * 2. page_idle only operates on LRU pages. Newly allocated pages are not
+ *    immediately allocated to LRU lists. Instead they are held in a "pagevec",
+ *    which is drained to LRU lists some time in the future. There is no
+ *    userspace API to force this drain to occur.
+ *
+ * These limitations are worked around in this test by using a large enough
+ * region of memory for each vCPU such that the number of translations cached in
+ * the TLB and the number of pages held in pagevecs are a small fraction of the
+ * overall workload. And if either of those conditions are not true this test
+ * will fail rather than silently passing.
+ */
+#include <inttypes.h>
+#include <limits.h>
+#include <pthread.h>
+#include <sys/mman.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+
+#include "kvm_util.h"
+#include "test_util.h"
+#include "perf_test_util.h"
+#include "guest_modes.h"
+
+/* Global variable used to synchronize all of the vCPU threads. */
+static int iteration = -1;
+
+/* Defines what vCPU threads should do during a given iteration. */
+static enum {
+	/* Run the vCPU to access all its memory. */
+	ITERATION_ACCESS_MEMORY,
+	/* Mark the vCPU's memory idle in page_idle. */
+	ITERATION_MARK_IDLE,
+} iteration_work;
+
+/* Set to true when vCPU threads should exit. */
+static bool done;
+
+/* The iteration that was last completed by each vCPU. */
+static int vcpu_last_completed_iteration[KVM_MAX_VCPUS];
+
+/* Whether to overlap the regions of memory vCPUs access. */
+static bool overlap_memory_access;
+
+struct test_params {
+	/* The backing source for the region of memory. */
+	enum vm_mem_backing_src_type backing_src;
+
+	/* The amount of memory to allocate for each vCPU. */
+	uint64_t vcpu_memory_bytes;
+
+	/* The number of vCPUs to create in the VM. */
+	int vcpus;
+};
+
+static uint64_t pread_uint64(int fd, const char *filename, uint64_t index)
+{
+	uint64_t value;
+	off_t offset = index * sizeof(value);
+
+	TEST_ASSERT(pread(fd, &value, sizeof(value), offset) == sizeof(value),
+		    "pread from %s offset 0x%" PRIx64 " failed!",
+		    filename, offset);
+
+	return value;
+
+}
+
+#define PAGEMAP_PRESENT (1ULL << 63)
+#define PAGEMAP_PFN_MASK ((1ULL << 55) - 1)
+
+static uint64_t lookup_pfn(int pagemap_fd, struct kvm_vm *vm, uint64_t gva)
+{
+	uint64_t hva = (uint64_t) addr_gva2hva(vm, gva);
+	uint64_t entry;
+	uint64_t pfn;
+
+	entry = pread_uint64(pagemap_fd, "pagemap", hva / getpagesize());
+	if (!(entry & PAGEMAP_PRESENT))
+		return 0;
+
+	pfn = entry & PAGEMAP_PFN_MASK;
+	if (!pfn) {
+		print_skip("Looking up PFNs requires CAP_SYS_ADMIN");
+		exit(KSFT_SKIP);
+	}
+
+	return pfn;
+}
+
+static bool is_page_idle(int page_idle_fd, uint64_t pfn)
+{
+	uint64_t bits = pread_uint64(page_idle_fd, "page_idle", pfn / 64);
+
+	return !!((bits >> (pfn % 64)) & 1);
+}
+
+static void mark_page_idle(int page_idle_fd, uint64_t pfn)
+{
+	uint64_t bits = 1ULL << (pfn % 64);
+
+	TEST_ASSERT(pwrite(page_idle_fd, &bits, 8, 8 * (pfn / 64)) == 8,
+		    "Set page_idle bits for PFN 0x%" PRIx64, pfn);
+}
+
+static void mark_vcpu_memory_idle(struct kvm_vm *vm, int vcpu_id)
+{
+	uint64_t base_gva = perf_test_args.vcpu_args[vcpu_id].gva;
+	uint64_t pages = perf_test_args.vcpu_args[vcpu_id].pages;
+	uint64_t page;
+	uint64_t still_idle = 0;
+	uint64_t no_pfn = 0;
+	int page_idle_fd;
+	int pagemap_fd;
+
+	/* If vCPUs are using an overlapping region, let vCPU 0 mark it idle. */
+	if (overlap_memory_access && vcpu_id)
+		return;
+
+	page_idle_fd = open("/sys/kernel/mm/page_idle/bitmap", O_RDWR);
+	TEST_ASSERT(page_idle_fd > 0, "Failed to open page_idle.");
+
+	pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
+	TEST_ASSERT(pagemap_fd > 0, "Failed to open pagemap.");
+
+	for (page = 0; page < pages; page++) {
+		uint64_t gva = base_gva + page * perf_test_args.guest_page_size;
+		uint64_t pfn = lookup_pfn(pagemap_fd, vm, gva);
+
+		if (!pfn) {
+			no_pfn++;
+			continue;
+		}
+
+		if (is_page_idle(page_idle_fd, pfn)) {
+			still_idle++;
+			continue;
+		}
+
+		mark_page_idle(page_idle_fd, pfn);
+	}
+
+	/*
+	 * Assumption: Less than 1% of pages are going to be swapped out from
+	 * under us during this test.
+	 */
+	TEST_ASSERT(no_pfn < pages / 100,
+		    "vCPU %d: No PFN for %" PRIu64 " out of %" PRIu64 " pages.",
+		    vcpu_id, no_pfn, pages);
+
+	/*
+	 * Test that at least 90% of memory has been marked idle (the rest might
+	 * not be marked idle because the pages have not yet made it to an LRU
+	 * list or the translations are still cached in the TLB). 90% is
+	 * arbitrary; high enough that we ensure most memory access went through
+	 * access tracking but low enough as to not make the test too brittle
+	 * over time and across architectures.
+	 */
+	TEST_ASSERT(still_idle < pages / 10,
+		    "vCPU%d: Too many pages still idle (%"PRIu64 " out of %"
+		    PRIu64 ").\n",
+		    vcpu_id, still_idle, pages);
+
+	close(page_idle_fd);
+	close(pagemap_fd);
+}
+
+static void assert_ucall(struct kvm_vm *vm, uint32_t vcpu_id,
+			 uint64_t expected_ucall)
+{
+	struct ucall uc;
+	uint64_t actual_ucall = get_ucall(vm, vcpu_id, &uc);
+
+	TEST_ASSERT(expected_ucall == actual_ucall,
+		    "Guest exited unexpectedly (expected ucall %" PRIu64
+		    ", got %" PRIu64 ")",
+		    expected_ucall, actual_ucall);
+}
+
+static bool spin_wait_for_next_iteration(int *current_iteration)
+{
+	int last_iteration = *current_iteration;
+
+	do {
+		if (READ_ONCE(done))
+			return false;
+
+		*current_iteration = READ_ONCE(iteration);
+	} while (last_iteration == *current_iteration);
+
+	return true;
+}
+
+static void *vcpu_thread_main(void *arg)
+{
+	struct perf_test_vcpu_args *vcpu_args = arg;
+	struct kvm_vm *vm = perf_test_args.vm;
+	int vcpu_id = vcpu_args->vcpu_id;
+	int current_iteration = -1;
+
+	vcpu_args_set(vm, vcpu_id, 1, vcpu_id);
+
+	while (spin_wait_for_next_iteration(&current_iteration)) {
+		switch (READ_ONCE(iteration_work)) {
+		case ITERATION_ACCESS_MEMORY:
+			vcpu_run(vm, vcpu_id);
+			assert_ucall(vm, vcpu_id, UCALL_SYNC);
+			break;
+		case ITERATION_MARK_IDLE:
+			mark_vcpu_memory_idle(vm, vcpu_id);
+			break;
+		};
+
+		vcpu_last_completed_iteration[vcpu_id] = current_iteration;
+	}
+
+	return NULL;
+}
+
+static void spin_wait_for_vcpu(int vcpu_id, int target_iteration)
+{
+	while (READ_ONCE(vcpu_last_completed_iteration[vcpu_id]) !=
+	       target_iteration) {
+		continue;
+	}
+}
+
+/* The type of memory accesses to perform in the VM. */
+enum access_type {
+	ACCESS_READ,
+	ACCESS_WRITE,
+};
+
+static void run_iteration(struct kvm_vm *vm, int vcpus, const char *description)
+{
+	struct timespec ts_start;
+	struct timespec ts_elapsed;
+	int next_iteration;
+	int vcpu_id;
+
+	/* Kick off the vCPUs by incrementing iteration. */
+	next_iteration = ++iteration;
+
+	clock_gettime(CLOCK_MONOTONIC, &ts_start);
+
+	/* Wait for all vCPUs to finish the iteration. */
+	for (vcpu_id = 0; vcpu_id < vcpus; vcpu_id++)
+		spin_wait_for_vcpu(vcpu_id, next_iteration);
+
+	ts_elapsed = timespec_elapsed(ts_start);
+	pr_info("%-30s: %ld.%09lds\n",
+		description, ts_elapsed.tv_sec, ts_elapsed.tv_nsec);
+}
+
+static void access_memory(struct kvm_vm *vm, int vcpus, enum access_type access,
+			  const char *description)
+{
+	perf_test_args.wr_fract = (access == ACCESS_READ) ? INT_MAX : 1;
+	sync_global_to_guest(vm, perf_test_args);
+	iteration_work = ITERATION_ACCESS_MEMORY;
+	run_iteration(vm, vcpus, description);
+}
+
+static void mark_memory_idle(struct kvm_vm *vm, int vcpus)
+{
+	/*
+	 * Even though this parallelizes the work across vCPUs, this is still a
+	 * very slow operation because page_idle forces the test to mark one pfn
+	 * at a time and the clear_young notifier serializes on the KVM MMU
+	 * lock.
+	 */
+	pr_debug("Marking VM memory idle (slow)...\n");
+	iteration_work = ITERATION_MARK_IDLE;
+	run_iteration(vm, vcpus, "Mark memory idle");
+}
+
+static pthread_t *create_vcpu_threads(int vcpus)
+{
+	pthread_t *vcpu_threads;
+	int i;
+
+	vcpu_threads = malloc(vcpus * sizeof(vcpu_threads[0]));
+	TEST_ASSERT(vcpu_threads, "Failed to allocate vcpu_threads.");
+
+	for (i = 0; i < vcpus; i++) {
+		vcpu_last_completed_iteration[i] = iteration;
+		pthread_create(&vcpu_threads[i], NULL, vcpu_thread_main,
+			       &perf_test_args.vcpu_args[i]);
+	}
+
+	return vcpu_threads;
+}
+
+static void terminate_vcpu_threads(pthread_t *vcpu_threads, int vcpus)
+{
+	int i;
+
+	/* Set done to signal the vCPU threads to exit */
+	done = true;
+
+	for (i = 0; i < vcpus; i++)
+		pthread_join(vcpu_threads[i], NULL);
+}
+
+static void run_test(enum vm_guest_mode mode, void *arg)
+{
+	struct test_params *params = arg;
+	struct kvm_vm *vm;
+	pthread_t *vcpu_threads;
+	int vcpus = params->vcpus;
+
+	vm = perf_test_create_vm(mode, vcpus, params->vcpu_memory_bytes,
+				 params->backing_src);
+
+	perf_test_setup_vcpus(vm, vcpus, params->vcpu_memory_bytes,
+			      !overlap_memory_access);
+
+	vcpu_threads = create_vcpu_threads(vcpus);
+
+	pr_info("\n");
+	access_memory(vm, vcpus, ACCESS_WRITE, "Populating memory");
+
+	/* As a control, read and write to the populated memory first. */
+	access_memory(vm, vcpus, ACCESS_WRITE, "Writing to populated memory");
+	access_memory(vm, vcpus, ACCESS_READ, "Reading from populated memory");
+
+	/* Repeat on memory that has been marked as idle. */
+	mark_memory_idle(vm, vcpus);
+	access_memory(vm, vcpus, ACCESS_WRITE, "Writing to idle memory");
+	mark_memory_idle(vm, vcpus);
+	access_memory(vm, vcpus, ACCESS_READ, "Reading from idle memory");
+
+	terminate_vcpu_threads(vcpu_threads, vcpus);
+	free(vcpu_threads);
+	perf_test_destroy_vm(vm);
+}
+
+static void help(char *name)
+{
+	puts("");
+	printf("usage: %s [-h] [-m mode] [-b vcpu_bytes] [-v vcpus] [-o]  [-s mem_type]\n",
+	       name);
+	puts("");
+	printf(" -h: Display this help message.");
+	guest_modes_help();
+	printf(" -b: specify the size of the memory region which should be\n"
+	       "     dirtied by each vCPU. e.g. 10M or 3G.\n"
+	       "     (default: 1G)\n");
+	printf(" -v: specify the number of vCPUs to run.\n");
+	printf(" -o: Overlap guest memory accesses instead of partitioning\n"
+	       "     them into a separate region of memory for each vCPU.\n");
+	printf(" -s: specify the type of memory that should be used to\n"
+	       "     back the guest data region.\n\n");
+	backing_src_help();
+	puts("");
+	exit(0);
+}
+
+int main(int argc, char *argv[])
+{
+	struct test_params params = {
+		.backing_src = VM_MEM_SRC_ANONYMOUS,
+		.vcpu_memory_bytes = DEFAULT_PER_VCPU_MEM_SIZE,
+		.vcpus = 1,
+	};
+	int page_idle_fd;
+	int opt;
+
+	guest_modes_append_default();
+
+	while ((opt = getopt(argc, argv, "hm:b:v:os:")) != -1) {
+		switch (opt) {
+		case 'm':
+			guest_modes_cmdline(optarg);
+			break;
+		case 'b':
+			params.vcpu_memory_bytes = parse_size(optarg);
+			break;
+		case 'v':
+			params.vcpus = atoi(optarg);
+			break;
+		case 'o':
+			overlap_memory_access = true;
+			break;
+		case 's':
+			params.backing_src = parse_backing_src_type(optarg);
+			break;
+		case 'h':
+		default:
+			help(argv[0]);
+			break;
+		}
+	}
+
+	page_idle_fd = open("/sys/kernel/mm/page_idle/bitmap", O_RDWR);
+	if (page_idle_fd < 0) {
+		print_skip("CONFIG_IDLE_PAGE_TRACKING is not enabled");
+		exit(KSFT_SKIP);
+	}
+	close(page_idle_fd);
+
+	for_each_guest_mode(run_test, &params);
+
+	return 0;
+}