KVM: selftests: access_tracking_perf_test: Use MGLRU for access tracking

Use MGLRU's debugfs interface to do access tracking instead of
page_idle. The logic to use the page_idle bitmap is left in, as it is
useful for kernels that do not have MGLRU built in.

When MGLRU is enabled, page_idle will report pages as still idle even
after being accessed, as MGLRU doesn't necessarily clear the Idle folio
flag when accessing an idle page, so the test will not attempt to use
page_idle if MGLRU is enabled but otherwise not usable.

Aging pages with MGLRU is much faster than marking pages as idle with
page_idle.

Co-developed-by: Axel Rasmussen <axelrasmussen@google.com>
Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
Signed-off-by: James Houghton <jthoughton@google.com>
Link: https://lore.kernel.org/r/20250508184649.2576210-8-jthoughton@google.com
[sean: print parsed features, not raw string]
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 199 insertions, 26 deletions

diff --git a/tools/testing/selftests/kvm/access_tracking_perf_test.c b/tools/testing/selftests/kvm/access_tracking_perf_test.c
index f273e4a33b11..da7196fd1b23 100644
--- a/tools/testing/selftests/kvm/access_tracking_perf_test.c
+++ b/tools/testing/selftests/kvm/access_tracking_perf_test.c
@@ -7,9 +7,11 @@
  * 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.
+ * however the clear_young notifier can be triggered either by
+ *   1. marking a pages as idle in /sys/kernel/mm/page_idle/bitmap OR
+ *   2. adding a new MGLRU generation using the lru_gen debugfs file.
+ * This test leverages page_idle to enable access tracking on guest memory
+ * unless MGLRU is enabled, in which case MGLRU is used.
  *
  * 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
@@ -17,10 +19,11 @@
  * 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:
+ * by using page_idle or MGLRU aging 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
+ * 1. page_idle and MGLRU only issue 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
@@ -48,9 +51,17 @@
 #include "guest_modes.h"
 #include "processor.h"
 
+#include "cgroup_util.h"
+#include "lru_gen_util.h"
+
+static const char *TEST_MEMCG_NAME = "access_tracking_perf_test";
+
 /* Global variable used to synchronize all of the vCPU threads. */
 static int iteration;
 
+/* The cgroup memory controller root. Needed for lru_gen-based aging. */
+char cgroup_root[PATH_MAX];
+
 /* Defines what vCPU threads should do during a given iteration. */
 static enum {
 	/* Run the vCPU to access all its memory. */
@@ -75,6 +86,15 @@ static bool overlap_memory_access;
  */
 static int idle_pages_warn_only = -1;
 
+/* Whether or not to use MGLRU instead of page_idle for access tracking */
+static bool use_lru_gen;
+
+/* Total number of pages to expect in the memcg after touching everything */
+static long test_pages;
+
+/* Last generation we found the pages in */
+static int lru_gen_last_gen = -1;
+
 struct test_params {
 	/* The backing source for the region of memory. */
 	enum vm_mem_backing_src_type backing_src;
@@ -133,8 +153,24 @@ static void mark_page_idle(int page_idle_fd, uint64_t pfn)
 		    "Set page_idle bits for PFN 0x%" PRIx64, pfn);
 }
 
-static void mark_vcpu_memory_idle(struct kvm_vm *vm,
-				  struct memstress_vcpu_args *vcpu_args)
+static void too_many_idle_pages(long idle_pages, long total_pages, int vcpu_idx)
+{
+	char prefix[18] = {};
+
+	if (vcpu_idx >= 0)
+		snprintf(prefix, 18, "vCPU%d: ", vcpu_idx);
+
+	TEST_ASSERT(idle_pages_warn_only,
+		    "%sToo many pages still idle (%lu out of %lu)",
+		    prefix, idle_pages, total_pages);
+
+	printf("WARNING: %sToo many pages still idle (%lu out of %lu), "
+	       "this will affect performance results.\n",
+	       prefix, idle_pages, total_pages);
+}
+
+static void pageidle_mark_vcpu_memory_idle(struct kvm_vm *vm,
+					   struct memstress_vcpu_args *vcpu_args)
 {
 	int vcpu_idx = vcpu_args->vcpu_idx;
 	uint64_t base_gva = vcpu_args->gva;
@@ -188,20 +224,78 @@ static void mark_vcpu_memory_idle(struct kvm_vm *vm,
 	 * access tracking but low enough as to not make the test too brittle
 	 * over time and across architectures.
 	 */
-	if (still_idle >= pages / 10) {
-		TEST_ASSERT(idle_pages_warn_only,
-			    "vCPU%d: Too many pages still idle (%lu out of %lu)",
-			    vcpu_idx, still_idle, pages);
-
-		printf("WARNING: vCPU%d: Too many pages still idle (%lu out of %lu), "
-		       "this will affect performance results.\n",
-		       vcpu_idx, still_idle, pages);
-	}
+	if (still_idle >= pages / 10)
+		too_many_idle_pages(still_idle, pages,
+				    overlap_memory_access ? -1 : vcpu_idx);
 
 	close(page_idle_fd);
 	close(pagemap_fd);
 }
 
+int find_generation(struct memcg_stats *stats, long total_pages)
+{
+	/*
+	 * For finding the generation that contains our pages, use the same
+	 * 90% threshold that page_idle uses.
+	 */
+	int gen = lru_gen_find_generation(stats, total_pages * 9 / 10);
+
+	if (gen >= 0)
+		return gen;
+
+	if (!idle_pages_warn_only) {
+		TEST_FAIL("Could not find a generation with 90%% of guest memory (%ld pages).",
+			   total_pages * 9 / 10);
+		return gen;
+	}
+
+	/*
+	 * We couldn't find a generation with 90% of guest memory, which can
+	 * happen if access tracking is unreliable. Simply look for a majority
+	 * of pages.
+	 */
+	puts("WARNING: Couldn't find a generation with 90% of guest memory. "
+	     "Performance results may not be accurate.");
+	gen = lru_gen_find_generation(stats, total_pages / 2);
+	TEST_ASSERT(gen >= 0,
+		    "Could not find a generation with 50%% of guest memory (%ld pages).",
+		    total_pages / 2);
+	return gen;
+}
+
+static void lru_gen_mark_memory_idle(struct kvm_vm *vm)
+{
+	struct timespec ts_start;
+	struct timespec ts_elapsed;
+	struct memcg_stats stats;
+	int new_gen;
+
+	/* Make a new generation */
+	clock_gettime(CLOCK_MONOTONIC, &ts_start);
+	lru_gen_do_aging(&stats, TEST_MEMCG_NAME);
+	ts_elapsed = timespec_elapsed(ts_start);
+
+	/* Check the generation again */
+	new_gen = find_generation(&stats, test_pages);
+
+	/*
+	 * This function should only be invoked with newly-accessed pages,
+	 * so pages should always move to a newer generation.
+	 */
+	if (new_gen <= lru_gen_last_gen) {
+		/* We did not move to a newer generation. */
+		long idle_pages = lru_gen_sum_memcg_stats_for_gen(lru_gen_last_gen,
+								  &stats);
+
+		too_many_idle_pages(min_t(long, idle_pages, test_pages),
+				    test_pages, -1);
+	}
+	pr_info("%-30s: %ld.%09lds\n",
+		"Mark memory idle (lru_gen)", ts_elapsed.tv_sec,
+		ts_elapsed.tv_nsec);
+	lru_gen_last_gen = new_gen;
+}
+
 static void assert_ucall(struct kvm_vcpu *vcpu, uint64_t expected_ucall)
 {
 	struct ucall uc;
@@ -241,7 +335,7 @@ static void vcpu_thread_main(struct memstress_vcpu_args *vcpu_args)
 			assert_ucall(vcpu, UCALL_SYNC);
 			break;
 		case ITERATION_MARK_IDLE:
-			mark_vcpu_memory_idle(vm, vcpu_args);
+			pageidle_mark_vcpu_memory_idle(vm, vcpu_args);
 			break;
 		}
 
@@ -293,15 +387,18 @@ static void access_memory(struct kvm_vm *vm, int nr_vcpus,
 
 static void mark_memory_idle(struct kvm_vm *vm, int nr_vcpus)
 {
+	if (use_lru_gen)
+		return lru_gen_mark_memory_idle(vm);
+
 	/*
 	 * 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
+	 * at a time and the clear_young notifier may serialize on the KVM MMU
 	 * lock.
 	 */
 	pr_debug("Marking VM memory idle (slow)...\n");
 	iteration_work = ITERATION_MARK_IDLE;
-	run_iteration(vm, nr_vcpus, "Mark memory idle");
+	run_iteration(vm, nr_vcpus, "Mark memory idle (page_idle)");
 }
 
 static void run_test(enum vm_guest_mode mode, void *arg)
@@ -313,11 +410,38 @@ static void run_test(enum vm_guest_mode mode, void *arg)
 	vm = memstress_create_vm(mode, nr_vcpus, params->vcpu_memory_bytes, 1,
 				 params->backing_src, !overlap_memory_access);
 
+	/*
+	 * If guest_page_size is larger than the host's page size, the
+	 * guest (memstress) will only fault in a subset of the host's pages.
+	 */
+	test_pages = params->nr_vcpus * params->vcpu_memory_bytes /
+		      max(memstress_args.guest_page_size,
+			  (uint64_t)getpagesize());
+
 	memstress_start_vcpu_threads(nr_vcpus, vcpu_thread_main);
 
 	pr_info("\n");
 	access_memory(vm, nr_vcpus, ACCESS_WRITE, "Populating memory");
 
+	if (use_lru_gen) {
+		struct memcg_stats stats;
+
+		/*
+		 * Do a page table scan now. Following initial population, aging
+		 * may not cause the pages to move to a newer generation. Do
+		 * an aging pass now so that future aging passes always move
+		 * pages to a newer generation.
+		 */
+		printf("Initial aging pass (lru_gen)\n");
+		lru_gen_do_aging(&stats, TEST_MEMCG_NAME);
+		TEST_ASSERT(lru_gen_sum_memcg_stats(&stats) >= test_pages,
+			    "Not all pages accounted for (looking for %ld). "
+			    "Was the memcg set up correctly?", test_pages);
+		access_memory(vm, nr_vcpus, ACCESS_WRITE, "Re-populating memory");
+		lru_gen_read_memcg_stats(&stats, TEST_MEMCG_NAME);
+		lru_gen_last_gen = find_generation(&stats, test_pages);
+	}
+
 	/* As a control, read and write to the populated memory first. */
 	access_memory(vm, nr_vcpus, ACCESS_WRITE, "Writing to populated memory");
 	access_memory(vm, nr_vcpus, ACCESS_READ, "Reading from populated memory");
@@ -354,7 +478,12 @@ static int access_tracking_unreliable(void)
 		puts("Skipping idle page count sanity check, because NUMA balancing is enabled");
 		return 1;
 	}
+	return 0;
+}
 
+static int run_test_for_each_guest_mode(const char *cgroup, void *arg)
+{
+	for_each_guest_mode(run_test, arg);
 	return 0;
 }
 
@@ -383,6 +512,11 @@ static void help(char *name)
 	exit(0);
 }
 
+void destroy_cgroup(char *cg)
+{
+	printf("Destroying cgroup: %s\n", cg);
+}
+
 int main(int argc, char *argv[])
 {
 	struct test_params params = {
@@ -390,6 +524,7 @@ int main(int argc, char *argv[])
 		.vcpu_memory_bytes = DEFAULT_PER_VCPU_MEM_SIZE,
 		.nr_vcpus = 1,
 	};
+	char *new_cg = NULL;
 	int page_idle_fd;
 	int opt;
 
@@ -424,15 +559,53 @@ int main(int argc, char *argv[])
 		}
 	}
 
-	page_idle_fd = open("/sys/kernel/mm/page_idle/bitmap", O_RDWR);
-	__TEST_REQUIRE(page_idle_fd >= 0,
-		       "CONFIG_IDLE_PAGE_TRACKING is not enabled");
-	close(page_idle_fd);
-
 	if (idle_pages_warn_only == -1)
 		idle_pages_warn_only = access_tracking_unreliable();
 
-	for_each_guest_mode(run_test, &params);
+	if (lru_gen_usable()) {
+		bool cg_created = true;
+		int ret;
+
+		puts("Using lru_gen for aging");
+		use_lru_gen = true;
+
+		if (cg_find_controller_root(cgroup_root, sizeof(cgroup_root), "memory"))
+			ksft_exit_skip("Cannot find memory cgroup controller\n");
+
+		new_cg = cg_name(cgroup_root, TEST_MEMCG_NAME);
+		printf("Creating cgroup: %s\n", new_cg);
+		if (cg_create(new_cg)) {
+			if (errno == EEXIST) {
+				printf("Found existing cgroup");
+				cg_created = false;
+			} else {
+				ksft_exit_skip("could not create new cgroup: %s\n", new_cg);
+			}
+		}
+
+		/*
+		 * This will fork off a new process to run the test within
+		 * a new memcg, so we need to properly propagate the return
+		 * value up.
+		 */
+		ret = cg_run(new_cg, &run_test_for_each_guest_mode, &params);
+		if (cg_created)
+			cg_destroy(new_cg);
+		if (ret < 0)
+			TEST_FAIL("child did not spawn or was abnormally killed");
+		if (ret)
+			return ret;
+	} else {
+		page_idle_fd = open("/sys/kernel/mm/page_idle/bitmap", O_RDWR);
+		__TEST_REQUIRE(page_idle_fd >= 0,
+			       "Couldn't open /sys/kernel/mm/page_idle/bitmap. "
+			       "Is CONFIG_IDLE_PAGE_TRACKING enabled?");
+
+		close(page_idle_fd);
+
+		puts("Using page_idle for aging");
+		run_test_for_each_guest_mode(NULL, &params);
+	}
 
 	return 0;
 }