Re: [PATCH 8/8] KVM: selftests: Introduce access_tracking_perf_test

[Ben Gardon <bgardon@google.com>]

On Fri, Jun 11, 2021 at 4:57 PM David Matlack <dmatlack@google.com> wrote:
>
> 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.
>
> Signed-off-by: David Matlack <dmatlack@google.com>

Reviewed-by: Ben Gardon <bgardon@google.com>

> ---
>  tools/testing/selftests/kvm/.gitignore        |   1 +
>  tools/testing/selftests/kvm/Makefile          |   1 +
>  .../selftests/kvm/access_tracking_perf_test.c | 419 ++++++++++++++++++
>  3 files changed, 421 insertions(+)
>  create mode 100644 tools/testing/selftests/kvm/access_tracking_perf_test.c
>
> diff --git a/tools/testing/selftests/kvm/.gitignore b/tools/testing/selftests/kvm/.gitignore
> index bd83158e0e0b..32a362d71e05 100644
> --- a/tools/testing/selftests/kvm/.gitignore
> +++ b/tools/testing/selftests/kvm/.gitignore
> @@ -34,6 +34,7 @@
>  /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 e439d027939d..9f1b478da92b 100644
> --- a/tools/testing/selftests/kvm/Makefile
> +++ b/tools/testing/selftests/kvm/Makefile
> @@ -67,6 +67,7 @@ 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..60828f2d780f
> --- /dev/null
> +++ b/tools/testing/selftests/kvm/access_tracking_perf_test.c
> @@ -0,0 +1,419 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * access_tracking_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;
> +
> +}
> +
> +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;
> +
> +       entry = pread_uint64(pagemap_fd, "pagemap", hva / getpagesize());
> +       if (!(entry & (1ULL << 63)))
> +               return 0;
> +
> +       return (entry & ((1ULL << 55) - 1));

It might be helpful to document these shifts and other constants in this test.

> +}
> +
> +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.");

Should this have an early skip-check too? I assume not all users of
this test will be running with the privileges required to access
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;
> +}
> --
> 2.32.0.272.g935e593368-goog
>