Re: [PATCH v3 12/12] KVM: arm64: selftests: arch_timer: Support vCPU migration

[Raghavendra Rao Ananta <rananta@google.com>]

On Fri, Sep 3, 2021 at 4:05 AM Andrew Jones <drjones@redhat.com> wrote:
>
> On Wed, Sep 01, 2021 at 09:14:12PM +0000, Raghavendra Rao Ananta wrote:
> > Since the timer stack (hardware and KVM) is per-CPU, there
> > are potential chances for races to occur when the scheduler
> > decides to migrate a vCPU thread to a different physical CPU.
> > Hence, include an option to stress-test this part as well by
> > forcing the vCPUs to migrate across physical CPUs in the
> > system at a particular rate.
> >
> > Originally, the bug for the fix with commit 3134cc8beb69d0d
> > ("KVM: arm64: vgic: Resample HW pending state on deactivation")
> > was discovered using arch_timer test with vCPU migrations and
> > can be easily reproduced.
> >
> > Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
> > ---
> >  .../selftests/kvm/aarch64/arch_timer.c        | 108 +++++++++++++++++-
> >  1 file changed, 107 insertions(+), 1 deletion(-)
> >
> > diff --git a/tools/testing/selftests/kvm/aarch64/arch_timer.c b/tools/testing/selftests/kvm/aarch64/arch_timer.c
> > index 1383f33850e9..de246c7afab2 100644
> > --- a/tools/testing/selftests/kvm/aarch64/arch_timer.c
> > +++ b/tools/testing/selftests/kvm/aarch64/arch_timer.c
> > @@ -14,6 +14,8 @@
> >   *
> >   * The test provides command-line options to configure the timer's
> >   * period (-p), number of vCPUs (-n), and iterations per stage (-i).
> > + * To stress-test the timer stack even more, an option to migrate the
> > + * vCPUs across pCPUs (-m), at a particular rate, is also provided.
> >   *
> >   * Copyright (c) 2021, Google LLC.
> >   */
> > @@ -24,6 +26,8 @@
> >  #include <pthread.h>
> >  #include <linux/kvm.h>
> >  #include <linux/sizes.h>
> > +#include <linux/bitmap.h>
> > +#include <sys/sysinfo.h>
> >
> >  #include "kvm_util.h"
> >  #include "processor.h"
> > @@ -41,12 +45,14 @@ struct test_args {
> >       int nr_vcpus;
> >       int nr_iter;
> >       int timer_period_ms;
> > +     int migration_freq_ms;
> >  };
> >
> >  static struct test_args test_args = {
> >       .nr_vcpus = NR_VCPUS_DEF,
> >       .nr_iter = NR_TEST_ITERS_DEF,
> >       .timer_period_ms = TIMER_TEST_PERIOD_MS_DEF,
> > +     .migration_freq_ms = 0,         /* Turn off migrations by default */
>
> I'd rather we enable good tests like these by default.
>
Well, that was my original idea, but I was concerned about the ease
for diagnosing
things since it'll become too noisy. And so I let it as a personal
preference. But I can
include it back and see how it goes.
> >  };
> >
> >  #define msecs_to_usecs(msec)         ((msec) * 1000LL)
> > @@ -81,6 +87,9 @@ struct test_vcpu {
> >  static struct test_vcpu test_vcpu[KVM_MAX_VCPUS];
> >  static struct test_vcpu_shared_data vcpu_shared_data[KVM_MAX_VCPUS];
> >
> > +static unsigned long *vcpu_done_map;
> > +static pthread_mutex_t vcpu_done_map_lock;
> > +
> >  static void
> >  guest_configure_timer_action(struct test_vcpu_shared_data *shared_data)
> >  {
> > @@ -216,6 +225,11 @@ static void *test_vcpu_run(void *arg)
> >
> >       vcpu_run(vm, vcpuid);
> >
> > +     /* Currently, any exit from guest is an indication of completion */
> > +     pthread_mutex_lock(&vcpu_done_map_lock);
> > +     set_bit(vcpuid, vcpu_done_map);
> > +     pthread_mutex_unlock(&vcpu_done_map_lock);
> > +
> >       switch (get_ucall(vm, vcpuid, &uc)) {
> >       case UCALL_SYNC:
> >       case UCALL_DONE:
> > @@ -235,9 +249,73 @@ static void *test_vcpu_run(void *arg)
> >       return NULL;
> >  }
> >
> > +static uint32_t test_get_pcpu(void)
> > +{
> > +     uint32_t pcpu;
> > +     unsigned int nproc_conf;
> > +     cpu_set_t online_cpuset;
> > +
> > +     nproc_conf = get_nprocs_conf();
> > +     sched_getaffinity(0, sizeof(cpu_set_t), &online_cpuset);
> > +
> > +     /* Randomly find an available pCPU to place a vCPU on */
> > +     do {
> > +             pcpu = rand() % nproc_conf;
> > +     } while (!CPU_ISSET(pcpu, &online_cpuset));
> > +
> > +     return pcpu;
> > +}
> > +static int test_migrate_vcpu(struct test_vcpu *vcpu)
> > +{
> > +     int ret;
> > +     cpu_set_t cpuset;
> > +     uint32_t new_pcpu = test_get_pcpu();
> > +
> > +     CPU_ZERO(&cpuset);
> > +     CPU_SET(new_pcpu, &cpuset);
> > +     ret = pthread_setaffinity_np(vcpu->pt_vcpu_run,
> > +                                     sizeof(cpuset), &cpuset);
> > +
> > +     /* Allow the error where the vCPU thread is already finished */
> > +     TEST_ASSERT(ret == 0 || ret == ESRCH,
> > +                     "Failed to migrate the vCPU:%u to pCPU: %u; ret: %d\n",
> > +                     vcpu->vcpuid, new_pcpu, ret);
>
> It'd be good to collect stats for the two cases so we know how many
> vcpus we actually migrated with a successful setaffinity and how many
> just completed too early. If our stats don't look good, then we can
> adjust our timeouts and frequencies.
>
I can do that, but since we don't attempt to migrate if the migration
thread learns
that the vCPU is already done, I'm guessing we may not hit ESRCH as much.
> > +
> > +     return ret;
> > +}
> > +static void *test_vcpu_migration(void *arg)
> > +{
> > +     unsigned int i, n_done;
> > +     bool vcpu_done;
> > +
> > +     do {
> > +             usleep(msecs_to_usecs(test_args.migration_freq_ms));
> > +
> > +             for (n_done = 0, i = 0; i < test_args.nr_vcpus; i++) {
> > +                     pthread_mutex_lock(&vcpu_done_map_lock);
> > +                     vcpu_done = test_bit(i, vcpu_done_map);
> > +                     pthread_mutex_unlock(&vcpu_done_map_lock);
> > +
> > +                     if (vcpu_done) {
> > +                             n_done++;
> > +                             continue;
> > +                     }
> > +
> > +                     test_migrate_vcpu(&test_vcpu[i]);
> > +             }
> > +     } while (test_args.nr_vcpus != n_done);
> > +
> > +     return NULL;
> > +}
> > +
> >  static void test_run(struct kvm_vm *vm)
> >  {
> >       int i, ret;
> > +     pthread_t pt_vcpu_migration;
> > +
> > +     pthread_mutex_init(&vcpu_done_map_lock, NULL);
> > +     vcpu_done_map = bitmap_alloc(test_args.nr_vcpus);
> > +     TEST_ASSERT(vcpu_done_map, "Failed to allocate vcpu done bitmap\n");
> >
> >       for (i = 0; i < test_args.nr_vcpus; i++) {
> >               ret = pthread_create(&test_vcpu[i].pt_vcpu_run, NULL,
> > @@ -245,8 +323,23 @@ static void test_run(struct kvm_vm *vm)
> >               TEST_ASSERT(!ret, "Failed to create vCPU-%d pthread\n", i);
> >       }
> >
> > +     /* Spawn a thread to control the vCPU migrations */
> > +     if (test_args.migration_freq_ms) {
> > +             srand(time(NULL));
> > +
> > +             ret = pthread_create(&pt_vcpu_migration, NULL,
> > +                                     test_vcpu_migration, NULL);
> > +             TEST_ASSERT(!ret, "Failed to create the migration pthread\n");
> > +     }
> > +
> > +
> >       for (i = 0; i < test_args.nr_vcpus; i++)
> >               pthread_join(test_vcpu[i].pt_vcpu_run, NULL);
> > +
> > +     if (test_args.migration_freq_ms)
> > +             pthread_join(pt_vcpu_migration, NULL);
> > +
> > +     bitmap_free(vcpu_done_map);
> >  }
> >
> >  static struct kvm_vm *test_vm_create(void)
> > @@ -286,6 +379,7 @@ static void test_print_help(char *name)
> >               NR_TEST_ITERS_DEF);
> >       pr_info("\t-p: Periodicity (in ms) of the guest timer (default: %u)\n",
> >               TIMER_TEST_PERIOD_MS_DEF);
> > +     pr_info("\t-m: Frequency (in ms) of vCPUs to migrate to different pCPU. 0 to turn off (default: 0)\n");
> >       pr_info("\t-h: print this help screen\n");
> >  }
> >
> > @@ -293,7 +387,7 @@ static bool parse_args(int argc, char *argv[])
> >  {
> >       int opt;
> >
> > -     while ((opt = getopt(argc, argv, "hn:i:p:")) != -1) {
> > +     while ((opt = getopt(argc, argv, "hn:i:p:m:")) != -1) {
> >               switch (opt) {
> >               case 'n':
> >                       test_args.nr_vcpus = atoi(optarg);
> > @@ -320,6 +414,13 @@ static bool parse_args(int argc, char *argv[])
> >                               goto err;
> >                       }
> >                       break;
> > +             case 'm':
> > +                     test_args.migration_freq_ms = atoi(optarg);
> > +                     if (test_args.migration_freq_ms < 0) {
> > +                             pr_info("0 or positive value needed for -m\n");
> > +                             goto err;
> > +                     }
> > +                     break;
> >               case 'h':
> >               default:
> >                       goto err;
> > @@ -343,6 +444,11 @@ int main(int argc, char *argv[])
> >       if (!parse_args(argc, argv))
> >               exit(KSFT_SKIP);
> >
> > +     if (get_nprocs() < 2) {
>
> if (test_args.migration_freq_ms && get_nprocs() < 2)
>
> > +             print_skip("At least two physical CPUs needed for vCPU migration");
> > +             exit(KSFT_SKIP);
> > +     }
> > +
> >       vm = test_vm_create();
> >       test_run(vm);
> >       kvm_vm_free(vm);
> > --
> > 2.33.0.153.gba50c8fa24-goog
> >
>
> Thanks,
> drew
>
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