Re: [PATCH v2 09/10] KVM: selftests: allow using UFFD minor faults for demand paging

[Axel Rasmussen]

On Wed, May 19, 2021 at 3:21 PM Ben Gardon <bgardon@xxxxxxxxxx> wrote:
>
> On Wed, May 19, 2021 at 1:04 PM Axel Rasmussen <axelrasmussen@xxxxxxxxxx> wrote:
> >
> > UFFD handling of MINOR faults is a new feature whose use case is to
> > speed up demand paging (compared to MISSING faults). So, it's
> > interesting to let this selftest exercise this new mode.
> >
> > Modify the demand paging test to have the option of using UFFD minor
> > faults, as opposed to missing faults. Now, when turning on userfaultfd
> > with '-u', the desired mode has to be specified ("MISSING" or "MINOR").
> >
> > If we're in minor mode, before registering, prefault via the *alias*.
> > This way, the guest will trigger minor faults, instead of missing
> > faults, and we can UFFDIO_CONTINUE to resolve them.
> >
> > Modify the page fault handler function to use the right ioctl depending
> > on the mode we're running in. In MINOR mode, use UFFDIO_CONTINUE.
> >
> > Signed-off-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
> > ---
> >  .../selftests/kvm/demand_paging_test.c        | 112 ++++++++++++------
> >  1 file changed, 79 insertions(+), 33 deletions(-)
> >
> > diff --git a/tools/testing/selftests/kvm/demand_paging_test.c b/tools/testing/selftests/kvm/demand_paging_test.c
> > index 01890a7b0155..df7190261923 100644
> > --- a/tools/testing/selftests/kvm/demand_paging_test.c
> > +++ b/tools/testing/selftests/kvm/demand_paging_test.c
> > @@ -74,33 +74,48 @@ static void *vcpu_worker(void *data)
> >         return NULL;
> >  }
> >
> > -static int handle_uffd_page_request(int uffd, uint64_t addr)
> > +static int handle_uffd_page_request(int uffd_mode, int uffd, uint64_t addr)
> >  {
> > -       pid_t tid;
> > +       pid_t tid = syscall(__NR_gettid);
> >         struct timespec start;
> >         struct timespec ts_diff;
> > -       struct uffdio_copy copy;
> >         int r;
> >
> > -       tid = syscall(__NR_gettid);
> > +       clock_gettime(CLOCK_MONOTONIC, &start);
> >
> > -       copy.src = (uint64_t)guest_data_prototype;
> > -       copy.dst = addr;
> > -       copy.len = demand_paging_size;
> > -       copy.mode = 0;
> > +       if (uffd_mode == UFFDIO_REGISTER_MODE_MISSING) {
> > +               struct uffdio_copy copy;
> >
> > -       clock_gettime(CLOCK_MONOTONIC, &start);
> > +               copy.src = (uint64_t)guest_data_prototype;
> > +               copy.dst = addr;
> > +               copy.len = demand_paging_size;
> > +               copy.mode = 0;
> > +
> > +               r = ioctl(uffd, UFFDIO_COPY, &copy);
> > +               if (r == -1) {
> > +                       pr_info("Failed UFFDIO_COPY in 0x%lx from thread %d with errno: %d\n",
> > +                               addr, tid, errno);
> > +                       return r;
> > +               }
> > +       } else if (uffd_mode == UFFDIO_REGISTER_MODE_MINOR) {
> > +               struct uffdio_continue cont = {0};
> > +
> > +               cont.range.start = addr;
> > +               cont.range.len = demand_paging_size;
> >
> > -       r = ioctl(uffd, UFFDIO_COPY, &copy);
> > -       if (r == -1) {
> > -               pr_info("Failed Paged in 0x%lx from thread %d with errno: %d\n",
> > -                       addr, tid, errno);
> > -               return r;
> > +               r = ioctl(uffd, UFFDIO_CONTINUE, &cont);
> > +               if (r == -1) {
> > +                       pr_info("Failed UFFDIO_CONTINUE in 0x%lx from thread %d with errno: %d\n",
> > +                               addr, tid, errno);
> > +                       return r;
> > +               }
> > +       } else {
> > +               TEST_FAIL("Invalid uffd mode %d", uffd_mode);
> >         }
> >
> >         ts_diff = timespec_elapsed(start);
> >
> > -       PER_PAGE_DEBUG("UFFDIO_COPY %d \t%ld ns\n", tid,
> > +       PER_PAGE_DEBUG("UFFD page-in %d \t%ld ns\n", tid,
> >                        timespec_to_ns(ts_diff));
> >         PER_PAGE_DEBUG("Paged in %ld bytes at 0x%lx from thread %d\n",
> >                        demand_paging_size, addr, tid);
> > @@ -111,6 +126,7 @@ static int handle_uffd_page_request(int uffd, uint64_t addr)
> >  bool quit_uffd_thread;
> >
> >  struct uffd_handler_args {
> > +       int uffd_mode;
> >         int uffd;
> >         int pipefd;
> >         useconds_t delay;
> > @@ -187,7 +203,7 @@ static void *uffd_handler_thread_fn(void *arg)
> >                 if (delay)
> >                         usleep(delay);
> >                 addr =  msg.arg.pagefault.address;
> > -               r = handle_uffd_page_request(uffd, addr);
> > +               r = handle_uffd_page_request(uffd_args->uffd_mode, uffd, addr);
> >                 if (r < 0)
> >                         return NULL;
> >                 pages++;
> > @@ -203,13 +219,32 @@ static void *uffd_handler_thread_fn(void *arg)
> >
> >  static void setup_demand_paging(struct kvm_vm *vm,
> >                                 pthread_t *uffd_handler_thread, int pipefd,
> > -                               useconds_t uffd_delay,
> > +                               int uffd_mode, useconds_t uffd_delay,
> >                                 struct uffd_handler_args *uffd_args,
> > -                               void *hva, uint64_t len)
> > +                               void *hva, void *alias, uint64_t len)
> >  {
> > +       bool is_minor = (uffd_mode == UFFDIO_REGISTER_MODE_MINOR);
> >         int uffd;
> >         struct uffdio_api uffdio_api;
> >         struct uffdio_register uffdio_register;
> > +       uint64_t expected_ioctls = ((uint64_t) 1) << _UFFDIO_COPY;
> > +
> > +       PER_PAGE_DEBUG("Userfaultfd %s mode, faults resolved with %s\n",
> > +                      is_minor ? "MINOR" : "MISSING",
> > +                      is_minor ? "UFFDIO_CONINUE" : "UFFDIO_COPY");
> > +
> > +       /* In order to get minor faults, prefault via the alias. */
> > +       if (is_minor) {
> > +               size_t p;
> > +
> > +               expected_ioctls = ((uint64_t) 1) << _UFFDIO_CONTINUE;
> > +
> > +               TEST_ASSERT(alias != NULL, "Alias required for minor faults");
> > +               for (p = 0; p < (len / demand_paging_size); ++p) {
> > +                       memcpy(alias + (p * demand_paging_size),
> > +                              guest_data_prototype, demand_paging_size);
> > +               }
> > +       }
>
> Would it be worth timing this operation? I think we'd need to know how
> long we spent prefaulting the memory to really be able to compare UDDF
> modes using this test.

It's easy to time it and print out a value, so I'm happy to add it.

As for how useful it is, I'm not so sure. In general the way I think
of it is, the prefaulting would happen during the precopy phase of
live migration. During this phase, the VM is still running on the
source machine, so the VM owner doesn't notice any performance
degradation or slowness in this phase.

>
> >
> >         uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
> >         TEST_ASSERT(uffd >= 0, "uffd creation failed, errno: %d", errno);
> > @@ -222,12 +257,13 @@ static void setup_demand_paging(struct kvm_vm *vm,
> >
> >         uffdio_register.range.start = (uint64_t)hva;
> >         uffdio_register.range.len = len;
> > -       uffdio_register.mode = UFFDIO_REGISTER_MODE_MISSING;
> > +       uffdio_register.mode = uffd_mode;
> >         TEST_ASSERT(ioctl(uffd, UFFDIO_REGISTER, &uffdio_register) != -1,
> >                     "ioctl UFFDIO_REGISTER failed");
> > -       TEST_ASSERT((uffdio_register.ioctls & UFFD_API_RANGE_IOCTLS) ==
> > -                   UFFD_API_RANGE_IOCTLS, "unexpected userfaultfd ioctl set");
> > +       TEST_ASSERT((uffdio_register.ioctls & expected_ioctls) ==
> > +                   expected_ioctls, "missing userfaultfd ioctls");
> >
> > +       uffd_args->uffd_mode = uffd_mode;
> >         uffd_args->uffd = uffd;
> >         uffd_args->pipefd = pipefd;
> >         uffd_args->delay = uffd_delay;
> > @@ -239,7 +275,7 @@ static void setup_demand_paging(struct kvm_vm *vm,
> >  }
> >
> >  struct test_params {
> > -       bool use_uffd;
> > +       int uffd_mode;
> >         useconds_t uffd_delay;
> >         enum vm_mem_backing_src_type src_type;
> >         bool partition_vcpu_memory_access;
> > @@ -276,7 +312,7 @@ static void run_test(enum vm_guest_mode mode, void *arg)
> >         perf_test_setup_vcpus(vm, nr_vcpus, guest_percpu_mem_size,
> >                               p->partition_vcpu_memory_access);
> >
> > -       if (p->use_uffd) {
> > +       if (p->uffd_mode) {
> >                 uffd_handler_threads =
> >                         malloc(nr_vcpus * sizeof(*uffd_handler_threads));
> >                 TEST_ASSERT(uffd_handler_threads, "Memory allocation failed");
> > @@ -290,6 +326,7 @@ static void run_test(enum vm_guest_mode mode, void *arg)
> >                 for (vcpu_id = 0; vcpu_id < nr_vcpus; vcpu_id++) {
> >                         vm_paddr_t vcpu_gpa;
> >                         void *vcpu_hva;
> > +                       void *vcpu_alias;
> >                         uint64_t vcpu_mem_size;
> >
> >
> > @@ -304,8 +341,9 @@ static void run_test(enum vm_guest_mode mode, void *arg)
> >                         PER_VCPU_DEBUG("Added VCPU %d with test mem gpa [%lx, %lx)\n",
> >                                        vcpu_id, vcpu_gpa, vcpu_gpa + vcpu_mem_size);
> >
> > -                       /* Cache the HVA pointer of the region */
> > +                       /* Cache the host addresses of the region */
> >                         vcpu_hva = addr_gpa2hva(vm, vcpu_gpa);
> > +                       vcpu_alias = addr_gpa2alias(vm, vcpu_gpa);
> >
> >                         /*
> >                          * Set up user fault fd to handle demand paging
> > @@ -316,8 +354,9 @@ static void run_test(enum vm_guest_mode mode, void *arg)
> >                         TEST_ASSERT(!r, "Failed to set up pipefd");
> >
> >                         setup_demand_paging(vm, &uffd_handler_threads[vcpu_id],
> > -                                           pipefds[vcpu_id * 2], p->uffd_delay,
> > -                                           &uffd_args[vcpu_id], vcpu_hva,
> > +                                           pipefds[vcpu_id * 2], p->uffd_mode,
> > +                                           p->uffd_delay, &uffd_args[vcpu_id],
> > +                                           vcpu_hva, vcpu_alias,
> >                                             vcpu_mem_size);
> >                 }
> >         }
> > @@ -346,7 +385,7 @@ static void run_test(enum vm_guest_mode mode, void *arg)
> >
> >         pr_info("All vCPU threads joined\n");
> >
> > -       if (p->use_uffd) {
> > +       if (p->uffd_mode) {
> >                 char c;
> >
> >                 /* Tell the user fault fd handler threads to quit */
> > @@ -368,7 +407,7 @@ static void run_test(enum vm_guest_mode mode, void *arg)
> >
> >         free(guest_data_prototype);
> >         free(vcpu_threads);
> > -       if (p->use_uffd) {
> > +       if (p->uffd_mode) {
> >                 free(uffd_handler_threads);
> >                 free(uffd_args);
> >                 free(pipefds);
> > @@ -378,11 +417,11 @@ static void run_test(enum vm_guest_mode mode, void *arg)
> >  static void help(char *name)
> >  {
> >         puts("");
> > -       printf("usage: %s [-h] [-m mode] [-u] [-d uffd_delay_usec]\n"
> > +       printf("usage: %s [-h] [-m mode] [-u mode] [-d uffd_delay_usec]\n"
>
> NIT: maybe use uffd_mode or some word other than mode here to
> disambiguate with -m
>
> >                "          [-b memory] [-t type] [-v vcpus] [-o]\n", name);
> >         guest_modes_help();
> > -       printf(" -u: use User Fault FD to handle vCPU page\n"
> > -              "     faults.\n");
> > +       printf(" -u: use userfaultfd to handle vCPU page faults. Mode is a\n"
> > +              "     UFFD registration mode: 'MISSING' or 'MINOR'.\n");
> >         printf(" -d: add a delay in usec to the User Fault\n"
> >                "     FD handler to simulate demand paging\n"
> >                "     overheads. Ignored without -u.\n");
> > @@ -409,13 +448,17 @@ int main(int argc, char *argv[])
> >
> >         guest_modes_append_default();
> >
> > -       while ((opt = getopt(argc, argv, "hm:ud:b:t:v:o")) != -1) {
> > +       while ((opt = getopt(argc, argv, "hm:u:d:b:t:v:o")) != -1) {
> >                 switch (opt) {
> >                 case 'm':
> >                         guest_modes_cmdline(optarg);
> >                         break;
> >                 case 'u':
> > -                       p.use_uffd = true;
> > +                       if (!strcmp("MISSING", optarg))
> > +                               p.uffd_mode = UFFDIO_REGISTER_MODE_MISSING;
> > +                       else if (!strcmp("MINOR", optarg))
> > +                               p.uffd_mode = UFFDIO_REGISTER_MODE_MINOR;
> > +                       TEST_ASSERT(p.uffd_mode, "UFFD mode must be 'MISSING' or 'MINOR'.");
> >                         break;
> >                 case 'd':
> >                         p.uffd_delay = strtoul(optarg, NULL, 0);
> > @@ -442,6 +485,9 @@ int main(int argc, char *argv[])
> >                 }
> >         }
> >
> > +       TEST_ASSERT(p.uffd_mode != UFFDIO_REGISTER_MODE_MINOR || p.src_type == VM_MEM_SRC_SHMEM,
> > +                   "userfaultfd MINOR mode requires shared memory; pick a different -t");
> > +
> >         for_each_guest_mode(run_test, &p);
> >
> >         return 0;
> > --
> > 2.31.1.751.gd2f1c929bd-goog
> >