One of the hardest priority inversion tasks to both handle and to
simulate in testing is inversion due to resource contention. The
challenge is that a high priority context should never be blocked by a
low priority context, even if both are starving for resources --
ideally, at least for some RT OSes, the higher priority context has
first pick of the meagre resources so that it can be executed with
minimum latency.
userfaultfd allows us to handle a page fault in userspace, and so
arbitrary impose a delay on the fault handler, creating a situation
where a low priority context is blocked waiting for the fault. This
blocked context should not prevent a high priority context from being
executed. While the userfault tries to emulate a slow fault (e.g. from a
failing swap device), it is unfortunately limited to a single object
type: the userptr. Hopefully, we will find other ways to impose other
starvation conditions on global resources.
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
---
tests/i915/gem_exec_schedule.c | 155 +++++++++++++++++++++++++++++++++
1 file changed, 155 insertions(+)
diff --git a/tests/i915/gem_exec_schedule.c b/tests/i915/gem_exec_schedule.c
index 84581bffe..d98434123 100644
--- a/tests/i915/gem_exec_schedule.c
+++ b/tests/i915/gem_exec_schedule.c
@@ -23,10 +23,16 @@
#include "config.h"
+#include <linux/userfaultfd.h>
+
+#include <pthread.h>
#include <sys/poll.h>
#include <sys/ioctl.h>
+#include <sys/mman.h>
+#include <sys/syscall.h>
#include <sched.h>
#include <signal.h>
+#include <unistd.h>
#include "igt.h"
#include "igt_rand.h"
@@ -1625,6 +1631,152 @@ static void test_pi_ringfull(int fd, unsigned int engine, unsigned int flags)
munmap(result, 4096);
}
+static int userfaultfd(int flags)
+{
+ return syscall(SYS_userfaultfd, flags);
+}
+
+struct ufd_thread {
+ uint32_t batch;
+ uint32_t *page;
+ unsigned int engine;
+ int i915;
+};
+
+static uint32_t create_userptr(int i915, void *page)
+{
+ uint32_t handle;
+
+ gem_userptr(i915, page, 4096, 0, 0, &handle);
+ return handle;
+}
+
+static void *ufd_thread(void *arg)
+{
+ struct ufd_thread *t = arg;
+ struct drm_i915_gem_exec_object2 obj[2] = {
+ { .handle = create_userptr(t->i915, t->page) },
+ { .handle = t->batch },
+ };
+ struct drm_i915_gem_execbuffer2 eb = {
+ .buffers_ptr = to_user_pointer(obj),
+ .buffer_count = ARRAY_SIZE(obj),
+ .flags = t->engine,
+ .rsvd1 = gem_context_create(t->i915),
+ };
+ gem_context_set_priority(t->i915, eb.rsvd1, MIN_PRIO);
+
+ igt_debug("submitting fault\n");
+ gem_execbuf(t->i915, &eb);
+ gem_sync(t->i915, obj[0].handle);
+ gem_close(t->i915, obj[0].handle);
+
+ gem_context_destroy(t->i915, eb.rsvd1);
+
+ t->i915 = -1;
+ return NULL;
+}
+
+static void test_pi_userfault(int i915, unsigned int engine)
+{
+ struct uffdio_api api = { .api = UFFD_API };
+ struct uffdio_register reg;
+ struct uffdio_copy copy;
+ struct uffd_msg msg;
+ struct ufd_thread t;
+ pthread_t thread;
+ char poison[4096];
+ int ufd;
+
+ /*
+ * Resource contention can easily lead to priority inversion problems,
+ * that we wish to avoid. Here, we simulate one simple form of resource
+ * starvation by using an arbitrary slow userspace fault handler to cause
+ * the low priority context to block waiting for its resource. While it
+ * is blocked, it should not prevent a higher priority context from
+ * executing.
+ *
+ * This is only a very simple scenario, in more general tests we will
+ * need to simulate contention on the shared resource such that both
+ * low and high priority contexts are starving and must fight over
+ * the meagre resources. One step at a time.
+ */
+
+ ufd = userfaultfd(0);
+ igt_require_f(ufd != -1, "kernel support for userfaultfd\n");
+ igt_require_f(ioctl(ufd, UFFDIO_API, &api) == 0 && api.api == UFFD_API,
+ "userfaultfd API v%lld:%lld\n", UFFD_API, api.api);
+
+ t.i915 = i915;
+ t.engine = engine;
+
+ t.page = mmap(NULL, 4096, PROT_WRITE, MAP_SHARED | MAP_ANON, 0, 0);
+ igt_assert(t.page != MAP_FAILED);
+
+ t.batch = gem_create(i915, 4096);
+ memset(poison, 0xff, sizeof(poison));
+ gem_write(i915, t.batch, 0, poison, 4096);
+
+ /* Register our fault handler for t.page */
+ memset(®, 0, sizeof(reg));
+ reg.mode = UFFDIO_REGISTER_MODE_MISSING;
+ reg.range.start = to_user_pointer(t.page);
+ reg.range.len = 4096;
+ do_ioctl(ufd, UFFDIO_REGISTER, ®);
+ igt_assert(reg.ioctls == UFFD_API_RANGE_IOCTLS);
+
+ /* Kick off the low priority submission */
+ igt_assert(pthread_create(&thread, NULL, ufd_thread, &t) == 0);
+
+ /* Wait until the low priority thread is blocked on a fault */
+ igt_assert_eq(read(ufd, &msg, sizeof(msg)), sizeof(msg));
+ igt_assert_eq(msg.event, UFFD_EVENT_PAGEFAULT);
+ igt_assert(from_user_pointer(msg.arg.pagefault.address) == t.page);
+
+ /* While the low priority context is blocked; execute a vip */
+ if (1) {
+ const uint32_t bbe = MI_BATCH_BUFFER_END;
+ struct drm_i915_gem_exec_object2 obj = {
+ .handle = t.batch,
+ };
+ struct pollfd pfd;
+ struct drm_i915_gem_execbuffer2 eb = {
+ .buffers_ptr = to_user_pointer(&obj),
+ .buffer_count = 1,
+ .flags = engine | I915_EXEC_FENCE_OUT,
+ .rsvd1 = gem_context_create(i915),
+ };
+ gem_context_set_priority(i915, eb.rsvd1, MAX_PRIO);
+ gem_write(i915, obj.handle, 0, &bbe, sizeof(bbe));
+ gem_execbuf_wr(i915, &eb);
+
+ memset(&pfd, 0, sizeof(pfd));
+ pfd.fd = eb.rsvd2 >> 32;
+ pfd.events = POLLIN;
+ poll(&pfd, 1, -1);
+ igt_assert_eq(sync_fence_status(pfd.fd), 1);
+ close(pfd.fd);
+
+ gem_context_destroy(i915, eb.rsvd1);
+ }
+
+ /* Confirm the low priority context is still waiting */
+ igt_assert_eq(t.i915, i915);
+
+ /* Service the fault; releasing the low priority context */
+ memset(©, 0, sizeof(copy));
+ copy.dst = msg.arg.pagefault.address;
+ copy.src = to_user_pointer(memset(poison, 0xc5, sizeof(poison)));
+ copy.len = 4096;
+ do_ioctl(ufd, UFFDIO_COPY, ©);
+
+ pthread_join(thread, NULL);
+
+ gem_close(i915, t.batch);
+ munmap(t.page, 4096);
+ close(ufd);
+}
+
static void measure_semaphore_power(int i915)
{
struct rapl gpu, pkg;
@@ -1864,6 +2016,9 @@ igt_main
igt_subtest_f("pi-common-%s", e->name)
test_pi_ringfull(fd, eb_ring(e), SHARED);
+
+ igt_subtest_f("pi-userfault-%s", e->name)
+ test_pi_userfault(fd, eb_ring(e));
}
}
}
--
2.24.0
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