If we have multiple contexts of equal priority pending execution,
activate a timer to demote the currently executing context in favour of
the next in the queue when that timeslice expires. This enforces
fairness between contexts (so long as they allow preemption -- forced
preemption, in the future, will kick those who do not obey) and allows
us to avoid userspace blocking forward progress with e.g. unbounded
MI_SEMAPHORE_WAIT.
For the starting point here, we use the jiffie as our timeslice so that
we should be reasonably efficient wrt frequent CPU wakeups.
Testcase: igt/gem_exec_scheduler/semaphore-resolve
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
---
drivers/gpu/drm/i915/gt/intel_engine_types.h | 6 +
drivers/gpu/drm/i915/gt/intel_lrc.c | 111 +++++++++
drivers/gpu/drm/i915/gt/selftest_lrc.c | 223 +++++++++++++++++++
drivers/gpu/drm/i915/i915_scheduler.c | 1 +
drivers/gpu/drm/i915/i915_scheduler_types.h | 1 +
5 files changed, 342 insertions(+)
diff --git a/drivers/gpu/drm/i915/gt/intel_engine_types.h b/drivers/gpu/drm/i915/gt/intel_engine_types.h
index dd0082df42cc..11a25f060fed 100644
--- a/drivers/gpu/drm/i915/gt/intel_engine_types.h
+++ b/drivers/gpu/drm/i915/gt/intel_engine_types.h
@@ -12,6 +12,7 @@
#include <linux/kref.h>
#include <linux/list.h>
#include <linux/llist.h>
+#include <linux/timer.h>
#include <linux/types.h>
#include "i915_gem.h"
@@ -137,6 +138,11 @@ struct intel_engine_execlists {
*/
struct tasklet_struct tasklet;
+ /**
+ * @timer: kick the current context if its timeslice expires
+ */
+ struct timer_list timer;
+
/**
* @default_priolist: priority list for I915_PRIORITY_NORMAL
*/
diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
index 137948927c63..109b32c6aead 100644
--- a/drivers/gpu/drm/i915/gt/intel_lrc.c
+++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
@@ -255,6 +255,7 @@ static int effective_prio(const struct i915_request *rq)
prio |= I915_PRIORITY_NOSEMAPHORE;
/* Restrict mere WAIT boosts from triggering preemption */
+ BUILD_BUG_ON(__NO_PREEMPTION & ~I915_PRIORITY_MASK); /* only internal */
return prio | __NO_PREEMPTION;
}
@@ -811,6 +812,81 @@ last_active(const struct intel_engine_execlists *execlists)
return *last;
}
+static void
+defer_request(struct i915_request * const rq, struct list_head * const pl)
+{
+ struct i915_dependency *p;
+
+ /*
+ * We want to move the interrupted request to the back of
+ * the round-robin list (i.e. its priority level), but
+ * in doing so, we must then move all requests that were in
+ * flight and were waiting for the interrupted request to
+ * be run after it again.
+ */
+ list_move_tail(&rq->sched.link, pl);
+
+ list_for_each_entry(p, &rq->sched.waiters_list, wait_link) {
+ struct i915_request *w =
+ container_of(p->waiter, typeof(*w), sched);
+
+ /* Leave semaphores spinning on the other engines */
+ if (w->engine != rq->engine)
+ continue;
+
+ /* No waiter should start before the active request completed */
+ GEM_BUG_ON(i915_request_started(w));
+
+ GEM_BUG_ON(rq_prio(w) > rq_prio(rq));
+ if (rq_prio(w) < rq_prio(rq))
+ continue;
+
+ if (list_empty(&w->sched.link))
+ continue; /* Not yet submitted; unready */
+
+ /*
+ * This should be very shallow as it is limited by the
+ * number of requests that can fit in a ring (<64) and
+ * the number of contexts that can be in flight on this
+ * engine.
+ */
+ defer_request(w, pl);
+ }
+}
+
+static void defer_active(struct intel_engine_cs *engine)
+{
+ struct i915_request *rq;
+
+ rq = __unwind_incomplete_requests(engine);
+ if (!rq)
+ return;
+
+ defer_request(rq, i915_sched_lookup_priolist(engine, rq_prio(rq)));
+}
+
+static bool
+need_timeslice(struct intel_engine_cs *engine, const struct i915_request *rq)
+{
+ int hint;
+
+ if (list_is_last(&rq->sched.link, &engine->active.requests))
+ return false;
+
+ hint = max(rq_prio(list_next_entry(rq, sched.link)),
+ engine->execlists.queue_priority_hint);
+
+ return hint >= rq_prio(rq);
+}
+
+static bool
+enable_timeslice(struct intel_engine_cs *engine)
+{
+ struct i915_request *last = last_active(&engine->execlists);
+
+ return last && need_timeslice(engine, last);
+}
+
static void execlists_dequeue(struct intel_engine_cs *engine)
{
struct intel_engine_execlists * const execlists = &engine->execlists;
@@ -904,6 +980,27 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
*/
last->hw_context->lrc_desc |= CTX_DESC_FORCE_RESTORE;
last = NULL;
+ } else if (need_timeslice(engine, last) &&
+ !timer_pending(&engine->execlists.timer)) {
+ GEM_TRACE("%s: expired last=%llx:%lld, prio=%d, hint=%d\n",
+ engine->name,
+ last->fence.context,
+ last->fence.seqno,
+ last->sched.attr.priority,
+ execlists->queue_priority_hint);
+
+ ring_pause(engine) = 1;
+ defer_active(engine);
+
+ /*
+ * Unlike for preemption, if we rewind and continue
+ * executing the same context as previously active,
+ * the order of execution will remain the same and
+ * the tail will only advance. We do not need to
+ * force a full context restore, as a lite-restore
+ * is sufficient to resample the monotonic TAIL.
+ */
+ last = NULL;
} else {
/*
* Otherwise if we already have a request pending
@@ -1226,6 +1323,9 @@ static void process_csb(struct intel_engine_cs *engine)
sizeof(*execlists->pending));
execlists->pending[0] = NULL;
+ if (enable_timeslice(engine))
+ mod_timer(&execlists->timer, jiffies + 1);
+
if (!inject_preempt_hang(execlists))
ring_pause(engine) = 0;
} else if (status & GEN8_CTX_STATUS_PREEMPTED) {
@@ -1296,6 +1396,15 @@ static void execlists_submission_tasklet(unsigned long data)
spin_unlock_irqrestore(&engine->active.lock, flags);
}
+static void execlists_submission_timer(struct timer_list *timer)
+{
+ struct intel_engine_cs *engine =
+ from_timer(engine, timer, execlists.timer);
+
+ /* Kick the tasklet for some interrupt coalescing and reset handling */
+ tasklet_hi_schedule(&engine->execlists.tasklet);
+}
+
static void queue_request(struct intel_engine_cs *engine,
struct i915_sched_node *node,
int prio)
@@ -2525,6 +2634,7 @@ static int gen8_init_rcs_context(struct i915_request *rq)
static void execlists_park(struct intel_engine_cs *engine)
{
+ del_timer_sync(&engine->execlists.timer);
intel_engine_park(engine);
}
@@ -2622,6 +2732,7 @@ int intel_execlists_submission_setup(struct intel_engine_cs *engine)
tasklet_init(&engine->execlists.tasklet,
execlists_submission_tasklet, (unsigned long)engine);
+ timer_setup(&engine->execlists.timer, execlists_submission_timer, 0);
logical_ring_default_vfuncs(engine);
logical_ring_default_irqs(engine);
diff --git a/drivers/gpu/drm/i915/gt/selftest_lrc.c b/drivers/gpu/drm/i915/gt/selftest_lrc.c
index d162b0d3ec8e..2a54c076a769 100644
--- a/drivers/gpu/drm/i915/gt/selftest_lrc.c
+++ b/drivers/gpu/drm/i915/gt/selftest_lrc.c
@@ -79,6 +79,225 @@ static int live_sanitycheck(void *arg)
return err;
}
+static int
+emit_semaphore_chain(struct i915_request *rq, struct i915_vma *vma, int idx)
+{
+ u32 *cs;
+
+ cs = intel_ring_begin(rq, 10);
+ if (IS_ERR(cs))
+ return PTR_ERR(cs);
+
+ *cs++ = MI_ARB_ON_OFF | MI_ARB_ENABLE;
+
+ *cs++ = MI_SEMAPHORE_WAIT |
+ MI_SEMAPHORE_GLOBAL_GTT |
+ MI_SEMAPHORE_POLL |
+ MI_SEMAPHORE_SAD_NEQ_SDD;
+ *cs++ = 0;
+ *cs++ = i915_ggtt_offset(vma) + 4 * idx;
+ *cs++ = 0;
+
+ if (idx > 0) {
+ *cs++ = MI_STORE_DWORD_IMM_GEN4 | MI_USE_GGTT;
+ *cs++ = i915_ggtt_offset(vma) + 4 * (idx - 1);
+ *cs++ = 0;
+ *cs++ = 1;
+ } else {
+ *cs++ = MI_NOOP;
+ *cs++ = MI_NOOP;
+ *cs++ = MI_NOOP;
+ *cs++ = MI_NOOP;
+ }
+
+ *cs++ = MI_ARB_ON_OFF | MI_ARB_DISABLE;
+
+ intel_ring_advance(rq, cs);
+ return 0;
+}
+
+static struct i915_request *
+semaphore_queue(struct intel_engine_cs *engine, struct i915_vma *vma, int idx)
+{
+ struct i915_gem_context *ctx;
+ struct i915_request *rq;
+ int err;
+
+ ctx = kernel_context(engine->i915);
+ if (!ctx)
+ return ERR_PTR(-ENOMEM);
+
+ rq = igt_request_alloc(ctx, engine);
+ if (IS_ERR(rq))
+ goto out_ctx;
+
+ err = emit_semaphore_chain(rq, vma, idx);
+ i915_request_add(rq);
+ if (err)
+ rq = ERR_PTR(err);
+
+out_ctx:
+ kernel_context_close(ctx);
+ return rq;
+}
+
+static int
+release_queue(struct intel_engine_cs *engine,
+ struct i915_vma *vma,
+ int idx)
+{
+ struct i915_sched_attr attr = {
+ .priority = I915_USER_PRIORITY(I915_PRIORITY_MAX),
+ };
+ struct i915_request *rq;
+ u32 *cs;
+
+ rq = i915_request_create(engine->kernel_context);
+ if (IS_ERR(rq))
+ return PTR_ERR(rq);
+
+ cs = intel_ring_begin(rq, 4);
+ if (IS_ERR(cs)) {
+ i915_request_add(rq);
+ return PTR_ERR(cs);
+ }
+
+ *cs++ = MI_STORE_DWORD_IMM_GEN4 | MI_USE_GGTT;
+ *cs++ = i915_ggtt_offset(vma) + 4 * (idx - 1);
+ *cs++ = 0;
+ *cs++ = 1;
+
+ intel_ring_advance(rq, cs);
+ i915_request_add(rq);
+
+ engine->schedule(rq, &attr);
+
+ return 0;
+}
+
+static int
+slice_semaphore_queue(struct intel_engine_cs *outer,
+ struct i915_vma *vma,
+ int count)
+{
+ struct intel_engine_cs *engine;
+ struct i915_request *head;
+ enum intel_engine_id id;
+ int err, i, n = 0;
+
+ head = semaphore_queue(outer, vma, n++);
+ if (IS_ERR(head))
+ return PTR_ERR(head);
+
+ i915_request_get(head);
+ for_each_engine(engine, outer->i915, id) {
+ for (i = 0; i < count; i++) {
+ struct i915_request *rq;
+
+ rq = semaphore_queue(engine, vma, n++);
+ if (IS_ERR(rq)) {
+ err = PTR_ERR(rq);
+ goto out;
+ }
+ }
+ }
+
+ err = release_queue(outer, vma, n);
+ if (err)
+ goto out;
+
+ if (i915_request_wait(head,
+ I915_WAIT_LOCKED,
+ 2 * RUNTIME_INFO(outer->i915)->num_engines * (count + 2) * (count + 3)) < 0) {
+ pr_err("Failed to slice along semaphore chain of length (%d, %d)!\n",
+ count, n);
+ GEM_TRACE_DUMP();
+ i915_gem_set_wedged(outer->i915);
+ err = -EIO;
+ }
+
+out:
+ i915_request_put(head);
+ return err;
+}
+
+static int live_timeslice_preempt(void *arg)
+{
+ struct drm_i915_private *i915 = arg;
+ struct drm_i915_gem_object *obj;
+ intel_wakeref_t wakeref;
+ struct i915_vma *vma;
+ void *vaddr;
+ int err = 0;
+ int count;
+
+ /*
+ * If a request takes too long, we would like to give other users
+ * a fair go on the GPU. In particular, users may create batches
+ * that wait upon external input, where that input may even be
+ * supplied by another GPU job. To avoid blocking forever, we
+ * need to preempt the current task and replace it with another
+ * ready task.
+ */
+
+ mutex_lock(&i915->drm.struct_mutex);
+ wakeref = intel_runtime_pm_get(i915);
+
+ obj = i915_gem_object_create_internal(i915, PAGE_SIZE);
+ if (IS_ERR(obj)) {
+ err = PTR_ERR(obj);
+ goto err_unlock;
+ }
+
+ vma = i915_vma_instance(obj, &i915->ggtt.vm, NULL);
+ if (IS_ERR(vma)) {
+ err = PTR_ERR(vma);
+ goto err_obj;
+ }
+
+ vaddr = i915_gem_object_pin_map(obj, I915_MAP_WC);
+ if (IS_ERR(vaddr)) {
+ err = PTR_ERR(vaddr);
+ goto err_obj;
+ }
+
+ err = i915_vma_pin(vma, 0, 0, PIN_GLOBAL);
+ if (err)
+ goto err_map;
+
+ for_each_prime_number_from(count, 1, 16) {
+ struct intel_engine_cs *engine;
+ enum intel_engine_id id;
+
+ for_each_engine(engine, i915, id) {
+ memset(vaddr, 0, PAGE_SIZE);
+
+ err = slice_semaphore_queue(engine, vma, count);
+ if (err)
+ goto err_pin;
+
+ if (igt_flush_test(i915, I915_WAIT_LOCKED)) {
+ err = -EIO;
+ goto err_pin;
+ }
+ }
+ }
+
+err_pin:
+ i915_vma_unpin(vma);
+err_map:
+ i915_gem_object_unpin_map(obj);
+err_obj:
+ i915_gem_object_put(obj);
+err_unlock:
+ if (igt_flush_test(i915, I915_WAIT_LOCKED))
+ err = -EIO;
+ intel_runtime_pm_put(i915, wakeref);
+ mutex_unlock(&i915->drm.struct_mutex);
+
+ return err;
+}
+
static int live_busywait_preempt(void *arg)
{
struct drm_i915_private *i915 = arg;
@@ -398,6 +617,9 @@ static int live_late_preempt(void *arg)
if (!ctx_lo)
goto err_ctx_hi;
+ /* Make sure ctx_lo stays before ctx_hi until we trigger preemption. */
+ ctx_lo->sched.priority = I915_USER_PRIORITY(1);
+
for_each_engine(engine, i915, id) {
struct igt_live_test t;
struct i915_request *rq;
@@ -1818,6 +2040,7 @@ int intel_execlists_live_selftests(struct drm_i915_private *i915)
{
static const struct i915_subtest tests[] = {
SUBTEST(live_sanitycheck),
+ SUBTEST(live_timeslice_preempt),
SUBTEST(live_busywait_preempt),
SUBTEST(live_preempt),
SUBTEST(live_late_preempt),
diff --git a/drivers/gpu/drm/i915/i915_scheduler.c b/drivers/gpu/drm/i915/i915_scheduler.c
index b1ba3e65cd52..0bd452e851d8 100644
--- a/drivers/gpu/drm/i915/i915_scheduler.c
+++ b/drivers/gpu/drm/i915/i915_scheduler.c
@@ -394,6 +394,7 @@ bool __i915_sched_node_add_dependency(struct i915_sched_node *node,
list_add(&dep->wait_link, &signal->waiters_list);
list_add(&dep->signal_link, &node->signalers_list);
dep->signaler = signal;
+ dep->waiter = node;
dep->flags = flags;
/* Keep track of whether anyone on this chain has a semaphore */
diff --git a/drivers/gpu/drm/i915/i915_scheduler_types.h b/drivers/gpu/drm/i915/i915_scheduler_types.h
index 3e309631bd0b..aad81acba9dc 100644
--- a/drivers/gpu/drm/i915/i915_scheduler_types.h
+++ b/drivers/gpu/drm/i915/i915_scheduler_types.h
@@ -62,6 +62,7 @@ struct i915_sched_node {
struct i915_dependency {
struct i915_sched_node *signaler;
+ struct i915_sched_node *waiter;
struct list_head signal_link;
struct list_head wait_link;
struct list_head dfs_link;
--
2.20.1
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