Quoting Tvrtko Ursulin (2020-07-15 13:39:56)
>
> On 14/07/2020 10:47, Chris Wilson wrote:
> > We are using the i915_request.lock to serialise adding an execution
> > callback with __i915_request_submit. However, if we use an atomic
> > llist_add to serialise multiple waiters and then check to see if the
> > request is already executing, we can remove the irq-spinlock.
> >
> > Fixes: 1d9221e9d395 ("drm/i915: Skip signaling a signaled request")
> > Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> > ---
> > drivers/gpu/drm/i915/i915_request.c | 38 +++++++----------------------
> > 1 file changed, 9 insertions(+), 29 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
> > index 0b2fe55e6194..c59315def07d 100644
> > --- a/drivers/gpu/drm/i915/i915_request.c
> > +++ b/drivers/gpu/drm/i915/i915_request.c
> > @@ -190,13 +190,11 @@ static void __notify_execute_cb(struct i915_request *rq)
> > {
> > struct execute_cb *cb, *cn;
> >
> > - lockdep_assert_held(&rq->lock);
> > -
> > - GEM_BUG_ON(!i915_request_is_active(rq));
> > if (llist_empty(&rq->execute_cb))
> > return;
> >
> > - llist_for_each_entry_safe(cb, cn, rq->execute_cb.first, work.llnode)
> > + llist_for_each_entry_safe(cb, cn,
> > + llist_del_all(&rq->execute_cb), work.llnode)
> > irq_work_queue(&cb->work);
> >
> > /*
> > @@ -209,7 +207,6 @@ static void __notify_execute_cb(struct i915_request *rq)
> > * preempt-to-idle cycle on the target engine, all the while the
> > * master execute_cb may refire.
> > */
> > - init_llist_head(&rq->execute_cb);
> > }
> >
> > static inline void
> > @@ -276,6 +273,7 @@ static void remove_from_engine(struct i915_request *rq)
> > list_del_init(&rq->sched.link);
> > clear_bit(I915_FENCE_FLAG_PQUEUE, &rq->fence.flags);
> > clear_bit(I915_FENCE_FLAG_HOLD, &rq->fence.flags);
> > + set_bit(I915_FENCE_FLAG_ACTIVE, &rq->fence.flags);
> > spin_unlock_irq(&locked->active.lock);
> > }
> >
> > @@ -323,12 +321,8 @@ bool i915_request_retire(struct i915_request *rq)
> > GEM_BUG_ON(!atomic_read(&rq->engine->gt->rps.num_waiters));
> > atomic_dec(&rq->engine->gt->rps.num_waiters);
> > }
> > - if (!test_bit(I915_FENCE_FLAG_ACTIVE, &rq->fence.flags)) {
> > - set_bit(I915_FENCE_FLAG_ACTIVE, &rq->fence.flags);
> > - __notify_execute_cb(rq);
> > - }
> > - GEM_BUG_ON(!llist_empty(&rq->execute_cb));
> > spin_unlock_irq(&rq->lock);
> > + __notify_execute_cb(rq);
> >
> > remove_from_client(rq);
> > __list_del_entry(&rq->link); /* poison neither prev/next (RCU walks) */
> > @@ -357,12 +351,6 @@ void i915_request_retire_upto(struct i915_request *rq)
> > } while (i915_request_retire(tmp) && tmp != rq);
> > }
> >
> > -static void __llist_add(struct llist_node *node, struct llist_head *head)
> > -{
> > - node->next = head->first;
> > - head->first = node;
> > -}
> > -
> > static struct i915_request * const *
> > __engine_active(struct intel_engine_cs *engine)
> > {
> > @@ -439,18 +427,11 @@ __await_execution(struct i915_request *rq,
> > cb->work.func = irq_execute_cb_hook;
> > }
> >
> > - spin_lock_irq(&signal->lock);
> > - if (i915_request_is_active(signal) || __request_in_flight(signal)) {
> > - if (hook) {
> > - hook(rq, &signal->fence);
> > - i915_request_put(signal);
> > - }
> > - i915_sw_fence_complete(cb->fence);
> > - kmem_cache_free(global.slab_execute_cbs, cb);
> > - } else {
> > - __llist_add(&cb->work.llnode, &signal->execute_cb);
> > + if (llist_add(&cb->work.llnode, &signal->execute_cb)) {
> > + if (i915_request_is_active(signal) ||
> > + __request_in_flight(signal))
> > + __notify_execute_cb(signal);
>
> Any reason why the hook couldn't be called straight away but needs to
> always go through the worker now?
>
> Maybe it would be easier to figure out if it is race free that way..
>
> if (llist_add(..)) {
> llist_for_each_entry_safe(.., llist_del_all(..), .)
Then you would tell me off for open coding __notify_execute_cb for the
benefit of not going through the irq_work. Something like
@@ -186,7 +186,7 @@ static void irq_execute_cb_hook(struct irq_work *wrk)
irq_execute_cb(wrk);
}
-static void __notify_execute_cb(struct i915_request *rq)
+static void __execute_cb_irq(struct i915_request *rq)
{
struct execute_cb *cb, *cn;
@@ -209,6 +209,15 @@ static void __notify_execute_cb(struct i915_request *rq)
*/
}
+static void __execute_cb_imm(struct i915_request *rq)
+{
+ struct execute_cb *cb, *cn;
+
+ llist_for_each_entry_safe(cb, cn,
+ llist_del_all(&rq->execute_cb), work.llnode)
+ cb->work.func(&cb->work);
+}
+
static inline void
remove_from_client(struct i915_request *request)
{
@@ -323,7 +332,7 @@ bool i915_request_retire(struct i915_request *rq)
atomic_dec(&rq->engine->gt->rps.num_waiters);
}
spin_unlock_irq(&rq->lock);
- __notify_execute_cb(rq);
+ __execute_cb_imm(rq);
remove_from_client(rq);
__list_del_entry(&rq->link); /* poison neither prev/next (RCU walks) */
@@ -431,7 +440,7 @@ __await_execution(struct i915_request *rq,
if (llist_add(&cb->work.llnode, &signal->execute_cb)) {
if (i915_request_is_active(signal) ||
__request_in_flight(signal))
- __notify_execute_cb(signal);
+ __execute_cb_imm(signal);
}
return 0;
@@ -547,7 +556,7 @@ bool __i915_request_submit(struct i915_request *request)
list_move_tail(&request->sched.link, &engine->active.requests);
clear_bit(I915_FENCE_FLAG_PQUEUE, &request->fence.flags);
}
- __notify_execute_cb(request);
+ __execute_cb_irq(request);
/* We may be recursing from the signal callback of another i915 fence */
if (!i915_request_signaled(request)) {
-Chris
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