Quoting Tvrtko Ursulin (2020-01-14 13:48:11)
>
> On 14/01/2020 11:20, Chris Wilson wrote:
> > Quoting Tvrtko Ursulin (2020-01-14 11:13:06)
> >> On 13/01/2020 10:44, Chris Wilson wrote:
> >>> Since commit 422d7df4f090 ("drm/i915: Replace engine->timeline with a
> >>> plain list"), we used the default embedded priotree slot for the virtual
> >>> engine request queue, which means we can also use the same solitary slot
> >>> with the scheduler. However, the priolist is expected to be guarded by
> >>> the engine->active.lock, but this is not true for the virtual engine
> >>>
> >>> References: 422d7df4f090 ("drm/i915: Replace engine->timeline with a plain list")
> >>> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> >>> Cc: Mika Kuoppala <mika.kuoppala@xxxxxxxxxxxxxxx>
> >>> Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> >>> ---
> >>> drivers/gpu/drm/i915/gt/intel_lrc.c | 3 +++
> >>> drivers/gpu/drm/i915/i915_request.c | 4 +++-
> >>> drivers/gpu/drm/i915/i915_request.h | 16 ++++++++++++++++
> >>> drivers/gpu/drm/i915/i915_scheduler.c | 3 +--
> >>> 4 files changed, 23 insertions(+), 3 deletions(-)
> >>>
> >>> diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
> >>> index a6ac37dece0a..685659f079a2 100644
> >>> --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
> >>> +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
> >>> @@ -985,6 +985,8 @@ __unwind_incomplete_requests(struct intel_engine_cs *engine)
> >>> GEM_BUG_ON(RB_EMPTY_ROOT(&engine->execlists.queue.rb_root));
> >>>
> >>> list_move(&rq->sched.link, pl);
> >>> + set_bit(I915_FENCE_FLAG_PQUEUE, &rq->fence.flags);
> >>> +
> >>> active = rq;
> >>> } else {
> >>> struct intel_engine_cs *owner = rq->context->engine;
> >>> @@ -2473,6 +2475,7 @@ static void execlists_submit_request(struct i915_request *request)
> >>> spin_lock_irqsave(&engine->active.lock, flags);
> >>>
> >>> queue_request(engine, &request->sched, rq_prio(request));
> >>> + set_bit(I915_FENCE_FLAG_PQUEUE, &request->fence.flags);
> >>
> >> Move into queue_request so it is closer to priolist management, just like at other call sites?
> >>
> >> Also, these are all under the engine active lock so non-atomic set/clear could be used, no?
> >
> > It's not the bit that is important, but if there may be any other
> > concurrent access to the dword.
> >
> > Thread A: Thread B:
> > __set_bit(0, &rq->flags) __set_bit(31, &rq->flags)
> >
> > *does* cause an issue, speaking from sad experience. So if in doubt, and
> > here there's always doubt with preempt-to-busy and background signaling,
> > go atomic.
> >
> >>
> >>>
> >>> GEM_BUG_ON(RB_EMPTY_ROOT(&engine->execlists.queue.rb_root));
> >>> GEM_BUG_ON(list_empty(&request->sched.link));
> >>> diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
> >>> index be185886e4fc..9ed0d3bc7249 100644
> >>> --- a/drivers/gpu/drm/i915/i915_request.c
> >>> +++ b/drivers/gpu/drm/i915/i915_request.c
> >>> @@ -408,8 +408,10 @@ bool __i915_request_submit(struct i915_request *request)
> >>> xfer: /* We may be recursing from the signal callback of another i915 fence */
> >>> spin_lock_nested(&request->lock, SINGLE_DEPTH_NESTING);
> >>>
> >>> - if (!test_and_set_bit(I915_FENCE_FLAG_ACTIVE, &request->fence.flags))
> >>> + if (!test_and_set_bit(I915_FENCE_FLAG_ACTIVE, &request->fence.flags)) {
> >>> list_move_tail(&request->sched.link, &engine->active.requests);
> >>> + clear_bit(I915_FENCE_FLAG_PQUEUE, &request->fence.flags);
> >>> + }
> >>>
> >>> if (test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &request->fence.flags) &&
> >>> !test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &request->fence.flags) &&
> >>> diff --git a/drivers/gpu/drm/i915/i915_request.h b/drivers/gpu/drm/i915/i915_request.h
> >>> index 031433691a06..f3e50ec989b8 100644
> >>> --- a/drivers/gpu/drm/i915/i915_request.h
> >>> +++ b/drivers/gpu/drm/i915/i915_request.h
> >>> @@ -70,6 +70,17 @@ enum {
> >>> */
> >>> I915_FENCE_FLAG_ACTIVE = DMA_FENCE_FLAG_USER_BITS,
> >>>
> >>> + /*
> >>> + * I915_FENCE_FLAG_PQUEUE - this request is ready for execution
> >>> + *
> >>> + * Using the scheduler, when a request is ready for execution it is put
> >>> + * into the priority queue. We want to track its membership within that
> >>> + * queue so that we can easily check before rescheduling.
> >>> + *
> >>> + * See i915_request_in_priority_queue()
> >>> + */
> >>> + I915_FENCE_FLAG_PQUEUE,
> >>> +
> >>> /*
> >>> * I915_FENCE_FLAG_SIGNAL - this request is currently on signal_list
> >>> *
> >>> @@ -361,6 +372,11 @@ static inline bool i915_request_is_active(const struct i915_request *rq)
> >>> return test_bit(I915_FENCE_FLAG_ACTIVE, &rq->fence.flags);
> >>> }
> >>>
> >>> +static inline bool i915_request_in_priority_queue(const struct i915_request *rq)
> >>> +{
> >>> + return test_bit(I915_FENCE_FLAG_PQUEUE, &rq->fence.flags);
> >>> +}
> >>> +
> >>> /**
> >>> * Returns true if seq1 is later than seq2.
> >>> */
> >>> diff --git a/drivers/gpu/drm/i915/i915_scheduler.c b/drivers/gpu/drm/i915/i915_scheduler.c
> >>> index bf87c70bfdd9..4f6e4d6c590a 100644
> >>> --- a/drivers/gpu/drm/i915/i915_scheduler.c
> >>> +++ b/drivers/gpu/drm/i915/i915_scheduler.c
> >>> @@ -338,8 +338,7 @@ static void __i915_schedule(struct i915_sched_node *node,
> >>> continue;
> >>> }
> >>>
> >>> - if (!intel_engine_is_virtual(engine) &&
> >>> - !i915_request_is_active(node_to_request(node))) {
> >>> + if (i915_request_in_priority_queue(node_to_request(node))) {
> >>
> >> Not shown in this diff before this if block we have:
> >>
> >> if (list_empty(&node->link)) {
> >> /*
> >> * If the request is not in the priolist queue because
> >> * it is not yet runnable, then it doesn't contribute
> >> * to our preemption decisions. On the other hand,
> >> * if the request is on the HW, it too is not in the
> >> * queue; but in that case we may still need to reorder
> >> * the inflight requests.
> >
> > This second sentence is obsolete, we now use node->link for active.
>
> Which active? I915_FENCE_FLAG_ACTIVE is set when request is not on the
> priority queue any more, no?
In the next patch, we introduce an inbetween state, ready but not active
and not in the queue.
>
> >
> >> */
> >> continue;
> >> }
> >>
> >> What is the difference between list_empty(&node->link) and !i915_request_in_priority_queue?
> >
> > list_empty() -> prior to being ready, we will put into the plist upon
> > submit_request()
> >
> > Once ready, we only want to fiddle with its place in the priority lists,
> > if it is in the plist.
>
> Yes brain fart on the list_empty check.
>
> However I need to go a step back and ask what is the whole point of this
> block:
>
> if (!intel_engine_is_virtual(engine) &&
> !i915_request_is_active(node_to_request(node))) {
>
> For active request, they are already on the hw so no need to push them
> up. But I forgot why are virtual ones special? VE is single context so
> in order, or in other words no need to track prio levels for it, is that it?
Yeah, it's because the lookup_prio didn't used work for virtual as it
didn't have the tree.
> But then also commit says "Use common priotree lists for virtual
> engine". It already uses that before the patch because of the
> !intel_engine_is_virtual condition. So I am confused.
First iteration of this patch tweaked it so this block worked on virtual
with no hassle. Then I realised I had a problem with on-hold requests,
so took the patch a step further.
-Chris
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