Quoting Tvrtko Ursulin (2020-05-18 15:55:46)
>
> On 18/05/2020 14:00, Chris Wilson wrote:
> > Quoting Tvrtko Ursulin (2020-05-18 13:53:29)
> >>
> >> On 18/05/2020 09:14, Chris Wilson wrote:
> >>> Once a virtual engine has been bound to a sibling, it will remain bound
> >>> until we finally schedule out the last active request. We can not rebind
> >>> the context to a new sibling while it is inflight as the context save
> >>> will conflict, hence we wait. As we cannot then use any other sibliing
> >>> while the context is inflight, only kick the bound sibling while it
> >>> inflight and upon scheduling out the kick the rest (so that we can swap
> >>> engines on timeslicing if the previously bound engine becomes
> >>> oversubscribed).
> >>>
> >>> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> >>> ---
> >>> drivers/gpu/drm/i915/gt/intel_lrc.c | 30 +++++++++++++----------------
> >>> 1 file changed, 13 insertions(+), 17 deletions(-)
> >>>
> >>> diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
> >>> index 7a5ac3375225..fe8f3518d6b8 100644
> >>> --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
> >>> +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
> >>> @@ -1398,9 +1398,8 @@ execlists_schedule_in(struct i915_request *rq, int idx)
> >>> static void kick_siblings(struct i915_request *rq, struct intel_context *ce)
> >>> {
> >>> struct virtual_engine *ve = container_of(ce, typeof(*ve), context);
> >>> - struct i915_request *next = READ_ONCE(ve->request);
> >>>
> >>> - if (next == rq || (next && next->execution_mask & ~rq->execution_mask))
> >>> + if (READ_ONCE(ve->request))
> >>> tasklet_hi_schedule(&ve->base.execlists.tasklet);
> >>> }
> >>>
> >>> @@ -1821,18 +1820,14 @@ first_virtual_engine(struct intel_engine_cs *engine)
> >>> rb_entry(rb, typeof(*ve), nodes[engine->id].rb);
> >>> struct i915_request *rq = READ_ONCE(ve->request);
> >>>
> >>> - if (!rq) { /* lazily cleanup after another engine handled rq */
> >>> + /* lazily cleanup after another engine handled rq */
> >>> + if (!rq || !virtual_matches(ve, rq, engine)) {
> >>> rb_erase_cached(rb, &el->virtual);
> >>> RB_CLEAR_NODE(rb);
> >>> rb = rb_first_cached(&el->virtual);
> >>> continue;
> >>> }
> >>>
> >>> - if (!virtual_matches(ve, rq, engine)) {
> >>> - rb = rb_next(rb);
> >>> - continue;
> >>> - }
> >>> -
> >>> return ve;
> >>> }
> >>>
> >>> @@ -5478,7 +5473,6 @@ static void virtual_submission_tasklet(unsigned long data)
> >>> if (unlikely(!mask))
> >>> return;
> >>>
> >>> - local_irq_disable();
> >>> for (n = 0; n < ve->num_siblings; n++) {
> >>> struct intel_engine_cs *sibling = READ_ONCE(ve->siblings[n]);
> >>> struct ve_node * const node = &ve->nodes[sibling->id];
> >>> @@ -5488,20 +5482,19 @@ static void virtual_submission_tasklet(unsigned long data)
> >>> if (!READ_ONCE(ve->request))
> >>> break; /* already handled by a sibling's tasklet */
> >>>
> >>> + spin_lock_irq(&sibling->active.lock);
> >>> +
> >>> if (unlikely(!(mask & sibling->mask))) {
> >>> if (!RB_EMPTY_NODE(&node->rb)) {
> >>> - spin_lock(&sibling->active.lock);
> >>> rb_erase_cached(&node->rb,
> >>> &sibling->execlists.virtual);
> >>> RB_CLEAR_NODE(&node->rb);
> >>> - spin_unlock(&sibling->active.lock);
> >>> }
> >>> - continue;
> >>> - }
> >>>
> >>> - spin_lock(&sibling->active.lock);
> >>> + goto unlock_engine;
> >>> + }
> >>>
> >>> - if (!RB_EMPTY_NODE(&node->rb)) {
> >>> + if (unlikely(!RB_EMPTY_NODE(&node->rb))) {
> >>> /*
> >>> * Cheat and avoid rebalancing the tree if we can
> >>> * reuse this node in situ.
> >>> @@ -5541,9 +5534,12 @@ static void virtual_submission_tasklet(unsigned long data)
> >>> if (first && prio >= sibling->execlists.queue_priority_hint)
> >>> tasklet_hi_schedule(&sibling->execlists.tasklet);
> >>>
> >>> - spin_unlock(&sibling->active.lock);
> >>> +unlock_engine:
> >>> + spin_unlock_irq(&sibling->active.lock);
> >>> +
> >>> + if (intel_context_inflight(&ve->context))
> >>> + break;
> >>
> >> So virtual request may not be added to all siblings any longer. Will it
> >> still be able to schedule it on any if time slicing kicks in under these
> >> conditions?
> >
> > Yes.
> >
> >> This is equivalent to the hunk in first_virtual_engine which also
> >> removes it from all other siblings.
> >>
> >> I guess it's inline with what the commit messages says - that new
> >> sibling will be picked upon time slicing. I just don't quite see the
> >> path which would do it. Only path which shuffles the siblings array
> >> around is in dequeue, and dequeue on other that the engine which first
> >> picked it will not happen any more. I must be missing something..
> >
> > It's all on the execlists_schedule_out. During timeslicing we call
> > unwind_incomplete_requests which moves the requests back to the priotree
> > (and in this patch back to the virtual engine).
> >
> > But... We cannot use the virtual request on any other engine until it has
> > been scheduled out. That only happens after we complete execlists_dequeue()
> > and submit a new ELSP[]. Once the HW acks the change, we call
> > execlists_schedule_out on the virtual_request.
> >
> > Now we known that intel_context_inflight() will return false so any
> > engine can pick up the request, and so it's time to kick the virtual
> > tasklet and in turn kick all the siblings.
> >
> > So timeslicing works by not submitting the virtual request again and
> > when it expires on this sibling[0], we wake up all the other siblings
> > and the first that is idle wins the race.
>
> If a virtual request is on hw and timeslice expires:
>
> 1. Unwinds the request.
> -> kicks the virtual tasklet
> 2. Virtual tasklet runs and puts the request back on siblings.
> -> kicks the physical tasklets
> 3. Siblings tasklet runs and submits the request.
>
> So two tasklets latency even if no other runnable requests?
Yes. It's worse than that when you look at it. Since the other
execlists_submission_tasklet will run *before* we schedule out, they do
nothing as they cannot use the virtual request. So they will only
function when we kick them again after the schedule-out.
This was "bug" number 3 I mentioned you found with your tiny snippet.
In the next patch we optimise away the ineffective timeslice preemptions
of the virtual request. The inter-engine inefficiencies are inherent to
the system though -- we can only schedule the other engines once we have
scheduled out after the context save of the virtual engine.
-Chris
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