Quoting Tvrtko Ursulin (2020-05-12 16:17:30)
>
> On 12/05/2020 14:22, Chris Wilson wrote:
> > The second try at staging the transfer of the breadcrumb. In part one,
> > we realised we could not simply move to the second engine as we were
> > only holding the breadcrumb lock on the first. So in commit 6c81e21a4742
> > ("drm/i915/gt: Stage the transfer of the virtual breadcrumb"), we
> > removed it from the first engine and marked up this request to reattach
> > the signaling on the new engine. However, this failed to take into
> > account that we only attach the breadcrumb if the new request is added
> > at the start of the queue, which if we are transferring, it is because
> > we know there to be a request to be signaled (and hence we would not be
> > attached).
> >
> > In this attempt, we try to transfer the completed requests to the
> > irq_worker on its rq->engine->breadcrumbs. This preserves the coupling
> > between the rq and its breadcrumbs, so that
> > i915_request_cancel_breadcrumb() does not attempt to manipulate the list
> > under the wrong lock.
> >
> > Fixes: 6c81e21a4742 ("drm/i915/gt: Stage the transfer of the virtual breadcrumb")
> > Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> > ---
> > v2: rewrite from scratch with a new idea
> > ---
> > drivers/gpu/drm/i915/gt/intel_breadcrumbs.c | 33 ++++++++++++++++++++
> > drivers/gpu/drm/i915/gt/intel_engine.h | 3 ++
> > drivers/gpu/drm/i915/gt/intel_engine_types.h | 2 ++
> > drivers/gpu/drm/i915/gt/intel_lrc.c | 26 ++-------------
> > 4 files changed, 41 insertions(+), 23 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
> > index cbedba857d43..e09dc162b508 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
> > +++ b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
> > @@ -155,6 +155,8 @@ static void signal_irq_work(struct irq_work *work)
> > if (b->irq_armed && list_empty(&b->signalers))
> > __intel_breadcrumbs_disarm_irq(b);
> >
> > + list_splice_init(&b->signaled_requests, &signal);
> > +
> > list_for_each_entry_safe(ce, cn, &b->signalers, signal_link) {
> > GEM_BUG_ON(list_empty(&ce->signals));
> >
> > @@ -255,6 +257,7 @@ void intel_engine_init_breadcrumbs(struct intel_engine_cs *engine)
> >
> > spin_lock_init(&b->irq_lock);
> > INIT_LIST_HEAD(&b->signalers);
> > + INIT_LIST_HEAD(&b->signaled_requests);
> >
> > init_irq_work(&b->irq_work, signal_irq_work);
> > }
> > @@ -274,6 +277,36 @@ void intel_engine_reset_breadcrumbs(struct intel_engine_cs *engine)
> > spin_unlock_irqrestore(&b->irq_lock, flags);
> > }
> >
> > +void intel_engine_transfer_breadcrumbs(struct intel_engine_cs *engine,
> > + struct intel_context *ce)
> > +{
> > + struct intel_breadcrumbs *b = &engine->breadcrumbs;
> > + unsigned long flags;
> > +
> > + spin_lock_irqsave(&b->irq_lock, flags);
> > + if (!list_empty(&ce->signals)) {
> > + struct i915_request *rq, *next;
> > +
> > + list_for_each_entry_safe(rq, next, &ce->signals, signal_link) {
> > + GEM_BUG_ON(rq->engine != engine);
> > + GEM_BUG_ON(!i915_request_completed(rq));
>
> Do you remember why the breadcrumbs code uses local __request_completed
> helper?
It knows the hwsp isn't going to disappear, we know the same here, just
this code was first written in intel_lrc.c
Fwiw, the rcu_read_lock() we have in i915_request_completed() is one of
our worst lockdep hotspots
> From here vvv
>
> > +
> > + clear_bit(I915_FENCE_FLAG_SIGNAL, &rq->fence.flags);
> > + if (!__dma_fence_signal(&rq->fence))
> > + continue;
> > +
> > + i915_request_get(rq);
> > + list_add_tail(&rq->signal_link, &b->signaled_requests);
>
> ^^^ to here looks like a block which could be shared with signal_irq_work.
And not even a suggestion of a function name.
> > + }
> > +
> > + INIT_LIST_HEAD(&ce->signals);
>
> Hm because list_add and not list_move you can't assert all have been
> unlinked.
Which is the point, we don't want to have to repeatedly do the same
unlinks when we can do them en masse.
> > + list_del_init(&ce->signal_link);
> > +
> > + irq_work_queue(&b->irq_work);
> > + }
> > + spin_unlock_irqrestore(&b->irq_lock, flags);
> > +}
> > +
> > void intel_engine_fini_breadcrumbs(struct intel_engine_cs *engine)
> > {
> > }
> > diff --git a/drivers/gpu/drm/i915/gt/intel_engine.h b/drivers/gpu/drm/i915/gt/intel_engine.h
> > index cb789c8bf06b..45418f887953 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_engine.h
> > +++ b/drivers/gpu/drm/i915/gt/intel_engine.h
> > @@ -238,6 +238,9 @@ intel_engine_signal_breadcrumbs(struct intel_engine_cs *engine)
> > void intel_engine_reset_breadcrumbs(struct intel_engine_cs *engine);
> > void intel_engine_fini_breadcrumbs(struct intel_engine_cs *engine);
> >
> > +void intel_engine_transfer_breadcrumbs(struct intel_engine_cs *engine,
> > + struct intel_context *ce);
> > +
> > void intel_engine_print_breadcrumbs(struct intel_engine_cs *engine,
> > struct drm_printer *p);
> >
> > diff --git a/drivers/gpu/drm/i915/gt/intel_engine_types.h b/drivers/gpu/drm/i915/gt/intel_engine_types.h
> > index c113b7805e65..e20b39eefd79 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_engine_types.h
> > +++ b/drivers/gpu/drm/i915/gt/intel_engine_types.h
> > @@ -377,6 +377,8 @@ struct intel_engine_cs {
> > spinlock_t irq_lock;
> > struct list_head signalers;
> >
> > + struct list_head signaled_requests;
> > +
> > struct irq_work irq_work; /* for use from inside irq_lock */
> >
> > unsigned int irq_enabled;
> > diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
> > index 15716e4d6b76..ac32d494b07d 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
> > +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
> > @@ -1821,30 +1821,10 @@ static bool virtual_matches(const struct virtual_engine *ve,
> > return true;
> > }
> >
> > -static void virtual_xfer_breadcrumbs(struct virtual_engine *ve,
> > - struct i915_request *rq)
> > +static void virtual_xfer_breadcrumbs(struct virtual_engine *ve)
> > {
> > - struct intel_engine_cs *old = ve->siblings[0];
> > -
> > /* All unattached (rq->engine == old) must already be completed */
>
> This comments feels a bit out of place now.
It's still true, just phrasing is hard.
> > -
> > - spin_lock(&old->breadcrumbs.irq_lock);
> > - if (!list_empty(&ve->context.signal_link)) {
> > - list_del_init(&ve->context.signal_link);
> > -
> > - /*
> > - * We cannot acquire the new engine->breadcrumbs.irq_lock
> > - * (as we are holding a breadcrumbs.irq_lock already),
> > - * so attach this request to the signaler on submission.
> > - * The queued irq_work will occur when we finally drop
> > - * the engine->active.lock after dequeue.
> > - */
> > - set_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &rq->fence.flags);
> > -
> > - /* Also transfer the pending irq_work for the old breadcrumb. */
> > - intel_engine_signal_breadcrumbs(rq->engine);
> > - }
> > - spin_unlock(&old->breadcrumbs.irq_lock);
> > + intel_engine_transfer_breadcrumbs(ve->siblings[0], &ve->context);
>
> But isn't ve->siblings[0] the old engine at this point so new target
> engine would have to be explicitly passed in?
ve->siblings[0] is the old engine, which is holding the completed
requests and their signals. Since their rq->engine == ve->siblings[0]
and we can't update rq->engine as we can't take the required locks, we
need to keep the breadcrumbs relative to ve->siblings[0] and not the new
engine (the i915_request_cancel_breadcrumb conundrum).
-Chris
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