Re: [PATCH v2] drm/i915/gt: Transfer old virtual breadcrumbs to irq_worker

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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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