Re: [Intel-gfx] [PATCH 05/41] drm/i915: Restructure priority inheritance

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Tvrtko Ursulin (2021-01-26 11:12:53)
> 
> 
> On 25/01/2021 14:01, Chris Wilson wrote:
> > +static void ipi_schedule(struct work_struct *wrk)
> > +{
> > +     struct i915_sched_ipi *ipi = container_of(wrk, typeof(*ipi), work);
> > +     struct i915_request *rq = xchg(&ipi->list, NULL);
> > +
> > +     do {
> > +             struct i915_request *rn = xchg(&rq->sched.ipi_link, NULL);
> > +             int prio;
> > +
> > +             prio = ipi_get_prio(rq);
> > +
> > +             /*
> > +              * For cross-engine scheduling to work we rely on one of two
> > +              * things:
> > +              *
> > +              * a) The requests are using dma-fence fences and so will not
> > +              * be scheduled until the previous engine is completed, and
> > +              * so we cannot cross back onto the original engine and end up
> > +              * queuing an earlier request after the first (due to the
> > +              * interrupted DFS).
> > +              *
> > +              * b) The requests are using semaphores and so may be already
> > +              * be in flight, in which case if we cross back onto the same
> > +              * engine, we will already have put the interrupted DFS into
> > +              * the priolist, and the continuation will now be queued
> > +              * afterwards [out-of-order]. However, since we are using
> > +              * semaphores in this case, we also perform yield on semaphore
> > +              * waits and so will reorder the requests back into the correct
> > +              * sequence. This occurrence (of promoting a request chain
> > +              * that crosses the engines using semaphores back unto itself)
> > +              * should be unlikely enough that it probably does not matter...
> > +              */
> > +             local_bh_disable();
> > +             i915_request_set_priority(rq, prio);
> > +             local_bh_enable();
> 
> Is it that important and wouldn't the priority order restore eventually 
> due timeslicing?

There would be a window in which we executed userspace code
out-of-order. That's enough to scare me! However, for our PI dependency
chains it should not matter as the only time we do submit out-of-order,
we are stuck on _our_ semaphore that cannot be resolved until the
requests are back in-order.

I've tried to trick this into causing problems with the
i915_selftest/igt_schedule_cycle and gem_exec_schedule/noreorder.
Fortunately for my sanity, neither test have caught any problems.

This is the handwaving part of removing the global lock.

> > +     /*
> > +      * If we are setting the priority before being submitted, see if we
> > +      * can quickly adjust our own priority in-situ and avoid taking
> > +      * the contended engine->active.lock. If we need priority inheritance,
> > +      * take the slow route.
> > +      */
> > +     if (rq_prio(rq) == I915_PRIORITY_INVALID) {
> > +             struct i915_dependency *p;
> > +
> > +             rcu_read_lock();
> > +             for_each_signaler(p, rq) {
> > +                     struct i915_request *s =
> > +                             container_of(p->signaler, typeof(*s), sched);
> > +
> > +                     if (rq_prio(s) >= prio)
> > +                             continue;
> > +
> > +                     if (__i915_request_is_complete(s))
> > +                             continue;
> > +
> > +                     break;
> > +             }
> > +             rcu_read_unlock();
> 
> Exit this loop with a first lower priority incomplete signaler. What 
> does the block below then do? Feels like it needs a comment.

I thought I had sufficiently explained that in the comment above.

/* Update priority in place if no PI required */
> > +             if (&p->signal_link == &rq->sched.signalers_list &&
> > +                 cmpxchg(&rq->sched.attr.priority,
> > +                         I915_PRIORITY_INVALID,
> > +                         prio) == I915_PRIORITY_INVALID)
> > +                     return;

It could do a few more tricks to change the priority in-place a second
time, but I did not think that would be frequent enough to matter.
Whereas we always adjust the priority from INVALID once before
submission, and avoiding taking the lock then does make a difference to
the profiles.
-Chris
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