[Intel-gfx] [PATCH 08/31] drm/i915: Move rate-limiting request retire to after submission

[Chris Wilson]

Quoting Tvrtko Ursulin (2018-06-27 11:57:39)
> 
> On 25/06/2018 10:48, Chris Wilson wrote:
> > Our long standing defense against a single client from flooding the
> > system with requests (causing mempressure and stalls across the whole
> > system) is to retire the old request on every allocation. (By retiring
> > the oldest, we try to keep returning requests back to the system in a
> > steady flow.) This adds an extra step into the submission path that we
> > can reduce simply by moving it to after the submission itself.
> > 
> > We already do try to clean up a stale request list after submission, so
> > always retiring all completed requests fits in as a natural extension.
> > 
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> > ---
> >   drivers/gpu/drm/i915/i915_request.c | 26 ++++++++++++++++++--------
> >   1 file changed, 18 insertions(+), 8 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
> > index e1dbb544046f..e6e5eea87629 100644
> > --- a/drivers/gpu/drm/i915/i915_request.c
> > +++ b/drivers/gpu/drm/i915/i915_request.c
> > @@ -694,12 +694,6 @@ i915_request_alloc(struct intel_engine_cs *engine, struct i915_gem_context *ctx)
> >       if (ret)
> >               goto err_unreserve;
> >   
> > -     /* Move our oldest request to the slab-cache (if not in use!) */
> > -     rq = list_first_entry(&ce->ring->request_list, typeof(*rq), ring_link);
> > -     if (!list_is_last(&rq->ring_link, &ce->ring->request_list) &&
> > -         i915_request_completed(rq))
> > -             i915_request_retire(rq);
> > -
> >       /*
> >        * Beware: Dragons be flying overhead.
> >        *
> > @@ -1110,6 +1104,8 @@ void i915_request_add(struct i915_request *request)
> >       local_bh_enable(); /* Kick the execlists tasklet if just scheduled */
> >   
> >       /*
> > +      * Move our oldest requests to the slab-cache (if not in use!)
> > +      *
> >        * In typical scenarios, we do not expect the previous request on
> >        * the timeline to be still tracked by timeline->last_request if it
> >        * has been completed. If the completed request is still here, that
> > @@ -1126,8 +1122,22 @@ void i915_request_add(struct i915_request *request)
> >        * work on behalf of others -- but instead we should benefit from
> >        * improved resource management. (Well, that's the theory at least.)
> >        */
> > -     if (prev && i915_request_completed(prev))
> > -             i915_request_retire_upto(prev);
> > +     do {
> > +             prev = list_first_entry(&ring->request_list,
> > +                                     typeof(*prev), ring_link);
> > +
> > +             /*
> > +              * Keep the current request, the caller may not be
> > +              * expecting it to be retired (and freed!) immediately,
> > +              * and preserving one request from the client allows us to
> > +              * carry forward frequently reused state onto the next
> > +              * submission.
> > +              */
> > +             if (prev == request || !i915_request_completed(prev))
> > +                     break;
> > +
> > +             i915_request_retire(prev);
> > +     } while (1);
> 
> Maybe new helper i915_request_try_retire_upto(prev)?

try_retire_before() I'm just feeling confusion in the name. Not yet
sold, and certainly don't want to invite more users :)

> >   static unsigned long local_clock_us(unsigned int *cpu)
> > 
> 
> Cost benefit? Is it really so interesting to keep tweaking this? I feel 
> like I can stamp an r-b with the "yeah whatever" approach.. but the 
> commit doesn't say what we gain to explain why it is useful to spend 
> time reviewing it.

The true cost was the contention the earlier retirement was causing with
the still inflight ELSP. The cost of that contention is less with the
current series, but the implication was made.
-Chris