[Intel-gfx] [PATCH 15/40] drm/i915: Priority boost for new clients

[Chris Wilson]

Quoting Tvrtko Ursulin (2018-09-24 11:29:52)
> 
> On 19/09/2018 20:55, Chris Wilson wrote:
> > Taken from an idea used for FQ_CODEL, we give the first request of a
> > new request flows a small priority boost. These flows are likely to
> > correspond with short, interactive tasks and so be more latency sensitive
> > than the longer free running queues. As soon as the client has more than
> > one request in the queue, further requests are not boosted and it settles
> > down into ordinary steady state behaviour.  Such small kicks dramatically
> > help combat the starvation issue, by allowing each client the opportunity
> > to run even when the system is under heavy throughput load (within the
> > constraints of the user selected priority).
> > 
> > v2: Mark the preempted request as the start of a new flow, to prevent a
> > single client being continually gazumped by its peers.
> > 
> > Testcase: igt/benchmarks/rrul
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> > Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> > ---
> >   drivers/gpu/drm/i915/i915_request.c   | 16 ++++++++++++++--
> >   drivers/gpu/drm/i915/i915_scheduler.h |  4 +++-
> >   drivers/gpu/drm/i915/intel_lrc.c      | 25 +++++++++++++++++++------
> >   3 files changed, 36 insertions(+), 9 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
> > index a492385b2089..56140ca054e8 100644
> > --- a/drivers/gpu/drm/i915/i915_request.c
> > +++ b/drivers/gpu/drm/i915/i915_request.c
> > @@ -1127,8 +1127,20 @@ void i915_request_add(struct i915_request *request)
> >        */
> >       local_bh_disable();
> >       rcu_read_lock(); /* RCU serialisation for set-wedged protection */
> > -     if (engine->schedule)
> > -             engine->schedule(request, &request->gem_context->sched);
> > +     if (engine->schedule) {
> > +             struct i915_sched_attr attr = request->gem_context->sched;
> > +
> > +             /*
> > +              * Boost priorities to new clients (new request flows).
> > +              *
> > +              * Allow interactive/synchronous clients to jump ahead of
> > +              * the bulk clients. (FQ_CODEL)
> > +              */
> > +             if (!prev || i915_request_completed(prev))
> > +                     attr.priority |= I915_PRIORITY_NEWCLIENT;
> > +
> > +             engine->schedule(request, &attr);
> > +     }
> >       rcu_read_unlock();
> >       i915_sw_fence_commit(&request->submit);
> >       local_bh_enable(); /* Kick the execlists tasklet if just scheduled */
> > diff --git a/drivers/gpu/drm/i915/i915_scheduler.h b/drivers/gpu/drm/i915/i915_scheduler.h
> > index 7edfad0abfd7..93e43e263d8c 100644
> > --- a/drivers/gpu/drm/i915/i915_scheduler.h
> > +++ b/drivers/gpu/drm/i915/i915_scheduler.h
> > @@ -19,12 +19,14 @@ enum {
> >       I915_PRIORITY_INVALID = INT_MIN
> >   };
> >   
> > -#define I915_USER_PRIORITY_SHIFT 0
> > +#define I915_USER_PRIORITY_SHIFT 1
> >   #define I915_USER_PRIORITY(x) ((x) << I915_USER_PRIORITY_SHIFT)
> >   
> >   #define I915_PRIORITY_COUNT BIT(I915_USER_PRIORITY_SHIFT)
> >   #define I915_PRIORITY_MASK (-I915_PRIORITY_COUNT)
> >   
> > +#define I915_PRIORITY_NEWCLIENT      ((u8)BIT(0))
> 
> Is the cast important and why?

Unreliable memory says there was something iffy about the code generation
at one point.
-Chris