[Intel-gfx] [PATCH 2/3] drm/i915/gem: Treat submit-fence as weak dependency for new clients

[Chris Wilson]

Quoting Tvrtko Ursulin (2020-05-07 16:10:37)
> 
> On 07/05/2020 16:05, Chris Wilson wrote:
> > Quoting Tvrtko Ursulin (2020-05-07 15:59:56)
> >>
> >> On 07/05/2020 09:21, Chris Wilson wrote:
> >>> The submit-fence adds a weak dependency to the requests, and for the
> >>> purpose of our FQ_CODEL hinting we do not want to treat as a
> >>> restriction. This is primarily because clients may treat submit-fences
> >>> as a bidirectional bonding between a pair of co-ordinating requests.
> >>>
> >>> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> >>> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> >>> ---
> >>>    drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c | 13 ++++++++++++-
> >>>    1 file changed, 12 insertions(+), 1 deletion(-)
> >>>
> >>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> >>> index 966523a8503f..e8bf0cf02fd7 100644
> >>> --- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> >>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> >>> @@ -2565,6 +2565,17 @@ static void retire_requests(struct intel_timeline *tl, struct i915_request *end)
> >>>                        break;
> >>>    }
> >>>    
> >>> +static bool new_client(struct i915_request *rq)
> >>> +{
> >>> +     struct i915_dependency *p;
> >>> +
> >>> +     list_for_each_entry(p, &rq->sched.signalers_list, signal_link)
> >>> +             if (!(p->flags & I915_DEPENDENCY_WEAK))
> >>> +                     return false;
> >>> +
> >>> +     return true;
> >>> +}
> >>> +
> >>>    static void eb_request_add(struct i915_execbuffer *eb)
> >>>    {
> >>>        struct i915_request *rq = eb->request;
> >>> @@ -2604,7 +2615,7 @@ static void eb_request_add(struct i915_execbuffer *eb)
> >>>                 * Allow interactive/synchronous clients to jump ahead of
> >>>                 * the bulk clients. (FQ_CODEL)
> >>>                 */
> >>> -             if (list_empty(&rq->sched.signalers_list))
> >>> +             if (new_client(rq))
> >>>                        attr.priority |= I915_PRIORITY_WAIT;
> >>>        } else {
> >>>                /* Serialise with context_close via the add_to_timeline */
> >>>
> >>
> >> Did absence of this have any functional effect? I hope not, but anyway:
> > 
> > Bah, I have a new test case where this WAIT bumping is still upsetting us.
> > 
> > I don't think I have any choice but to rip it out if we have timeslicing
> > enabled.
> > 
> > Would you prefer a complete remission of I915_PRIORITY_WAIT or keep it
> > under if (!intel_engine_has_timeslicing(rq->engine)) ?
> 
> Doesn't feel worthwhile to keep it for just BDW right?

No. There's ivb waiting in the rings (as I haven't figured out
preemption for it yet), but similarly just doesn't feel worth the
complications. :(
-Chris