[Intel-gfx] [PATCH] drm/i915: Remove second level open-coded rcu work

[Chris Wilson]

Quoting Tvrtko Ursulin (2019-02-28 09:55:46)
> 
> On 28/02/2019 09:43, Chris Wilson wrote:
> > Quoting Tvrtko Ursulin (2019-02-28 09:38:36)
> >>
> >> On 28/02/2019 08:29, Chris Wilson wrote:
> >>> @@ -143,7 +141,15 @@ static u32 __i915_gem_park(struct drm_i915_private *i915)
> >>>    
> >>>        intel_display_power_put(i915, POWER_DOMAIN_GT_IRQ, wakeref);
> >>>    
> >>> -     return i915->gt.epoch;
> >>> +     /*
> >>> +      * When we are idle, it is an opportune time to reap our caches.
> >>> +      * However, we have many objects that utilise RCU and the ordered
> >>> +      * i915->wq that this work is executing on. To try and flush any
> >>> +      * pending frees now we are idle, we first wait for an RCU grace
> >>> +      * period, and then queue a task (that will run last on the wq) to
> >>> +      * shrink and re-optimize the caches.
> >>> +      */
> >>> +     i915_globals_park();
> >>
> >> I think this comment would be better placed in i915_globals_park.
> > 
> > In i915_globals_park(), we have
> > 
> >          /*
> >           * Defer shrinking the global slab caches (and other work) until
> >           * after a RCU grace period has completed with no activity. This
> >           * is to try and reduce the latency impact on the consumers caused
> >           * by us shrinking the caches the same time as they are trying to
> >           * allocate, with the assumption being that if we idle long enough
> >           * for an RCU grace period to elapse since the last use, it is likely
> >           * to be longer until we need the caches again.
> >           */
> > 
> > Yeah, the old comment is a bit too verbose now, but I think it's still
> > worth having a comment there to explain what's about to be done and why
> > we call it now.
> > 
> > /* Tell the world we are idle and reap the benefits! */
> > 
> > Too subtle?
> 
> Maybe even no comment needed (gasp!). When I looks at the function I see:
> 
> __i915_gem_park(...)
> {
>         ...
>         intel_engines_park(i915);
>         i915_timelines_park(i915);
> 
>         i915_pmu_gt_parked(i915);
>         i915_vma_parked(i915);
>         ...
>         i915_globals_park();
>         ...
> }
> 
> So it seems pretty obvious different components are parked from here. :)

No comment. ;)
-Chris