[Intel-gfx] [PATCH v5] drm/i915/icl: Preempt-to-idle support in execlists.

[Joonas Lahtinen]

Quoting Lis, Tomasz (2018-10-19 19:00:15)
> 
> 
> On 2018-10-16 12:53, Joonas Lahtinen wrote:
> > Quoting Tomasz Lis (2018-10-15 20:29:18)
> >> The patch adds support of preempt-to-idle requesting by setting a proper
> >> bit within Execlist Control Register, and receiving preemption result from
> >> Context Status Buffer.
> >>
> >> Preemption in previous gens required a special batch buffer to be executed,
> >> so the Command Streamer never preempted to idle directly. In Icelake it is
> >> possible, as there is a hardware mechanism to inform the kernel about
> >> status of the preemption request.
> >>
> >> This patch does not cover using the new preemption mechanism when GuC is
> >> active.
> >>
> >> v2: Added needs_preempt_context() change so that it is not created when
> >>      preempt-to-idle is supported. (Chris)
> >>      Updated setting HWACK flag so that it is cleared after
> >>      preempt-to-dle. (Chris, Daniele)
> >>      Updated to use I915_ENGINE_HAS_PREEMPTION flag. (Chris)
> >>
> >> v3: Fixed needs_preempt_context() change. (Chris)
> >>      Merged preemption trigger functions to one. (Chris)
> >>      Fixed conyext state tonot assume COMPLETED_MASK after preemption,
> >>      since idle-to-idle case will not have it set.
> >>
> >> v4: Simplified needs_preempt_context() change. (Daniele)
> >>      Removed clearing HWACK flag in idle-to-idle preempt. (Daniele)
> >>
> >> v5: Renamed inject_preempt_context(). (Daniele)
> >>      Removed duplicated GEM_BUG_ON() on HWACK (Daniele)
> >>
> >> Bspec: 18922
> >> Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> >> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> >> Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio at intel.com>
> >> Cc: Michal Winiarski <michal.winiarski at intel.com>
> >> Cc: Mika Kuoppala <mika.kuoppala at intel.com>
> >> Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio at intel.com>
> > This R-b was on v4, and should be indicated with # v4 comment.
> >
> > The commit message doesn't say much about why preempting to idle is
> > beneficial? The pre-Gen11 codepath needs to be maintained anyway.
> >
> > Regards, Joonas
> The benefit is one less context switch - there is no "preempt context".

Yes.

But that still doesn't quite explain what material benefits there are? :)

Is there some actual workloads/microbenchmarks that get an improvement?

This alters the behavior between different platforms for a very delicate
feature, probably resulting in slightly different bugs. So there should
be some more reasoning than just because we can.

Regards, Joonas