[Mesa-dev] [PATCH] i965: Perform manual preemption checks between commands

[Chris Wilson]

Quoting Rafael Antognolli (2019-03-05 19:33:03)
> On Tue, Mar 05, 2019 at 09:40:20AM +0000, Chris Wilson wrote:
> > Not all commands support being preempted as they execute, and for those
> > make sure we at least check for being preempted before we start so as to
> > try and minimise the latency of whomever is more important than
> > ourselves.
> > 
> > Cc: Jari Tahvanainen <jari.tahvanainen at intel.com>,
> > Cc: Rafael Antognolli <rafael.antognolli at intel.com>
> > Cc: Kenneth Graunke <kenneth at whitecape.org>
> > ---
> > Always double check before you hit send.
> > ---
> >  src/mesa/drivers/dri/i965/brw_defines.h | 1 +
> >  src/mesa/drivers/dri/i965/brw_draw.c    | 7 +++++++
> >  2 files changed, 8 insertions(+)
> > 
> > diff --git a/src/mesa/drivers/dri/i965/brw_defines.h b/src/mesa/drivers/dri/i965/brw_defines.h
> > index 2729a54e144..ef71c556cca 100644
> > --- a/src/mesa/drivers/dri/i965/brw_defines.h
> > +++ b/src/mesa/drivers/dri/i965/brw_defines.h
> > @@ -1420,6 +1420,7 @@ enum brw_pixel_shader_coverage_mask_mode {
> >  
> >  #define MI_NOOP                              (CMD_MI | 0)
> >  
> > +#define MI_ARB_CHECK                 (CMD_MI | 0x5 << 23)
> >  #define MI_BATCH_BUFFER_END          (CMD_MI | 0xA << 23)
> >  
> >  #define MI_FLUSH                     (CMD_MI | (4 << 23))
> > diff --git a/src/mesa/drivers/dri/i965/brw_draw.c b/src/mesa/drivers/dri/i965/brw_draw.c
> > index d07349419cc..a04e334ffc4 100644
> > --- a/src/mesa/drivers/dri/i965/brw_draw.c
> > +++ b/src/mesa/drivers/dri/i965/brw_draw.c
> > @@ -196,6 +196,13 @@ brw_emit_prim(struct brw_context *brw,
> >     if (verts_per_instance == 0 && !prim->is_indirect && !xfb_obj)
> >        return;
> >  
> > +   /* If this object is not itself preemptible, check before we begin. */
> > +   if (!brw->object_preemption) {
> > +      BEGIN_BATCH(1);
> > +      OUT_BATCH(MI_ARB_CHECK);
> > +      ADVANCE_BATCH();
> > +   }
> > +
> 
> "The command streamer will preempt in the case arbitration is enabled,
> there is a pending execution list and this command is currently being
> parsed."
> 
> If there is a pending execution list, shouldn't we have been preempted
> already, since mid-batch preemption is supposed to be enabled?

No, it still only occurs on certain instructions, not every instruction
boundary.
-Chris