[Intel-gfx] [PATCH] drm/i915/perf: don't forget noa wait after oa config

[Chris Wilson]

Quoting Chris Wilson (2019-11-13 18:10:22)
> Quoting Lionel Landwerlin (2019-11-13 18:07:59)
> > On 13/11/2019 18:35, Chris Wilson wrote:
> > > Quoting Lionel Landwerlin (2019-11-13 15:46:39)
> > >> I'm observing incoherence metric values, changing from run to run.
> > >>
> > >> It appears the patches introducing noa wait & reconfiguration from
> > >> command stream switched places in the series multiple times during the
> > >> review. This lead to the dependency of one onto the order to go
> > >> missing...
> > > I don't think I dropped it; if I did my apologies. I do feel the
> > > egg-on-face for writing a selftest to verify that noa_wait does what you
> > > said it did, but completely missing that it went unused :)
> > >   
> > >> Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
> > >> Fixes: 15d0ace1f876 ("drm/i915/perf: execute OA configuration from command stream")
> > >> ---
> > >>   drivers/gpu/drm/i915/i915_perf.c | 9 +++++++--
> > >>   1 file changed, 7 insertions(+), 2 deletions(-)
> > >>
> > >> diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c
> > >> index 507236bd41ae..31e47ee23357 100644
> > >> --- a/drivers/gpu/drm/i915/i915_perf.c
> > >> +++ b/drivers/gpu/drm/i915/i915_perf.c
> > >> @@ -1870,7 +1870,7 @@ alloc_oa_config_buffer(struct i915_perf_stream *stream,
> > >>          config_length += num_lri_dwords(oa_config->mux_regs_len);
> > >>          config_length += num_lri_dwords(oa_config->b_counter_regs_len);
> > >>          config_length += num_lri_dwords(oa_config->flex_regs_len);
> > >> -       config_length++; /* MI_BATCH_BUFFER_END */
> > >> +       config_length += 3; /* MI_BATCH_BUFFER_START */
> > >>          config_length = ALIGN(sizeof(u32) * config_length, I915_GTT_PAGE_SIZE);
> > >>   
> > >>          obj = i915_gem_object_create_shmem(stream->perf->i915, config_length);
> > >> @@ -1895,7 +1895,12 @@ alloc_oa_config_buffer(struct i915_perf_stream *stream,
> > >>                               oa_config->flex_regs,
> > >>                               oa_config->flex_regs_len);
> > >>   
> > >> -       *cs++ = MI_BATCH_BUFFER_END;
> > >> +       /* Jump into the active wait. */
> > >> +       *cs++ = (INTEL_GEN(stream->perf->i915) < 8 ?
> > >> +                MI_BATCH_BUFFER_START :
> > >> +                MI_BATCH_BUFFER_START_GEN8);
> > >> +       *cs++ = i915_ggtt_offset(stream->noa_wait);
> > >> +       *cs++ = 0;
> > > Yikes, stream->noa_wait is unused.
> > >
> > > Hmm, the noa_wait doesn't have any arbitration points internally, so we
> > > probably do need to make it non-preemptable as well?
> > >
> > > With a rq->flags |= I915_REQUEST_NOPREEMPT in emit_oa_config,
> > > Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
> > >
> > > We need to wrap emit_oa_config() in a similar selftest and verify
> > > that a read of the oa regs are correct and that the TIMESTAMP indicates
> > > the appropriate delay before the read. If you feel bored.
> > > -Chris
> > 
> > 
> > As long as we wait long enough, it should be okay.
> > 
> > Why making it nopreempt?
> 
> Aiui, the batch buffer has no arbitration points so the delay may incur
> the wrath of the forced preemption. That is another request (of higher
> priority) wishing to run, but not being able to.

The alternative would be adding a MI_ARB_CHECK at the start of the loop
if you happy with being preempted out.
-Chris