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

[Lionel Landwerlin <lionel.g.landwerlin@intel.com>]

On 13/11/2019 20:11, Chris Wilson wrote:
> 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 :)


It was probably my mistake :)


>>>>    
>>>>> Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@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@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

I guess I'll do that :)


-Lionel

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