Re: [PATCH 4/7] drm/i915/perf: lock powergating configuration to default when active

Lionel Landwerlin
2018-09-06T10:18:04Z

On 06/09/2018 11:10, Chris Wilson wrote:
> Quoting Lionel Landwerlin (2018-09-06 10:57:47)
>> On 05/09/2018 15:22, Tvrtko Ursulin wrote:
>>> From: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
>>>
>>> If some of the contexts submitting workloads to the GPU have been
>>> configured to shutdown slices/subslices, we might loose the NOA
>>> configurations written in the NOA muxes.
>>>
>>> One possible solution to this problem is to reprogram the NOA muxes
>>> when we switch to a new context. We initially tried this in the
>>> workaround batchbuffer but some concerns where raised about the cost
>>> of reprogramming at every context switch. This solution is also not
>>> without consequences from the userspace point of view. Reprogramming
>>> of the muxes can only happen once the powergating configuration has
>>> changed (which happens after context switch). This means for a window
>>> of time during the recording, counters recorded by the OA unit might
>>> be invalid. This requires userspace dealing with OA reports to discard
>>> the invalid values.
>>>
>>> Minimizing the reprogramming could be implemented by tracking of the
>>> last programmed configuration somewhere in GGTT and use MI_PREDICATE
>>> to discard some of the programming commands, but the command streamer
>>> would still have to parse all the MI_LRI instructions in the
>>> workaround batchbuffer.
>>>
>>> Another solution, which this change implements, is to simply disregard
>>> the user requested configuration for the period of time when i915/perf
>>> is active. There is no known issue with this apart from a performance
>>> penality for some media workloads that benefit from running on a
>>> partially powergated GPU. We already prevent RC6 from affecting the
>>> programming so it doesn't sound completely unreasonable to hold on
>>> powergating for the same reason.
>>>
>>> v2: Leave RPCS programming in intel_lrc.c (Lionel)
>>>
>>> v3: Update for s/union intel_sseu/struct intel_sseu/ (Lionel)
>>> More to_intel_context() (Tvrtko)
>>> s/dev_priv/i915/ (Tvrtko)
>>>
>>> Tvrtko Ursulin:
>>>
>>> v4:
>>> * Rebase for make_rpcs changes.
>>>
>>> v5:
>>> * Apply OA restriction from make_rpcs directly.
>>>
>>> v6:
>>> * Rebase for context image setup changes.
>>>
>>> Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
>>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>> ---
>>> drivers/gpu/drm/i915/i915_perf.c | 5 +++++
>>> drivers/gpu/drm/i915/intel_lrc.c | 30 ++++++++++++++++++++----------
>>> drivers/gpu/drm/i915/intel_lrc.h | 3 +++
>>> 3 files changed, 28 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c
>>> index ccb20230df2c..dd65b72bddd4 100644
>>> --- a/drivers/gpu/drm/i915/i915_perf.c
>>> +++ b/drivers/gpu/drm/i915/i915_perf.c
>>> @@ -1677,6 +1677,11 @@ static void gen8_update_reg_state_unlocked(struct i915_gem_context *ctx,
>>>
>>> CTX_REG(reg_state, state_offset, flex_regs[i], value);
>>> }
>>> +
>>> + CTX_REG(reg_state, CTX_R_PWR_CLK_STATE, GEN8_R_PWR_CLK_STATE,
>>> + gen8_make_rpcs(dev_priv,
>>> + &to_intel_context(ctx,
>>> + dev_priv->engine[RCS])->sseu));
>>
>> I think there is one issue I missed on the previous iterations of this
>> patch.
>>
>> This gen8_update_reg_state_unlocked() is called when the GPU is parked
>> on the kernel context.
>>
>> It's supposed to update all contexts, but I think we might not be able
>> to update the kernel context image while the GPU is using it.
> The kernel context is only ever taken in extremis (you are either
> parking or stalling userspace) so I don't care.

The patch exposing the RPCS configuration to userspace will make use of
the kernel context while OA/perf is enabled. Even if it reprograms the
locked value that will break the power configuration stability on Gen11
(because the locked configuration will be different from the kernel
context configuration).

-

Lionel

>
>> Context save might happen after we edited the image and that would
>> override the values we just put in there.
>>
>>
>> The OA config is emitted through context image edition in this function
>> but also through the ring buffer in
>> gen8_switch_to_updated_kernel_context() for the kernel context.
>>
>> Since we can't have a context modify its own RCPS value, we'll have to
>> resort to yet another context to do that for the kernel context.
>>
>>
>> I remember having a patch that created yet another kernel context (let's
>> call it rpcs edition context), which is used to reconfigure rpcs for
>> every context but itself and then have the kernel context reconfigure
>> this rpcs edition context.
>>
>> Or alternatively not do anything to it, because it's only going to run
>> to edit other contexts at a time when we don't care about power
>> configuration stability.
> Exactly.
> -Chris