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

Tvrtko Ursulin
2018-09-07T08:26:32Z

On 06/09/2018 11:36, Lionel Landwerlin wrote:
> On 06/09/2018 11:22, Chris Wilson wrote:
>> Quoting Lionel Landwerlin (2018-09-06 11:18:01)
>>> 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).
>> Sure, but as you point out that's only on changing configuration.
>>
>> What's missing in the patch is that we only bail early if the new sseu
>> matches the ce->sseu, but that doesn't necessarily match whats in the
>> context due to OA. (Or maybe I missed the conversion to rpcs value and
>> checking.)
>> -Chris
>>
>
> Yep, because the gen8_make_rpcs() post processes the values store at the
> gem context level, we risk rerunning the kernel context to write the
> exiting value.
> Sorry this is all so messy :(

Lets see if I managed to follow here.

The current code indeed bails out at the set ctx param level if the
requested state matches the ce->state. My thinking was that ce->state is
the master state and whatever happens in "post processing" via
gen8_make_rpcs should be hidden from it since the design is that the
i915_perf.c will re-configure all contexts when the OA active status
changes (to either direction).

So I don't see a problem in those two interactions.

Apart from one, get_param_sseu will lie a bit - we can discuss about
this one more. At one point I suggested we have two sets of masks in the
uAPI, requested and active in a way. So userspace could query what it
set and what is actually active.

Now second issue is if i915_perf.c is able to reprogram the kernel config.

Here its true, it will write to the context image and that will get
overwritten by context save.

If that is a problem for OA, I was initially if a throw-away second
"kernel" context could be use to re-program the real one, but perhaps
even simpler - what about a mmio write to program the RPCS while kernel
context is active?

Regards,

Tvrtko