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

Chris Wilson
2018-09-07T08:59:24Z

Quoting Tvrtko Ursulin (2018-09-07 09:26:27)
>
> 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.

Our muttering was just along the lines that we can skip the update via
GPU if oa was already active.

> 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.

In essence, the context should only get to see its own value, not the
system value since that is privileged information (of the OA user in
this case). It's always a nasty dilemma and I think idempotence of the
user interface is far more important (i.e. the current paired setparam,
getparam is the correct starting point for the API).

> 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?

I object to OA reporting on the kernel context. I think it should never
provide information about the system contexts as that is privileged
information.
* crawls back under his rock
-Chris