Quoting Tvrtko Ursulin (2018-04-10 10:23:28)
> From: Tvrtko Ursulin <tvrtko.ursu...@intel.com>
> 
> While thinking about sporadic failures of perf_pmu/rc6-runtime-pm* tests
> on some CI machines I have concluded that: a) the PMU readout of RC6 can
> race against runtime PM transitions, and b) there are other reasons than
> being runtime suspended which can cause intel_runtime_pm_get_if_in_use to
> fail.
> 
> Therefore when estimating RC6 the code needs to assert we are indeed in
> suspended state and if not the best we can do is return the last known RC6
> value.
> 
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursu...@intel.com>
> Fixes: 1fe699e30113 ("drm/i915/pmu: Fix sleep under atomic in RC6 readout")
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105010
> Cc: Tvrtko Ursulin <tvrtko.ursu...@intel.com>
> Cc: Chris Wilson <ch...@chris-wilson.co.uk>
> Cc: Imre Deak <imre.d...@intel.com>
> ---
> I was able to trigger state != RPM_SUSPENDED on the shards, but not yet
> the actual estimation overaccounting. As such this fix is based partially
> on speculation that it will fix the sporadic perf_pmu/rc6* failures.
> Nevertheless I think it is correct to add this check regardless.
> ---
>  drivers/gpu/drm/i915/i915_pmu.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_pmu.c b/drivers/gpu/drm/i915/i915_pmu.c
> index bd7e695fc663..e92a9571db77 100644
> --- a/drivers/gpu/drm/i915/i915_pmu.c
> +++ b/drivers/gpu/drm/i915/i915_pmu.c
> @@ -473,6 +473,30 @@ static u64 get_rc6(struct drm_i915_private *i915)
>                 spin_lock_irqsave(&i915->pmu.lock, flags);
>                 spin_lock(&kdev->power.lock);
>  
> +               /*
> +                * After the above branch intel_runtime_pm_get_if_in_use 
> failed
> +                * to get the runtime PM reference we cannot assume we are in
> +                * runtime suspend since we can either: a) race with coming 
> out
> +                * of it before we took the power.lock, or b) there are other
> +                * states than suspended which can bring us here.
> +                *
> +                * We need to double-check that we are indeed currently 
> runtime
> +                * suspended and if not we cannot do better than report the 
> last
> +                * known RC6 value.
> +                */
> +               if (kdev->power.runtime_status != RPM_SUSPENDED) {
> +                       spin_unlock(&kdev->power.lock);
> +
> +                       if (i915->pmu.sample[__I915_SAMPLE_RC6_ESTIMATED].cur)
> +                               val = 
> i915->pmu.sample[__I915_SAMPLE_RC6_ESTIMATED].cur;
> +                       else
> +                               val = i915->pmu.sample[__I915_SAMPLE_RC6].cur;

If rpm awake, but having lost the race to read the regs, report the last
known value.

This is because we don't know if another thread is in the other branch,
and so we will have one updating the estimate while it being compared
against.

But I'm not understanding the failure -- why is the estimate bad? At the
very least we still ensure that it is monotonic? Is it just the jitter
you are worrying about? (If the estimate is bad here, isn't it always
bad?)

> +
> +                       spin_unlock_irqrestore(&i915->pmu.lock, flags);
> +
> +                       return val;
> +               }

I'd prefer moving the RPM_SUSPENDED code into an else branch to avoid
another unlock/early return here. (It just fits into 80cols, so no
excuses ;)
-Chris
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