Re: [PATCH 1/3] drm/i915: Allow kswapd to pause the device whilst reaping

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Mika Kuoppala (2017-06-02 13:38:34)
> Chris Wilson <chris@chris-wilson.co.uk> writes:
> 
> > Quoting Mika Kuoppala (2017-06-02 13:02:57)
> >> Chris Wilson <chris@chris-wilson.co.uk> writes:
> >> 
> >> > In commit 5763ff04dc4e ("drm/i915: Avoid GPU stalls from kswapd") we
> >> > stopped direct reclaim and kswapd from triggering GPU/client stalls
> >> > whilst running (by restricting the objects they could reap to be idle).
> >> >
> >> > However with abusive GPU usage, it becomes quite easy to starve kswapd
> >> > of memory and prevent it from making forward progress towards obtaining
> >> > enough free memory (thus driving the system closer to swap exhaustion).
> >> > Relax the previous restriction to allow kswapd (but not direct reclaim)
> >> > to stall the device whilst reaping purgeable pages.
> >> >
> >> > v2: Also acquire the rpm wakelock to allow kswapd to unbind buffers.
> >> >
> >> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> >> > Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> >> > ---
> >> >  drivers/gpu/drm/i915/i915_gem_shrinker.c | 9 +++++++++
> >> >  1 file changed, 9 insertions(+)
> >> >
> >> > diff --git a/drivers/gpu/drm/i915/i915_gem_shrinker.c b/drivers/gpu/drm/i915/i915_gem_shrinker.c
> >> > index 0fd2b58ce475..58f27369183c 100644
> >> > --- a/drivers/gpu/drm/i915/i915_gem_shrinker.c
> >> > +++ b/drivers/gpu/drm/i915/i915_gem_shrinker.c
> >> > @@ -332,6 +332,15 @@ i915_gem_shrinker_scan(struct shrinker *shrinker, struct shrink_control *sc)
> >> >                                        sc->nr_to_scan - freed,
> >> >                                        I915_SHRINK_BOUND |
> >> >                                        I915_SHRINK_UNBOUND);
> >> > +     if (freed < sc->nr_to_scan && current_is_kswapd()) {
> >> > +             intel_runtime_pm_get(dev_priv);
> >> 
> >> We take extra ref to force device wake and thus force bound objects out?
> >
> > Yes. The shrinker skips the unbind phase if it can't acquire the device
> > wakeref, so we ensure we enter the shrinker with it held.
> >  
> >> > +             freed += i915_gem_shrink(dev_priv,
> >> > +                                      sc->nr_to_scan - freed,
> >> > +                                      I915_SHRINK_ACTIVE |
> >> > +                                      I915_SHRINK_BOUND |
> >> > +                                      I915_SHRINK_UNBOUND);
> >> 
> >> Looking at the shrink code, I am pondering how the stall will happen?
> >> 
> >> There are other callpaths that force gpu idle before kicking out
> >> objects, but for this callpath it seems that we kick out
> >> objects that might be still currently accessed by the gpu.
> >
> > By unbinding an active object, we stall.
> 
> I see that now.
> 
> Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>

Pushed this as the wakeref alone should make kswapd much more effective.
Perhaps I should have included a fixes, but alas I did not.

I would like to get some traction on the NORETRY patch.
-Chris
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