Re: [PATCH 03/10] drm/i915: Shrink the GEM kmem_caches upon idling

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 15/01/2018 21:24, Chris Wilson wrote:
> When we finally decide the gpu is idle, that is a good time to shrink
> our kmem_caches.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>   drivers/gpu/drm/i915/i915_gem.c | 22 ++++++++++++++++++++++
>   1 file changed, 22 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index a8840a514377..8547f5214599 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -4709,6 +4709,21 @@ i915_gem_retire_work_handler(struct work_struct *work)
>   	}
>   }
>   
> +static void shrink_caches(struct drm_i915_private *i915)
> +{
> +	/*
> +	 * kmem_cache_shrink() discards empty slabs and reorders partially
> +	 * filled slabs to prioritise allocating from the mostly full slabs,
> +	 * with the aim of reducing fragmentation.
> +	 */

This makes it sound like it would be a very good thing in general.

> +	kmem_cache_shrink(i915->priorities);
> +	kmem_cache_shrink(i915->dependencies);
> +	kmem_cache_shrink(i915->requests);
> +	kmem_cache_shrink(i915->luts);
> +	kmem_cache_shrink(i915->vmas);
> +	kmem_cache_shrink(i915->objects);
> +}
> +
>   static inline bool
>   new_requests_since_last_retire(const struct drm_i915_private *i915)
>   {
> @@ -4796,6 +4811,13 @@ i915_gem_idle_work_handler(struct work_struct *work)
>   		GEM_BUG_ON(!dev_priv->gt.awake);
>   		i915_queue_hangcheck(dev_priv);
>   	}
> +
> +	rcu_barrier();

Ugh, more sprinkled around complexity we add the more difficult it 
becomes to maintain the code base for mere mortals. At the very minimum 
a comment is needed here.

What about activity other than requests? Like active mmap_gtt, that 
might at least create busyness on the vma and object caches and is not 
correlated to idle work handler firing.

Regards,

Tvrtko

> +
> +	if (!new_requests_since_last_retire(dev_priv)) {
> +		__i915_gem_free_work(&dev_priv->mm.free_work);
> +		shrink_caches(dev_priv);
> +	}
>   }
>   
>   int i915_gem_suspend(struct drm_i915_private *dev_priv)
> 
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