[Intel-gfx] [PATCH 2/2] drm/i915: Shrink the GEM kmem_caches upon idling

[Tvrtko Ursulin]

On 19/01/2018 15:23, Chris Wilson wrote:
> When we finally decide the gpu is idle, that is a good time to shrink
> our kmem_caches.
> 
> v3: Defer until an rcu grace period after we idle.
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
> ---
>   drivers/gpu/drm/i915/i915_gem.c | 65 +++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 65 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 7f0684ccc724..6a8fbcae835b 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -3341,12 +3341,59 @@ new_requests_since_last_retire(const struct drm_i915_private *i915)
>   		work_pending(&i915->gt.idle_work.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.
> +	 */
> +	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);
> +}
> +
> +struct sleep_rcu_work {
> +	struct drm_i915_private *i915;
> +	struct rcu_head rcu;
> +	struct work_struct work;
> +	u32 epoch;
> +};
> +
> +static void __sleep_work(struct work_struct *work)
> +{
> +	struct sleep_rcu_work *s = container_of(work, typeof(*s), work);
> +	struct drm_i915_private *i915 = s->i915;
> +	u32 epoch = s->epoch;
> +
> +	kfree(s);
> +	if (epoch == READ_ONCE(i915->gt.epoch))
> +		shrink_caches(i915);
> +}
> +
> +static void __sleep_rcu(struct rcu_head *rcu)
> +{
> +	struct sleep_rcu_work *s = container_of(rcu, typeof(*s), rcu);
> +	struct drm_i915_private *i915 = s->i915;
> +
> +	if (s->epoch == READ_ONCE(i915->gt.epoch)) {
> +		INIT_WORK(&s->work, __sleep_work);
> +		queue_work(i915->wq, &s->work);
> +	} else {
> +		kfree(s);
> +	}
> +}
> +
>   static void
>   i915_gem_idle_work_handler(struct work_struct *work)
>   {
>   	struct drm_i915_private *dev_priv =
>   		container_of(work, typeof(*dev_priv), gt.idle_work.work);
>   	bool rearm_hangcheck;
> +	u32 epoch = 0;
>   	ktime_t end;
>   
>   	if (!READ_ONCE(dev_priv->gt.awake))
> @@ -3406,6 +3453,7 @@ i915_gem_idle_work_handler(struct work_struct *work)
>   	GEM_BUG_ON(!dev_priv->gt.awake);
>   	dev_priv->gt.awake = false;
>   	rearm_hangcheck = false;
> +	epoch = dev_priv->gt.epoch;
>   
>   	if (INTEL_GEN(dev_priv) >= 6)
>   		gen6_rps_idle(dev_priv);
> @@ -3421,6 +3469,23 @@ i915_gem_idle_work_handler(struct work_struct *work)
>   		GEM_BUG_ON(!dev_priv->gt.awake);
>   		i915_queue_hangcheck(dev_priv);
>   	}
> +
> +	/*
> +	 * When we are idle, it is an opportune time to reap our caches.
> +	 * However, we have many objects that utilise RCU and the ordered
> +	 * i915->wq that this work is executing on. To try and flush any
> +	 * pending frees now we are idle, we first wait for an RCU grace
> +	 * period, and then queue a task (that will run last on the wq) to
> +	 * shrink and re-optimize the caches.
> +	 */
> +	if (epoch == READ_ONCE(dev_priv->gt.epoch)) {

Theoretically this can be true on epoch wrap-around, when trylock 
failed. It's one in four billion busy transitions but it could be just 
worth handling it explicitly. Simplest probably to ensure gt.epoch is 
never zero when incrementing?

> +		struct sleep_rcu_work *s = kmalloc(sizeof(*s), GFP_KERNEL);
> +		if (s) {
> +			s->i915 = dev_priv;
> +			s->epoch = epoch;
> +			call_rcu(&s->rcu, __sleep_rcu);
> +		}
> +	}
>   }
>   
>   void i915_gem_close_object(struct drm_gem_object *gem, struct drm_file *file)
> 

Otherwise it sounds believable and looks correct.

Regards,

Tvrtko