Re: [Intel-gfx] [PATCH v2 1/3] drm/i915: Enable lockless lookup of request tracking via RCU

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Dec 23, 2015 at 01:35:54PM +0000, Chris Wilson wrote:
> If we enable RCU for the requests (providing a grace period where we can
> inspect a "dead" request before it is freed), we can allow callers to
> carefully perform lockless lookup of an active request.
> 
> However, by enabling deferred freeing of requests, we can potentially
> hog a lot of memory when dealing with tens of thousands of requests per
> second - with a quick insertion of the a synchronize_rcu() inside our
> shrinker callback, that issue disappears.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>  drivers/gpu/drm/i915/i915_gem.c          |  3 ++-
>  drivers/gpu/drm/i915/i915_gem_request.c  |  2 +-
>  drivers/gpu/drm/i915/i915_gem_request.h  | 24 +++++++++++++++++++++++-
>  drivers/gpu/drm/i915/i915_gem_shrinker.c |  1 +
>  4 files changed, 27 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index c169574758d5..696ada3891ed 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -4222,7 +4222,8 @@ i915_gem_load(struct drm_device *dev)
>  	dev_priv->requests =
>  		kmem_cache_create("i915_gem_request",
>  				  sizeof(struct drm_i915_gem_request), 0,
> -				  SLAB_HWCACHE_ALIGN,
> +				  SLAB_HWCACHE_ALIGN |
> +				  SLAB_DESTROY_BY_RCU,
>  				  NULL);
>  
>  	INIT_LIST_HEAD(&dev_priv->context_list);

[snip i915 private changes, leave just slab/shrinker changes]

> diff --git a/drivers/gpu/drm/i915/i915_gem_shrinker.c b/drivers/gpu/drm/i915/i915_gem_shrinker.c
> index c561ed2b8287..03a8bbb3e31e 100644
> --- a/drivers/gpu/drm/i915/i915_gem_shrinker.c
> +++ b/drivers/gpu/drm/i915/i915_gem_shrinker.c
> @@ -142,6 +142,7 @@ i915_gem_shrink(struct drm_i915_private *dev_priv,
>  	}
>  
>  	i915_gem_retire_requests(dev_priv->dev);
> +	synchronize_rcu(); /* expedite the grace period to free the requests */

Shouldn't the slab subsystem do this for us if we request it delays the
actual kfree? Seems like a core bug to me ... Adding more folks.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

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