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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jan 05, 2016 at 03:59:51PM +0100, Daniel Vetter wrote:
> 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.

note that sync_rcu() can take a terribly long time.. but yes, I seem to
remember Paul talking about adding this to reclaim paths for just this
reason. Not sure that ever happened thouhg.

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