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@xxxxxxxxxxxxxxxxxx>
> > ---
> > 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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