Re: [Intel-gfx] [PATCH] drm/i915: Release i915_gem_context from a worker

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Aug 31, 2021 at 02:16:56PM +0200, Daniel Vetter wrote:
> On Tue, Aug 31, 2021 at 11:38:27AM +0200, Maarten Lankhorst wrote:
> > Op 14-08-2021 om 12:43 schreef Daniel Vetter:
> > > The only reason for this really is the i915_gem_engines->fence
> > > callback engines_notify(), which exists purely as a fairly funky
> > > reference counting scheme for that. Otherwise all other callers are
> > > from process context, and generally fairly benign locking context.
> > >
> > > Unfortunately untangling that requires some major surgery, and we have
> > > a few i915_gem_context reference counting bugs that need fixing, and
> > > they blow in the current hardirq calling context, so we need a
> > > stop-gap measure.
> > >
> > > Put a FIXME comment in when this should be removable again.
> > >
> > > v2: Fix mock_context(), noticed by intel-gfx-ci.
> > >
> > > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> > > Cc: Jon Bloomfield <jon.bloomfield@intel.com>
> > > Cc: Chris Wilson <chris@chris-wilson.co.uk>
> > > Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > > Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> > > Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> > > Cc: "Thomas Hellström" <thomas.hellstrom@linux.intel.com>
> > > Cc: Matthew Auld <matthew.auld@intel.com>
> > > Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
> > > Cc: Dave Airlie <airlied@redhat.com>
> > > Cc: Jason Ekstrand <jason@jlekstrand.net>
> > > ---
> > >  drivers/gpu/drm/i915/gem/i915_gem_context.c       | 13 +++++++++++--
> > >  drivers/gpu/drm/i915/gem/i915_gem_context_types.h | 12 ++++++++++++
> > >  drivers/gpu/drm/i915/gem/selftests/mock_context.c |  1 +
> > >  3 files changed, 24 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > > index fd169cf2f75a..051bc357ff65 100644
> > > --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > > +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > > @@ -986,9 +986,10 @@ static struct i915_gem_engines *user_engines(struct i915_gem_context *ctx,
> > >  	return err;
> > >  }
> > >  
> > > -void i915_gem_context_release(struct kref *ref)
> > > +static void i915_gem_context_release_work(struct work_struct *work)
> > >  {
> > > -	struct i915_gem_context *ctx = container_of(ref, typeof(*ctx), ref);
> > > +	struct i915_gem_context *ctx = container_of(work, typeof(*ctx),
> > > +						    release_work);
> > >  
> > >  	trace_i915_context_free(ctx);
> > >  	GEM_BUG_ON(!i915_gem_context_is_closed(ctx));
> > > @@ -1002,6 +1003,13 @@ void i915_gem_context_release(struct kref *ref)
> > >  	kfree_rcu(ctx, rcu);
> > >  }
> > >  
> > > +void i915_gem_context_release(struct kref *ref)
> > > +{
> > > +	struct i915_gem_context *ctx = container_of(ref, typeof(*ctx), ref);
> > > +
> > > +	queue_work(ctx->i915->wq, &ctx->release_work);
> > > +}
> > > +
> > >  static inline struct i915_gem_engines *
> > >  __context_engines_static(const struct i915_gem_context *ctx)
> > >  {
> > > @@ -1303,6 +1311,7 @@ i915_gem_create_context(struct drm_i915_private *i915,
> > >  	ctx->sched = pc->sched;
> > >  	mutex_init(&ctx->mutex);
> > >  	INIT_LIST_HEAD(&ctx->link);
> > > +	INIT_WORK(&ctx->release_work, i915_gem_context_release_work);
> > >  
> > >  	spin_lock_init(&ctx->stale.lock);
> > >  	INIT_LIST_HEAD(&ctx->stale.engines);
> > > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context_types.h b/drivers/gpu/drm/i915/gem/i915_gem_context_types.h
> > > index 94c03a97cb77..0c38789bd4a8 100644
> > > --- a/drivers/gpu/drm/i915/gem/i915_gem_context_types.h
> > > +++ b/drivers/gpu/drm/i915/gem/i915_gem_context_types.h
> > > @@ -288,6 +288,18 @@ struct i915_gem_context {
> > >  	 */
> > >  	struct kref ref;
> > >  
> > > +	/**
> > > +	 * @release_work:
> > > +	 *
> > > +	 * Work item for deferred cleanup, since i915_gem_context_put() tends to
> > > +	 * be called from hardirq context.
> > > +	 *
> > > +	 * FIXME: The only real reason for this is &i915_gem_engines.fence, all
> > > +	 * other callers are from process context and need at most some mild
> > > +	 * shuffling to pull the i915_gem_context_put() call out of a spinlock.
> > > +	 */
> > > +	struct work_struct release_work;
> > > +
> > >  	/**
> > >  	 * @rcu: rcu_head for deferred freeing.
> > >  	 */
> > > diff --git a/drivers/gpu/drm/i915/gem/selftests/mock_context.c b/drivers/gpu/drm/i915/gem/selftests/mock_context.c
> > > index fee070df1c97..067d68a6fe4c 100644
> > > --- a/drivers/gpu/drm/i915/gem/selftests/mock_context.c
> > > +++ b/drivers/gpu/drm/i915/gem/selftests/mock_context.c
> > > @@ -23,6 +23,7 @@ mock_context(struct drm_i915_private *i915,
> > >  	kref_init(&ctx->ref);
> > >  	INIT_LIST_HEAD(&ctx->link);
> > >  	ctx->i915 = i915;
> > > +	INIT_WORK(&ctx->release_work, i915_gem_context_release_work);
> > >  
> > >  	mutex_init(&ctx->mutex);
> > >  
> > 
> > ----
> > Is the workqueue really needed? I'm not sure you could still race in
> > drm_syncobj_free when refcount is zero, so in that case removing locking
> > from _release would work as well as a workqueue.
> > 
> > Something like below would keep the drm_sync_obj_put hardirq safe.
> > 
> > I assume when freeing, the  cb list is supposed to be empty, so I added a WARN_ON just to be sure, otherwise we should just tear down the list without locking too.
> > 
> > This should be a better alternative for patch 1.
> 
> This isn't enough, because the problem isn't just the syncobj put. It's
> also the i915_vm_put, and if we dercuify the intel_context stuff too, then
> there will be more intel_context_put on top.
> 
> So we really need the worker here I think. Trying to make every _unpin() and
> _put() work from hardirq context with clever locking tricks is why the
> current code is so incomprehensible.
> 
> Also vms are rare enough that we really don't care about some
> overhead/delay here.

Other reason is the one I explained in the commit message: Aside from the
engines i915_active there's no reason why anyone should call
i915_gem_context_put outside of process context. And I plan to fix that as
the next step. Or at least I'll try to untangle the context/engine
lifetime rules a bit.
-Daniel

> -Daniel
> 
> > ----8<-------
> > diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
> > index c9a9d74f338c..9d561decd97e 100644
> > --- a/drivers/gpu/drm/drm_syncobj.c
> > +++ b/drivers/gpu/drm/drm_syncobj.c
> > @@ -462,7 +462,13 @@ void drm_syncobj_free(struct kref *kref)
> >  	struct drm_syncobj *syncobj = container_of(kref,
> >  						   struct drm_syncobj,
> >  						   refcount);
> > -	drm_syncobj_replace_fence(syncobj, NULL);
> > +	struct dma_fence *old_fence;
> > +
> > +	old_fence = rcu_dereference_protected(syncobj->fence, !kref_read(&syncobj->refcount));
> > +	dma_fence_put(old_fence);
> > +
> > +	WARN_ON(!list_empty(&syncobj->cb_list));
> > +
> >  	kfree(syncobj);
> >  }
> >  EXPORT_SYMBOL(drm_syncobj_free);
> > 
> > 
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch