Re: [Intel-gfx] [PATCH 01/11] drm/i915: Release i915_gem_context from a worker

[Daniel Vetter <daniel.vetter@ffwll.ch>]

On Thu, Sep 2, 2021 at 2:42 PM Tvrtko Ursulin
<tvrtko.ursulin@linux.intel.com> wrote:
>
>
> On 13/08/2021 21:30, Daniel Vetter wrote:
> > 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.
>
> There is reset which definitely isn't process context.

gpu reset runs in process context. The tasklet context is the
engines_notify I'm talking about above.

> Otherwise I did not really get from the commit message is this patch
> fixing an existing problem or preparing something for the future. If the
> former then as I wrote above - I am pretty sure there are call sites
> from the tasklet already.
>
> Regards,
>
> Tvrtko
>
> > 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.

I guess this para wasn't clear, but subsequent patches fix the
refcount bugs and need this prep patch here.
-Daniel

> >
> > Put a FIXME comment in when this should be removable again.
> >
> > 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 ++++++++++++
> >   2 files changed, 23 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.
> >        */
> >



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