Re: [Intel-gfx] [PATCH] drm/i915: Use DRIVER_NAME for tracing unattached requests

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, May 20, 2021 at 4:28 PM Daniel Vetter <daniel@ffwll.ch> wrote:
>
> On Thu, May 20, 2021 at 08:35:14AM +0100, Matthew Auld wrote:
> > From: Chris Wilson <chris@chris-wilson.co.uk>
> >
> > The first tracepoint for a request is trace_dma_fence_init called before
> > we have associated the request with a device. The tracepoint uses
> > fence->ops->get_driver_name() as a pretty name, and as we try to report
> > the device name this oopses as it is then NULL. Support the early
> > tracepoint by reporting the DRIVER_NAME instead of the actual device
> > name.
> >
> > Note that rq->engine remains during the course of request recycling
> > (SLAB_TYPESAFE_BY_RCU). For the physical engines, the pointer remains
> > valid, however a virtual engine may be destroyed after the request is
> > retired. If we process a preempt-to-busy completed request along the
> > virtual engine, we should make sure we mark the request as no longer
> > belonging to the virtual engine to remove the dangling pointers from the
> > tracepoint.
>
> Why can't we assign the request beforehand? The idea behind these
> tracepoints is that they actually match up, if trace_dma_fence_init is
> different, then we're breaking that.

Ok I looked a bit more and pondered this a bit, and the initial
tracepoint is called from dma_fence_init, where we haven't yet set up
rq->engine properly. So that part makes sense, but should have a
bigger comment that explains this a bit more and why we can't solve
this in a neater way. Probably should also drop the unlikely(), this
isn't a performance critical path, ever.

The other changes thgouh feel like they should be split out into a
separate path, since they solve a conceptually totally different
issue: SLAB_TYPESAFE_BY_RCU recycling. And I'm honestly not sure about
that one whether it's even correct, there's another patch floating
around that sprinkles rcu_read_lock around some of these accesssors,
and that would be a breakage of dma_fence interaces where outside of
i915 rcu isn't required for this stuff. So imo should be split out,
and come with a wider analysis of what's going on there and why and
how exactly i915 works.

In generally SLAB_TYPESAFE_BY_RCU is extremely dangerous and I'm
frankly not sure we have the perf data (outside of contrived
microbenchmarks) showing that it's needed and justifies all the costs
it's encurring.
-Daniel

> -Daniel
>
> >
> > Fixes: 855e39e65cfc ("drm/i915: Initialise basic fence before acquiring seqno")
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
> > Cc: Chintan M Patel <chintan.m.patel@intel.com>
> > Cc: Andi Shyti <andi.shyti@intel.com>
> > Cc: <stable@vger.kernel.org> # v5.7+
> > Signed-off-by: Matthew Auld <matthew.auld@intel.com>
> > ---
> >  .../drm/i915/gt/intel_execlists_submission.c  | 20 ++++++++++++++-----
> >  drivers/gpu/drm/i915/i915_request.c           |  7 ++++++-
> >  2 files changed, 21 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/gt/intel_execlists_submission.c b/drivers/gpu/drm/i915/gt/intel_execlists_submission.c
> > index de124870af44..75604e927d34 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_execlists_submission.c
> > +++ b/drivers/gpu/drm/i915/gt/intel_execlists_submission.c
> > @@ -3249,6 +3249,18 @@ static struct list_head *virtual_queue(struct virtual_engine *ve)
> >       return &ve->base.execlists.default_priolist.requests;
> >  }
> >
> > +static void
> > +virtual_submit_completed(struct virtual_engine *ve, struct i915_request *rq)
> > +{
> > +     GEM_BUG_ON(!__i915_request_is_complete(rq));
> > +     GEM_BUG_ON(rq->engine != &ve->base);
> > +
> > +     __i915_request_submit(rq);
> > +
> > +     /* Remove the dangling pointer to the stale virtual engine */
> > +     WRITE_ONCE(rq->engine, ve->siblings[0]);
> > +}
> > +
> >  static void rcu_virtual_context_destroy(struct work_struct *wrk)
> >  {
> >       struct virtual_engine *ve =
> > @@ -3265,8 +3277,7 @@ static void rcu_virtual_context_destroy(struct work_struct *wrk)
> >
> >               old = fetch_and_zero(&ve->request);
> >               if (old) {
> > -                     GEM_BUG_ON(!__i915_request_is_complete(old));
> > -                     __i915_request_submit(old);
> > +                     virtual_submit_completed(ve, old);
> >                       i915_request_put(old);
> >               }
> >
> > @@ -3538,13 +3549,12 @@ static void virtual_submit_request(struct i915_request *rq)
> >
> >       /* By the time we resubmit a request, it may be completed */
> >       if (__i915_request_is_complete(rq)) {
> > -             __i915_request_submit(rq);
> > +             virtual_submit_completed(ve, rq);
> >               goto unlock;
> >       }
> >
> >       if (ve->request) { /* background completion from preempt-to-busy */
> > -             GEM_BUG_ON(!__i915_request_is_complete(ve->request));
> > -             __i915_request_submit(ve->request);
> > +             virtual_submit_completed(ve, ve->request);
> >               i915_request_put(ve->request);
> >       }
> >
> > diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
> > index 970d8f4986bb..aa124adb1051 100644
> > --- a/drivers/gpu/drm/i915/i915_request.c
> > +++ b/drivers/gpu/drm/i915/i915_request.c
> > @@ -61,7 +61,12 @@ static struct i915_global_request {
> >
> >  static const char *i915_fence_get_driver_name(struct dma_fence *fence)
> >  {
> > -     return dev_name(to_request(fence)->engine->i915->drm.dev);
> > +     struct i915_request *rq = to_request(fence);
> > +
> > +     if (unlikely(!rq->engine)) /* not yet attached to any device */
> > +             return DRIVER_NAME;
> > +
> > +     return dev_name(rq->engine->i915->drm.dev);
> >  }
> >
> >  static const char *i915_fence_get_timeline_name(struct dma_fence *fence)
> > --
> > 2.26.3
> >
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch



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