Re: [PATCH] drm/i915/execlists: Disable submission tasklets when rescheduling

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Chris Wilson (2018-05-01 10:49:17)
> As we reschedule the requests, we do not want the submission tasklet
> running until we finish updating the priority chains. (We start
> rewriting priorities from the oldest, but the dequeue looks at the most
> recent in-flight, so there is a small race condition where dequeue may
> decide that preemption is falsely required.) Combine the tasklet kicking
> from adding a new request with the set-wedge protection so that we only
> have to adjust the preempt-counter once to achieve both goals.
> 
> Reported-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> ---
>  drivers/gpu/drm/i915/i915_gem.c     | 4 ++--
>  drivers/gpu/drm/i915/i915_request.c | 5 +----
>  2 files changed, 3 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 438a2fc5bba0..a278e4dc5e09 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -577,10 +577,10 @@ static void __fence_set_priority(struct dma_fence *fence,
>         rq = to_request(fence);
>         engine = rq->engine;
>  
> -       rcu_read_lock();
> +       local_bh_disable(); /* RCU serialisation for set-wedged protection */
>         if (engine->schedule)
>                 engine->schedule(rq, attr);
> -       rcu_read_unlock();
> +       local_bh_enable(); /* kick the tasklets if queues were reprioritised */
>  }
>  
>  static void fence_set_priority(struct dma_fence *fence,
> diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
> index 7bb613c00cc3..eb66579db823 100644
> --- a/drivers/gpu/drm/i915/i915_request.c
> +++ b/drivers/gpu/drm/i915/i915_request.c
> @@ -1111,12 +1111,9 @@ void __i915_request_add(struct i915_request *request, bool flush_caches)
>          * decide whether to preempt the entire chain so that it is ready to
>          * run at the earliest possible convenience.
>          */
> -       rcu_read_lock();
> +       local_bh_disable();
>         if (engine->schedule)
>                 engine->schedule(request, &request->ctx->sched);
> -       rcu_read_unlock();
> -
> -       local_bh_disable();
>         i915_sw_fence_commit(&request->submit);
>         local_bh_enable(); /* Kick the execlists tasklet if just scheduled */

It's an improvement in that we don't mix and match rcu/bh anymore,
but it doesn't entirely close the race; as local-bh is local to this cpu
and the tasklet may be running on another. Bother.
-Chris
_______________________________________________
Intel-gfx mailing list
Intel-gfx@lists.freedesktop.org
https://lists.freedesktop.org/mailman/listinfo/intel-gfx