On Fri, Aug 05, 2016 at 10:14:18AM +0100, Chris Wilson wrote:
> By applying the same logic as for wait-ioctl, we can query whether a
> request has completed without holding struct_mutex. The biggest impact
> system-wide is removing the flush_active and the contention that causes.
>
> Testcase: igt/gem_busy
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Akash Goel <akash.goel@xxxxxxxxx>
> Reviewed-by: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
> ---
> drivers/gpu/drm/i915/i915_gem.c | 131 +++++++++++++++++++++++++++++++---------
> 1 file changed, 101 insertions(+), 30 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index ceb00970b2da..b99d64bfb7eb 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -3736,49 +3736,120 @@ i915_gem_object_ggtt_unpin_view(struct drm_i915_gem_object *obj,
> i915_vma_unpin(i915_gem_obj_to_ggtt_view(obj, view));
> }
>
> +static __always_inline unsigned __busy_read_flag(unsigned int id)
> +{
> + /* Note that we could alias engines in the execbuf API, but
> + * that would be very unwise as it prevents userspace from
> + * fine control over engine selection. Ahem.
> + *
> + * This should be something like EXEC_MAX_ENGINE instead of
> + * I915_NUM_ENGINES.
> + */
> + BUILD_BUG_ON(I915_NUM_ENGINES > 16);
> + return 0x10000 << id;
> +}
> +
> +static __always_inline unsigned int __busy_write_id(unsigned int id)
> +{
> + return id;
> +}
> +
> +static __always_inline unsigned
> +__busy_set_if_active(const struct i915_gem_active *active,
> + unsigned int (*flag)(unsigned int id))
> +{
> + /* For more discussion about the barriers and locking concerns,
> + * see __i915_gem_active_get_rcu().
> + */
> + do {
> + struct drm_i915_gem_request *request;
> + unsigned int id;
> +
> + request = rcu_dereference(active->request);
> + if (!request || i915_gem_request_completed(request))
> + return 0;
> +
> + id = request->engine->exec_id;
> +
> + /* Check that the pointer wasn't reassigned and overwritten. */
cf. our discussion in active_get_rcu - there's no fence_get_rcu in sight
anywhere here, hence this needs an smp_rmb(). Also nitpick: The two
rcu_dereference(actove->request) feel a bit silly. If we move the first in
front of the loop, and update the local request pointer (using a tmp) it
would look tidier, and we could even move the loop termination condition
into the while () check (and move the return flag(id) at the end of the
function).
-Daniel
> + if (request == rcu_access_pointer(active->request))
> + return flag(id);
> + } while (1);
> +}
> +
> +static inline unsigned
> +busy_check_reader(const struct i915_gem_active *active)
> +{
> + return __busy_set_if_active(active, __busy_read_flag);
> +}
> +
> +static inline unsigned
> +busy_check_writer(const struct i915_gem_active *active)
> +{
> + return __busy_set_if_active(active, __busy_write_id);
> +}
> +
> int
> i915_gem_busy_ioctl(struct drm_device *dev, void *data,
> struct drm_file *file)
> {
> struct drm_i915_gem_busy *args = data;
> struct drm_i915_gem_object *obj;
> - int ret;
> -
> - ret = i915_mutex_lock_interruptible(dev);
> - if (ret)
> - return ret;
> + unsigned long active;
>
> obj = i915_gem_object_lookup(file, args->handle);
> - if (!obj) {
> - ret = -ENOENT;
> - goto unlock;
> - }
> + if (!obj)
> + return -ENOENT;
>
> - /* Count all active objects as busy, even if they are currently not used
> - * by the gpu. Users of this interface expect objects to eventually
> - * become non-busy without any further actions.
> - */
> args->busy = 0;
> - if (i915_gem_object_is_active(obj)) {
> - struct drm_i915_gem_request *req;
> - int i;
> + active = __I915_BO_ACTIVE(obj);
> + if (active) {
> + int idx;
>
> - for (i = 0; i < I915_NUM_ENGINES; i++) {
> - req = i915_gem_active_peek(&obj->last_read[i],
> - &obj->base.dev->struct_mutex);
> - if (req)
> - args->busy |= 1 << (16 + req->engine->exec_id);
> - }
> - req = i915_gem_active_peek(&obj->last_write,
> - &obj->base.dev->struct_mutex);
> - if (req)
> - args->busy |= req->engine->exec_id;
> + /* Yes, the lookups are intentionally racy.
> + *
> + * First, we cannot simply rely on __I915_BO_ACTIVE. We have
> + * to regard the value as stale and as our ABI guarantees
> + * forward progress, we confirm the status of each active
> + * request with the hardware.
> + *
> + * Even though we guard the pointer lookup by RCU, that only
> + * guarantees that the pointer and its contents remain
> + * dereferencable and does *not* mean that the request we
> + * have is the same as the one being tracked by the object.
> + *
> + * Consider that we lookup the request just as it is being
> + * retired and freed. We take a local copy of the pointer,
> + * but before we add its engine into the busy set, the other
> + * thread reallocates it and assigns it to a task on another
> + * engine with a fresh and incomplete seqno.
> + *
> + * So after we lookup the engine's id, we double check that
> + * the active request is the same and only then do we add it
> + * into the busy set.
> + */
> + rcu_read_lock();
> +
> + for_each_active(active, idx)
> + args->busy |= busy_check_reader(&obj->last_read[idx]);
> +
> + /* For ABI sanity, we only care that the write engine is in
> + * the set of read engines. This is ensured by the ordering
> + * of setting last_read/last_write in i915_vma_move_to_active,
> + * and then in reverse in retire.
> + *
> + * We don't care that the set of active read/write engines
> + * may change during construction of the result, as it is
> + * equally liable to change before userspace can inspect
> + * the result.
> + */
> + args->busy |= busy_check_writer(&obj->last_write);
> +
> + rcu_read_unlock();
> }
>
> - i915_gem_object_put(obj);
> -unlock:
> - mutex_unlock(&dev->struct_mutex);
> - return ret;
> + i915_gem_object_put_unlocked(obj);
> + return 0;
> }
>
> int
> --
> 2.8.1
>
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--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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