On Wed, Nov 09, 2022 at 04:11:39PM +0000, Tvrtko Ursulin wrote:
> +DRM scheduling soft limits
> +~~~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +Because of the heterogenous hardware and driver DRM capabilities, soft limits
> +are implemented as a loose co-operative (bi-directional) interface between the
> +controller and DRM core.
> +
> +The controller configures the GPU time allowed per group and periodically scans
> +the belonging tasks to detect the over budget condition, at which point it
> +invokes a callback notifying the DRM core of the condition.
> +
> +DRM core provides an API to query per process GPU utilization and 2nd API to
> +receive notification from the cgroup controller when the group enters or exits
> +the over budget condition.
> +
> +Individual DRM drivers which implement the interface are expected to act on this
> +in the best-effort manner only. There are no guarantees that the soft limits
> +will be respected.
Soft limits is a bit of misnomer and can be confused with best-effort limits
such as memory.high. Prolly best to not use the term.
> +static bool
> +__start_scanning(struct drm_cgroup_state *root, unsigned int period_us)
> +{
> + struct cgroup_subsys_state *node;
> + bool ok = false;
> +
> + rcu_read_lock();
> +
> + css_for_each_descendant_post(node, &root->css) {
> + struct drm_cgroup_state *drmcs = css_to_drmcs(node);
> +
> + if (!css_tryget_online(node))
> + goto out;
> +
> + drmcs->active_us = 0;
> + drmcs->sum_children_weights = 0;
> +
> + if (node == &root->css)
> + drmcs->per_s_budget_ns =
> + DIV_ROUND_UP_ULL(NSEC_PER_SEC * period_us,
> + USEC_PER_SEC);
> + else
> + drmcs->per_s_budget_ns = 0;
> +
> + css_put(node);
> + }
> +
> + css_for_each_descendant_post(node, &root->css) {
> + struct drm_cgroup_state *drmcs = css_to_drmcs(node);
> + struct drm_cgroup_state *parent;
> + u64 active;
> +
> + if (!css_tryget_online(node))
> + goto out;
> + if (!node->parent) {
> + css_put(node);
> + continue;
> + }
> + if (!css_tryget_online(node->parent)) {
> + css_put(node);
> + goto out;
> + }
> + parent = css_to_drmcs(node->parent);
> +
> + active = drmcs_get_active_time_us(drmcs);
> + if (active > drmcs->prev_active_us)
> + drmcs->active_us += active - drmcs->prev_active_us;
> + drmcs->prev_active_us = active;
> +
> + parent->active_us += drmcs->active_us;
> + parent->sum_children_weights += drmcs->weight;
> +
> + css_put(node);
> + css_put(&parent->css);
> + }
> +
> + ok = true;
> +
> +out:
> + rcu_read_unlock();
> +
> + return ok;
> +}
A more conventional and scalable way to go about this would be using an
rbtree keyed by virtual time. Both CFS and blk-iocost are examples of this,
but I think for drm, it can be a lot simpler.
Thanks.
--
tejun
