Re: [RFC 11/13] cgroup/drm: Introduce weight based drm cgroup control

[Tejun Heo]

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