[RFC 10/12] cgroup/drm: Introduce weight based drm cgroup control

[Tvrtko Ursulin]

On 28/01/2023 01:11, Tejun Heo wrote:
> On Thu, Jan 12, 2023 at 04:56:07PM +0000, Tvrtko Ursulin wrote:
> ...
>> +	/*
>> +	 * 1st pass - reset working values and update hierarchical weights and
>> +	 * GPU utilisation.
>> +	 */
>> +	if (!__start_scanning(root, period_us))
>> +		goto out_retry; /*
>> +				 * Always come back later if scanner races with
>> +				 * core cgroup management. (Repeated pattern.)
>> +				 */
>> +
>> +	css_for_each_descendant_pre(node, &root->css) {
>> +		struct drm_cgroup_state *drmcs = css_to_drmcs(node);
>> +		struct cgroup_subsys_state *css;
>> +		unsigned int over_weights = 0;
>> +		u64 unused_us = 0;
>> +
>> +		if (!css_tryget_online(node))
>> +			goto out_retry;
>> +
>> +		/*
>> +		 * 2nd pass - calculate initial budgets, mark over budget
>> +		 * siblings and add up unused budget for the group.
>> +		 */
>> +		css_for_each_child(css, &drmcs->css) {
>> +			struct drm_cgroup_state *sibling = css_to_drmcs(css);
>> +
>> +			if (!css_tryget_online(css)) {
>> +				css_put(node);
>> +				goto out_retry;
>> +			}
>> +
>> +			sibling->per_s_budget_us  =
>> +				DIV_ROUND_UP_ULL(drmcs->per_s_budget_us *
>> +						 sibling->weight,
>> +						 drmcs->sum_children_weights);
>> +
>> +			sibling->over = sibling->active_us >
>> +					sibling->per_s_budget_us;
>> +			if (sibling->over)
>> +				over_weights += sibling->weight;
>> +			else
>> +				unused_us += sibling->per_s_budget_us -
>> +					     sibling->active_us;
>> +
>> +			css_put(css);
>> +		}
>> +
>> +		/*
>> +		 * 3rd pass - spread unused budget according to relative weights
>> +		 * of over budget siblings.
>> +		 */
>> +		css_for_each_child(css, &drmcs->css) {
>> +			struct drm_cgroup_state *sibling = css_to_drmcs(css);
>> +
>> +			if (!css_tryget_online(css)) {
>> +				css_put(node);
>> +				goto out_retry;
>> +			}
>> +
>> +			if (sibling->over) {
>> +				u64 budget_us =
>> +					DIV_ROUND_UP_ULL(unused_us *
>> +							 sibling->weight,
>> +							 over_weights);
>> +				sibling->per_s_budget_us += budget_us;
>> +				sibling->over = sibling->active_us  >
>> +						sibling->per_s_budget_us;
>> +			}
>> +
>> +			css_put(css);
>> +		}
>> +
>> +		css_put(node);
>> +	}
>> +
>> +	/*
>> +	 * 4th pass - send out over/under budget notifications.
>> +	 */
>> +	css_for_each_descendant_post(node, &root->css) {
>> +		struct drm_cgroup_state *drmcs = css_to_drmcs(node);
>> +
>> +		if (!css_tryget_online(node))
>> +			goto out_retry;
>> +
>> +		if (drmcs->over || drmcs->over_budget)
>> +			signal_drm_budget(drmcs,
>> +					  drmcs->active_us,
>> +					  drmcs->per_s_budget_us);
>> +		drmcs->over_budget = drmcs->over;
>> +
>> +		css_put(node);
>> +	}
> 
> It keeps bothering me that the distribution logic has no memory. Maybe this
> is good enough for coarse control with long cycle durations but it likely
> will get in trouble if pushed to finer grained control. State keeping
> doesn't require a lot of complexity. The only state that needs tracking is
> each cgroup's vtime and then the core should be able to tell specific
> drivers how much each cgroup is over or under fairly accurately at any given
> time.
> 
> That said, this isn't a blocker. What's implemented can work well enough
> with coarse enough time grain and that might be enough for the time being
> and we can get back to it later. I think Michal already mentioned it but it
> might be a good idea to track active and inactive cgroups and build the
> weight tree with only active ones. There are machines with a lot of mostly
> idle cgroups (> tens of thousands) and tree wide scanning even at low
> frequency can become a pretty bad bottleneck.

Right, that's the kind of experience (tens of thousands) I was missing, 
thank you. Another one item on my TODO list then but I have a question 
first.

When you say active/inactive - to what you are referring in the cgroup 
world? Offline/online? For those my understanding was offline was a 
temporary state while css is getting destroyed.

Also, I am really postponing implementing those changes until I hear at 
least something from the DRM community.

Regards,

Tvrtko