On 22/11/2022 21:29, Tejun Heo wrote:
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.
Are you suggesting "best effort limits" or "best effort <something>"? It
would sounds good to me if we found the right <something>. Best effort
budget perhaps?
+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.
It's well impressive you were able to figure out what I am doing there.
:) And probably you can see that this is the first time I am attempting
an algorithm like this one. I think I made it /dtrt/ with a few post/pre
walks so the right pieces of data propagate correctly.
Are you suggesting a parallel/shadow tree to be kept in the drm
controller (which would shadow the cgroup hierarchy)? Or something else?
The mention of rbtree is not telling me much, but I will look into the
referenced examples. (Although I will refrain from major rework until
more people start "biting" into all this.)
Also, when you mention scalability you are concerned about multiple tree
walks I have per iteration? I wasn't so much worried about that,
definitely not for the RFC, but even in general due relatively low
frequency of scanning and a good amount of less trivial cost being
outside the actual tree walks (drm client walks, GPU utilisation
calculations, maybe more). But perhaps I don't have the right idea on
how big cgroups hierarchies can be compared to number of drm clients etc.
Regards,
Tvrtko
