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

Michal Koutný
2023-01-27T13:01:34Z

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On Thu, Jan 12, 2023 at 04:56:07PM +0000, Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com> wrote:
> +static int drmcs_can_attach(struct cgroup_taskset *tset)
> +{
> + int ret;
> +
> + /*
> + * As processes are getting moved between groups we need to ensure
> + * both that the old group does not see a sudden downward jump in the
> + * GPU utilisation, and that the new group does not see a sudden jump
> + * up with all the GPU time clients belonging to the migrated process
> + * have accumulated.
> + *
> + * To achieve that we suspend the scanner until the migration is
> + * completed where the resume at the end ensures both groups start
> + * observing GPU utilisation from a reset state.
> + */
> +
> + ret = mutex_lock_interruptible(&drmcg_mutex);
> + if (ret)
> + return ret;
> + start_suspend_scanning();
> + mutex_unlock(&drmcg_mutex);
> +
> + finish_suspend_scanning();

Here's scanning suspension, communicated via

root_drmcs.scanning_suspended = true;
root_drmcs.suspended_period_us = root_drmcs.period_us;
root_drmcs.period_us = 0;

but I don't see those used in scan_worker() and the scanning traversal
can apparently run concurrently with a task migration.

> [...]
> +static bool
> +__start_scanning(struct drm_cgroup_state *root, unsigned int period_us)
> [...]
> + css_for_each_descendant_post(node, &root->css) {
> [...]
> + active = drmcs_get_active_time_us(drmcs);
> + if (period_us && active > drmcs->prev_active_us)
> + drmcs->active_us += active - drmcs->prev_active_us;
> + drmcs->prev_active_us = active;

drmcs_get_active_time_us() could count a task's contribution here,
the task would migrate to a different drmcs,
and it'd be counted 2nd time.

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