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SubjectRe: [PATCH resend 5/8] sched: cgroup cookie API for core scheduling
On Wed, Mar 24, 2021 at 05:40:17PM -0400, Joel Fernandes (Google) wrote:
> From: Josh Don <joshdon@google.com>
>
> This adds the API to set/get the cookie for a given cgroup. This
> interface lives at cgroup/cpu.core_tag.
>
> The cgroup interface can be used to toggle a unique cookie value for all
> descendent tasks, preventing these tasks from sharing with any others.
> See Documentation/admin-guide/hw-vuln/core-scheduling.rst for a full
> rundown of both this and the per-task API.

I refuse to read RST. Life's too short for that.

> +u64 cpu_core_tag_read_u64(struct cgroup_subsys_state *css,
> + struct cftype *cft)
> +{
> + return !!css_tg(css)->core_tagged;
> +}
> +
> +int cpu_core_tag_write_u64(struct cgroup_subsys_state *css, struct cftype *cft,
> + u64 val)
> +{
> + static DEFINE_MUTEX(sched_core_group_mutex);
> + struct task_group *tg = css_tg(css);
> + struct cgroup_subsys_state *css_tmp;
> + struct task_struct *p;
> + unsigned long group_cookie;
> + int ret = 0;
> +
> + if (val > 1)
> + return -ERANGE;
> +
> + if (!static_branch_likely(&sched_smt_present))
> + return -EINVAL;
> +
> + mutex_lock(&sched_core_group_mutex);
> +
> + if (!tg->core_tagged && val) {
> + /* Tag is being set. Check ancestors and descendants. */
> + if (cpu_core_get_group_cookie(tg) ||
> + cpu_core_check_descendants(tg, true /* tag */)) {
> + ret = -EBUSY;
> + goto out_unlock;
> + }

So the desired semantics is to only allow a single tag on any upwards
path? Isn't that in conflict with the cgroup requirements?

TJ?

> + } else if (tg->core_tagged && !val) {
> + /* Tag is being reset. Check descendants. */
> + if (cpu_core_check_descendants(tg, true /* tag */)) {

I'm struggling to understand this. If, per the above, you cannot set
when either a parent is already set or a child is set, then how can a
child be set to refuse clearing?

> + ret = -EBUSY;
> + goto out_unlock;
> + }
> + } else {
> + goto out_unlock;
> + }


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