Re: [PATCH resend 5/8] sched: cgroup cookie API for core scheduling

[Peter Zijlstra]

*sigh*, +tj

On Tue, Mar 30, 2021 at 11:23:10AM +0200, Peter Zijlstra wrote:
> On Wed, Mar 24, 2021 at 05:40:17PM -0400, Joel Fernandes (Google) wrote:
> > From: Josh Don <joshdon@xxxxxxxxxx>
> > 
> > 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;
> > +	}
> 
>