linux-kernel - Re: [PATCH 2/2] sched/psi: iterate through cgroups directly

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Date:   Wed, 8 Feb 2023 14:20:12 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     Kairui Song <kasong@...cent.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Chengming Zhou <zhouchengming@...edance.com>,
        Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, Kairui Song <ryncsn@...il.com>
Subject: Re: [PATCH 2/2] sched/psi: iterate through cgroups directly

On Wed, Feb 08, 2023 at 06:29:56PM +0100, Michal Koutný wrote:
> On Thu, Feb 09, 2023 at 12:16:54AM +0800, Kairui Song <ryncsn@...il.com> wrote:
> > Signed-off-by: Kairui Song <kasong@...cent.com>
> > Signed-off-by: Kairui Song <ryncsn@...il.com>
> 
> Typo?
> 
> > -static inline struct psi_group *task_psi_group(struct task_struct *task)
> > +static inline struct psi_group *psi_iter_first(struct task_struct *task, void **iter)
> >  {
> >  #ifdef CONFIG_CGROUPS
> > -	if (static_branch_likely(&psi_cgroups_enabled))
> > -		return cgroup_psi(task_dfl_cgroup(task));
> > +	if (static_branch_likely(&psi_cgroups_enabled)) {
> > +		struct cgroup *cgroup = task_dfl_cgroup(task);
> > +
> > +		*iter = cgroup_parent(cgroup);
> 
> This seems to skip a cgroup level -- maybe that's the observed
> performance gain?

Hm, I don't think it does. It sets up *iter to point to the parent for
the _next() call, but it returns task_dfl_cgroup()->psi. The next call
does the same: cgroup = *iter, *iter = parent, return cgroup->psi.

It could be a bit more readable to have *iter always point to the
current cgroup - but no strong preference either way from me:

psi_groups_first(task, iter)
{
#ifdef CONFIG_CGROUPS
	if (static_branch_likely(&psi_cgroups_enabled)) {
		struct cgroup *cgroup = task_dfl_cgroup(task);

		*iter = cgroup;
		return cgroup_psi(cgroup);
	}
#endif
	return &psi_system;
}

psi_groups_next(iter)
{
#ifdef CONFIG_CGROUPS
	if (static_branch_likely(&psi_cgroups_enabled)) {
		struct cgroup *cgroup = *iter;

		if (cgroup) {
			*iter = cgroup_parent(cgroup);
			return cgroup_psi(cgroup);
		}
	}
	return NULL;
#endif
}