Re: [PATCH 1/1] cgroup: make per-cgroup pressure stall tracking configurable

[Suren Baghdasaryan <surenb@google.com>]

On Mon, May 17, 2021 at 11:31 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Sun, May 16, 2021 at 12:52:32PM -0700, Suren Baghdasaryan wrote:
> > After reworking the code to add a static key I had to expand the
> > #ifdef CONFIG_CGROUPS section, so I think a code refactoring below
> > would make sense. It localizes config-specific code and it has the
> > same exact code for CONFIG_CGROUPS=n and for
> > cgroup_psi_enabled()==false. WDYT?:
> >
> > --- a/kernel/sched/psi.c
> > +++ b/kernel/sched/psi.c
> > @@ -181,6 +181,7 @@ struct psi_group psi_system = {
> >  };
> >
> >  static void psi_avgs_work(struct work_struct *work);
> > +static void cgroup_iterator_init(void);
> >
> >  static void group_init(struct psi_group *group)
> >  {
> > @@ -211,6 +212,8 @@ void __init psi_init(void)
> >                  return;
> >          }
> >
> > +        cgroup_iterator_init();
> > +
> >          psi_period = jiffies_to_nsecs(PSI_FREQ);
> >          group_init(&psi_system);
> >  }
> > @@ -742,11 +745,31 @@ static void psi_group_change(struct psi_group
> > *group, int cpu,
> >                  schedule_delayed_work(&group->avgs_work, PSI_FREQ);
> >  }
> >
> > -static struct psi_group *iterate_groups(struct task_struct *task, void **iter)
> > +static inline struct psi_group *sys_group_iterator(struct task_struct *task,
> > +                                                   void **iter)
> >  {
> > +        *iter = &psi_system;
> > +        return &psi_system;
> > +}
> > +
> >  #ifdef CONFIG_CGROUPS
> > +
> > +DEFINE_STATIC_KEY_FALSE(psi_cgroups_disabled);
> > +
> > +static void cgroup_iterator_init(void)
> > +{
> > +        if (!cgroup_psi_enabled())
> > +                static_branch_enable(&psi_cgroups_disabled);
> > +}
> > +
> > +static struct psi_group *iterate_groups(struct task_struct *task, void **iter)
> > +{
> >          struct cgroup *cgroup = NULL;
> >
> > +        /* Skip to psi_system if per-cgroup accounting is disabled */
> > +        if (static_branch_unlikely(&psi_cgroups_disabled))
> > +                return *iter ? NULL : sys_group_iterator(task, iter);
> > +
> >          if (!*iter)
> >                  cgroup = task->cgroups->dfl_cgrp;
>
> That looks over-engineered. You have to check iter whether cgroups are
> enabled or not. Pulling the jump label check up doesn't save anything,
> but it ends up duplicating code.
>
> What you had in the beginning was better, it just had the system label
> in an unexpected place where it would check iter twice in a row.
>
> The (*iter == &psi_system) check inside the cgroups branch has the
> same purpose as the (*iter) check in the else branch. We could
> consolidate that by pulling it up front.
>
> If we wrap the entire cgroup iteration block into the static branch,
> IMO it becomes a bit clearer as well.
>
> How about this?
>
> static struct psi_group *iterate_groups(struct task_struct *task, void **iter)
> {
>         if (*iter == &psi_system)
>                 return NULL;
>
> #ifdef CONFIG_CGROUPS
>         if (!static_branch_likely(&psi_cgroups_disabled)) {
>                 struct cgroup *cgroup = NULL;
>
>                 if (!*iter)
>                         cgroup = task->cgroups->dfl_cgrp;
>                 else
>                         cgroup = cgroup_parent(*iter);
>
>                 if (cgroup && cgroup_parent(cgroup)) {
>                         *iter = cgroup;
>                         return cgroup_psi(cgroup);
>                 }
>         }
> #endif
>
>         *iter = &psi_system;
>         return &psi_system;
> }

This looks great to me. Will use it in the next version. Thanks!