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

[Suren Baghdasaryan]

On Fri, May 14, 2021 at 11:50 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>
> On Fri, May 14, 2021 at 11:20 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> >
> > On Fri, May 14, 2021 at 10:52 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > >
> > > On Fri, May 14, 2021 at 08:54:47AM -0700, Suren Baghdasaryan wrote:
> > >
> > > > Correct, for this function CONFIG_CGROUPS=n and
> > > > cgroup_disable=pressure are treated the same. True, from the code it's
> > > > not very obvious. Do you have some refactoring in mind that would make
> > > > it more explicit?
> > >
> > > Does this make sense?
> > >
> > > --- a/kernel/sched/psi.c
> > > +++ b/kernel/sched/psi.c
> > > @@ -744,24 +744,26 @@ static void psi_group_change(struct psi_
> > >
> > >  static struct psi_group *iterate_groups(struct task_struct *task, void **iter)
> > >  {
> > > +       if (cgroup_psi_enabled()) {
> > >  #ifdef CONFIG_CGROUPS
> > > -       struct cgroup *cgroup = NULL;
> > > +               struct cgroup *cgroup = NULL;
> > >
> > > -       if (!*iter)
> > > -               cgroup = task->cgroups->dfl_cgrp;
> > > -       else if (*iter == &psi_system)
> > > -               return NULL;
> > > -       else
> > > -               cgroup = cgroup_parent(*iter);
> > > +               if (!*iter)
> > > +                       cgroup = task->cgroups->dfl_cgrp;
> > > +               else if (*iter == &psi_system)
> > > +                       return NULL;
> > > +               else
> > > +                       cgroup = cgroup_parent(*iter);
> > >
> > > -       if (cgroup && cgroup_parent(cgroup)) {
> > > -               *iter = cgroup;
> > > -               return cgroup_psi(cgroup);
> > > -       }
> > > -#else
> > > -       if (*iter)
> > > -               return NULL;
> > > +               if (cgroup && cgroup_parent(cgroup)) {
> > > +                       *iter = cgroup;
> > > +                       return cgroup_psi(cgroup);
> > > +               }
> > >  #endif
> > > +       } else {
> > > +               if (*iter)
> > > +                       return NULL;
> > > +       }
> > >         *iter = &psi_system;
> > >         return &psi_system;
> > >  }
> >
> > Hmm. Looks like the case when cgroup_psi_enabled()==true and
> > CONFIG_CGROUPS=n would miss the "if (*iter) return NULL;" condition.
> > Effectively with CONFIG_CGROUPS=n this becomes:
> >
> >        if (cgroup_psi_enabled()) {           <== assume this is true
> > #ifdef CONFIG_CGROUPS                <== compiled out
> > #endif
> >        } else {
> >                if (*iter)                                  <== this
> > statement will never execute
> >                        return NULL;
> >        }
> >        *iter = &psi_system;
> >         return &psi_system;
> >
>
> Ah, sorry. I forgot that CONFIG_CGROUPS=n would force
> cgroup_psi_enabled()==false (the way function is defined in cgroup.h),
> so (CONFIG_CGROUPS=n && cgroup_psi_enabled()==true) is an invalid
> configuration. I think adding a comment to your suggestion would make
> it more clear.
> So your suggestion seems to work. I'll test it and include it in the
> next revision. Thanks!

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;
         else if (*iter == &psi_system)
@@ -758,14 +781,20 @@ static struct psi_group *iterate_groups(struct
task_struct *task, void **iter)
                 *iter = cgroup;
                 return cgroup_psi(cgroup);
         }
-#else
-        if (*iter)
-                return NULL;
-#endif
-        *iter = &psi_system;
-        return &psi_system;
+
+        return sys_group_iterator(task, iter);
 }

+#else /* CONFIG_CGROUPS */
+static inline void cgroup_iterator_init(void) {}
+
+static struct psi_group *iterate_groups(struct task_struct *task, void **iter)
+{
+        return *iter ? NULL : sys_group_iterator(task, iter);
+}
+
+#endif /* CONFIG_CGROUPS */
+


>
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