On Wed, Oct 18, 2023 at 11:26:59AM -0700, Shakeel Butt wrote:
> On Wed, Oct 18, 2023 at 10:22 AM Roman Gushchin
> <roman.gushchin@xxxxxxxxx> wrote:
> >
> [...]
> > > > struct mem_cgroup *memcg;
> > > > @@ -3008,19 +3054,26 @@ __always_inline struct obj_cgroup *get_obj_cgroup_from_current(void)
> > > >
> > > > if (in_task()) {
> > > > memcg = current->active_memcg;
> > > > + if (unlikely(memcg))
> > > > + goto from_memcg;
> > > >
> > > > - /* Memcg to charge can't be determined. */
> > > > - if (likely(!memcg) && (!current->mm || (current->flags & PF_KTHREAD)))
> > >
> > > The checks for current->mm and PF_KTHREAD seem to be gone completely after
> > > the patch, was that intended and why?
> >
> > There is no need for those anymore because it's as cheap or cheaper
> > to check task->objcg for being NULL. Those were primarily used to rule out
> > kernel threads allocations early.
> >
>
> I have the same understanding but please correct my suspicions here.
> We can echo the kernel thread's pid to cgroup.procs which have
> PF_NO_SETAFFINITY and thus this will cause the lower bit of the kernel
> thread's task->objcg to be set. Please correct me if I am missing
> something.
Yes, you seem to be right. It's a gray zone because moving kernel threads out of
the root cgroup doesn't sound like a good idea, but I agree it's better to keep
the old behavior in place.
Does this fixlet look good to you?
Thanks!
--
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 1a2835448028..0b0d2dc7a7d4 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3021,6 +3021,10 @@ static struct obj_cgroup *current_objcg_update(void)
old = NULL;
}
+ /* If new objcg is NULL, no reason for the second atomic update. */
+ if (!current->mm || (current->flags & PF_KTHREAD))
+ return NULL;
+
/*
* Release the objcg pointer from the previous iteration,
* if try_cmpxcg() below fails.
