Re: [PATCH v3 2/5] mm: kmem: add direct objcg pointer to task_struct

[Roman Gushchin <roman.gushchin@linux.dev>]

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@linux.dev> 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.