linux-kernel - Re: [PATCH v3 2/5] mm: kmem: add direct objcg pointer to task

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Date:   Wed, 18 Oct 2023 11:26:59 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Roman Gushchin <roman.gushchin@...ux.dev>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Muchun Song <muchun.song@...ux.dev>,
        Dennis Zhou <dennis@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Naresh Kamboju <naresh.kamboju@...aro.org>
Subject: Re: [PATCH v3 2/5] mm: kmem: add direct objcg pointer to task_struct

On Wed, Oct 18, 2023 at 10:22 AM Roman Gushchin
<roman.gushchin@...ux.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.