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From:   Shakeel Butt <shakeelb@google.com>
Date:   Wed, 18 Oct 2023 11:26:59 -0700
Message-ID: <CALvZod6mb91o9pW57suovtW1UQ8G8j=2S3Tjoqzjh6L+jqz-EQ@mail.gmail.com>
Subject: Re: [PATCH v3 2/5] mm: kmem: add direct objcg pointer to task_struct
To:     Roman Gushchin <roman.gushchin@linux.dev>
Cc:     Vlastimil Babka <vbabka@suse.cz>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Muchun Song <muchun.song@linux.dev>,
        Dennis Zhou <dennis@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Naresh Kamboju <naresh.kamboju@linaro.org>
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On Wed, Oct 18, 2023 at 10:22=E2=80=AFAM Roman Gushchin
<roman.gushchin@linux.dev> wrote:
>
[...]
> > >     struct mem_cgroup *memcg;
> > > @@ -3008,19 +3054,26 @@ __always_inline struct obj_cgroup *get_obj_cg=
roup_from_current(void)
> > >
> > >     if (in_task()) {
> > >             memcg =3D 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 af=
ter
> > 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 ou=
t
> 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.