Re: [PATCH v3 1/3] mm, memcg: add mem_cgroup_disabled checks in vmpressure and swap-related functions

[Suren Baghdasaryan <surenb@google.com>]

On Fri, Jul 9, 2021 at 6:52 PM Miaohe Lin <linmiaohe@huawei.com> wrote:
>
> On 2021/7/10 8:36, Suren Baghdasaryan wrote:
> > Add mem_cgroup_disabled check in vmpressure, mem_cgroup_uncharge_swap and
> > cgroup_throttle_swaprate functions. This minimizes the memcg overhead in
> > the pagefault and exit_mmap paths when memcgs are disabled using
> > cgroup_disable=memory command-line option.
> > This change results in ~2.1% overhead reduction when running PFT test
> > comparing {CONFIG_MEMCG=n, CONFIG_MEMCG_SWAP=n} against {CONFIG_MEMCG=y,
> > CONFIG_MEMCG_SWAP=y, cgroup_disable=memory} configuration on an 8-core
> > ARM64 Android device.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > Reviewed-by: Shakeel Butt <shakeelb@google.com>
> > Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> > ---
> >  mm/memcontrol.c | 3 +++
> >  mm/swapfile.c   | 3 +++
> >  mm/vmpressure.c | 7 ++++++-
> >  3 files changed, 12 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index ae1f5d0cb581..a228cd51c4bd 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -7305,6 +7305,9 @@ void mem_cgroup_uncharge_swap(swp_entry_t entry, unsigned int nr_pages)
> >       struct mem_cgroup *memcg;
> >       unsigned short id;
> >
> > +     if (mem_cgroup_disabled())
> > +             return;
> > +
> >       id = swap_cgroup_record(entry, 0, nr_pages);
> >       rcu_read_lock();
> >       memcg = mem_cgroup_from_id(id);
> > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > index 1e07d1c776f2..707fa0481bb4 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -3778,6 +3778,9 @@ void cgroup_throttle_swaprate(struct page *page, gfp_t gfp_mask)
> >       struct swap_info_struct *si, *next;
> >       int nid = page_to_nid(page);
> >
> > +     if (mem_cgroup_disabled())
> > +             return;
> > +
>
> Many thanks for your patch. But I'am somewhat confused about this change.
> IMO, cgroup_throttle_swaprate() is only related to blk_cgroup and it seems
> it's irrelevant to mem_cgroup. Could you please have a explanation for me?

cgroup_throttle_swaprate() is a NoOp when CONFIG_MEMCG=n (see:
https://elixir.bootlin.com/linux/latest/source/include/linux/swap.h#L699),
therefore I assume we can safely skip it when memcgs are disabled via
"cgroup_disable=memory". From perf results I also see no hits on this
function when CONFIG_MEMCG=n.
However, looking into the code, I'm not sure why it should depend on
CONFIG_MEMCG. But it's Friday night and I might be missing some
details here...

>
> Thanks!
>
> >       if (!(gfp_mask & __GFP_IO))
> >               return;
> >
> > diff --git a/mm/vmpressure.c b/mm/vmpressure.c
> > index d69019fc3789..9b172561fded 100644
> > --- a/mm/vmpressure.c
> > +++ b/mm/vmpressure.c
> > @@ -240,7 +240,12 @@ static void vmpressure_work_fn(struct work_struct *work)
> >  void vmpressure(gfp_t gfp, struct mem_cgroup *memcg, bool tree,
> >               unsigned long scanned, unsigned long reclaimed)
> >  {
> > -     struct vmpressure *vmpr = memcg_to_vmpressure(memcg);
> > +     struct vmpressure *vmpr;
> > +
> > +     if (mem_cgroup_disabled())
> > +             return;
> > +
> > +     vmpr = memcg_to_vmpressure(memcg);
> >
> >       /*
> >        * Here we only want to account pressure that userland is able to
> >
>