linux-kernel - Re: [PATCH 2/3] mm/memcg: Simplify mem_cgroup_get

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Date:   Thu, 20 Aug 2020 14:25:41 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Cgroups <cgroups@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        Chris Down <chris@...isdown.name>,
        Roman Gushchin <guro@...com>,
        Yafang Shao <laoar.shao@...il.com>
Subject: Re: [PATCH 2/3] mm/memcg: Simplify mem_cgroup_get_max()

On Thu, Aug 20, 2020 at 1:29 PM Waiman Long <longman@...hat.com> wrote:
>
> On 8/20/20 1:35 PM, Johannes Weiner wrote:
> > On Thu, Aug 20, 2020 at 09:03:49AM -0400, Waiman Long wrote:
> >> The mem_cgroup_get_max() function used to get memory+swap max from
> >> both the v1 memsw and v2 memory+swap page counters & return the maximum
> >> of these 2 values. This is redundant and it is more efficient to just
> >> get either the v1 or the v2 values depending on which one is currently
> >> in use.
> >>
> >> Signed-off-by: Waiman Long <longman@...hat.com>
> >> ---
> >>   mm/memcontrol.c | 14 +++++---------
> >>   1 file changed, 5 insertions(+), 9 deletions(-)
> >>
> >> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> >> index 26b7a48d3afb..d219dca5239f 100644
> >> --- a/mm/memcontrol.c
> >> +++ b/mm/memcontrol.c
> >> @@ -1633,17 +1633,13 @@ void mem_cgroup_print_oom_meminfo(struct mem_cgroup *memcg)
> >>    */
> >>   unsigned long mem_cgroup_get_max(struct mem_cgroup *memcg)
> >>   {
> >> -    unsigned long max;
> >> +    unsigned long max = READ_ONCE(memcg->memory.max);
> >>
> >> -    max = READ_ONCE(memcg->memory.max);
> >>      if (mem_cgroup_swappiness(memcg)) {
> >> -            unsigned long memsw_max;
> >> -            unsigned long swap_max;
> >> -
> >> -            memsw_max = memcg->memsw.max;
> >> -            swap_max = READ_ONCE(memcg->swap.max);
> >> -            swap_max = min(swap_max, (unsigned long)total_swap_pages);
> >> -            max = min(max + swap_max, memsw_max);
> >> +            if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
> >> +                    max += READ_ONCE(memcg->swap.max);
> >> +            else
> >> +                    max = memcg->memsw.max;
> > I agree with the premise of the patch, but v1 and v2 have sufficiently
> > different logic, and the way v1 overrides max from the innermost
> > branch again also doesn't help in understanding what's going on.
> >
> > Can you please split out the v1 and v2 code?
> >
> >       if (cgroup_subsys_on_dfl(memory_cgrp_subsys)) {
> >               max = READ_ONCE(memcg->memory.max);
> >               if (mem_cgroup_swappiness(memcg))
> >                       max += READ_ONCE(memcg->swap.max);
> >       } else {
> >               if (mem_cgroup_swappiness(memcg))
> >                       max = memcg->memsw.max;
> >               else
> >                       max = READ_ONCE(memcg->memory.max);
> >       }
> >
> > It's slightly repetitive, but IMO much more readable.
> >
> Sure. That makes it even better.
>

Can you please also add in the commit message why it is ok to drop
total_swap_pages comparison from mem_cgroup_get_max()?