Re: [PATCH 2/3] mm/memcg: Simplify mem_cgroup_get_max()

[Waiman Long]

On 8/20/20 5:25 PM, Shakeel Butt wrote:
> On Thu, Aug 20, 2020 at 1:29 PM Waiman Long <longman@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > > ---
> > > >    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()?
My bad. I accidentally skipped the total_swap_pages check. Will add it 
back in v2.

Cheers,
Longman