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

[Waiman Long]

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.

Cheers,
Longman