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

Waiman Long
2020-09-14T00:49:18Z

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

Cheers,
Longman