Re: [PATCH] mm, memcg: fix wrong mem cgroup protection

[Yafang Shao <laoar.shao@gmail.com>]

On Fri, Apr 24, 2020 at 9:44 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Fri, Apr 24, 2020 at 09:14:52AM -0400, Johannes Weiner wrote:
> > However, mem_cgroup_protected() never expected anybody to look at the
> > effective protection values when it indicated that the cgroup is above
> > its protection. As a result, a query during limit reclaim may return
> > stale protection values that were calculated by a previous reclaim
> > cycle in which the cgroup did have siblings.
>
> Btw, I think there is opportunity to make this a bit less error prone.
>
> We have a mem_cgroup_protected() that returns yes or no, essentially,
> but protection isn't a binary state anymore.
>
> It's also been a bit iffy that it looks like a simple predicate
> function, but it indeed needs to run procedurally for each cgroup in
> order for the calculations throughout the tree to be correct.
>
> It might be better to have a
>
>         mem_cgroup_calculate_protection()
>
> that runs for every cgroup we visit and sets up the internal state;
> then have more self-explanatory query functions on top of that:
>
>         mem_cgroup_below_min()
>         mem_cgroup_below_low()
>         mem_cgroup_protection()
>
> What do you guys think?
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index e0f502b5fca6..dbd3f75d39b9 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2615,14 +2615,15 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
>                 unsigned long reclaimed;
>                 unsigned long scanned;
>
> -               switch (mem_cgroup_protected(target_memcg, memcg)) {
> -               case MEMCG_PROT_MIN:
> +               mem_cgroup_calculate_protection(target_memcg, memcg);
> +
> +               if (mem_cgroup_below_min(memcg)) {
>                         /*
>                          * Hard protection.
>                          * If there is no reclaimable memory, OOM.
>                          */
>                         continue;
> -               case MEMCG_PROT_LOW:
> +               } else if (mem_cgroup_below_low(memcg)) {
>                         /*
>                          * Soft protection.
>                          * Respect the protection only as long as
> @@ -2634,16 +2635,6 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
>                                 continue;
>                         }
>                         memcg_memory_event(memcg, MEMCG_LOW);
> -                       break;
> -               case MEMCG_PROT_NONE:
> -                       /*
> -                        * All protection thresholds breached. We may
> -                        * still choose to vary the scan pressure
> -                        * applied based on by how much the cgroup in
> -                        * question has exceeded its protection
> -                        * thresholds (see get_scan_count).
> -                        */
> -                       break;
>                 }
>
>                 reclaimed = sc->nr_reclaimed;


After my revist of the memcg protection. I have another idea.

The emin and elow is not decided by the memcg(struct mem_cgroup), but
they are really decided by the reclaim context(struct srhink_control).
So they should not be bound into struct  mem_cgroup, while they are
really should be bound into struct srhink_control.
IOW, we should move emin and elow from struct mem_cgroup into struct
srhink_control.
And they two members in shrink_control will be updated when a new
memcg is to be shrinked.
I haven't thought it deeply, but I think this should be the right thing to do.

Thanks
Yafang



-- 
Thanks
Yafang