Re: [PATCH] memcg: cleanup racy sum avoidance code

[Shakeel Butt]

On Wed, Jul 28, 2021 at 12:43 PM Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Tue, 27 Jul 2021 18:22:43 -0700 Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:
>
> > We used to have per-cpu memcg and lruvec stats and the readers have to
> > traverse and sum the stats from each cpu. This summing was racy and may
> > expose transient negative values. So, an explicit check was added to
> > avoid such scenarios. Now these stats are moved to rstat infrastructure
> > and are no more per-cpu, so we can remove the fixup for transient
> > negative values.
>
> We can't do anything about the same code in lruvec_page_state_local()?

lruvec_page_state_local() is used by cgroup v1's memory.numa_stat for
cgroup local stats (not hierarchical) and are still per-cpu. To make
it non-per-cpu, we have to add 'long
state_local[NR_VM_NODE_STAT_ITEMS]' in 'struct lruvec_stats' and do
aggregation in rstat flushing. So, paying the cpu traversal cost with
more memory usage. I am not sure if it is worth it.