Re: [PATCH 0/4] mm: memcontrol: memory.stat cost & correctness

[Roman Gushchin <guro@fb.com>]

On Fri, Apr 12, 2019 at 11:15:03AM -0400, Johannes Weiner wrote:
> The cgroup memory.stat file holds recursive statistics for the entire
> subtree. The current implementation does this tree walk on-demand
> whenever the file is read. This is giving us problems in production.
> 
> 1. The cost of aggregating the statistics on-demand is high. A lot of
> system service cgroups are mostly idle and their stats don't change
> between reads, yet we always have to check them. There are also always
> some lazily-dying cgroups sitting around that are pinned by a handful
> of remaining page cache; the same applies to them.
> 
> In an application that periodically monitors memory.stat in our fleet,
> we have seen the aggregation consume up to 5% CPU time.
> 
> 2. When cgroups die and disappear from the cgroup tree, so do their
> accumulated vm events. The result is that the event counters at
> higher-level cgroups can go backwards and confuse some of our
> automation, let alone people looking at the graphs over time.
> 
> To address both issues, this patch series changes the stat
> implementation to spill counts upwards when the counters change.
> 
> The upward spilling is batched using the existing per-cpu cache. In a
> sparse file stress test with 5 level cgroup nesting, the additional
> cost of the flushing was negligible (a little under 1% of CPU at 100%
> CPU utilization, compared to the 5% of reading memory.stat during
> regular operation).
> 
>  include/linux/memcontrol.h |  96 +++++++-------
>  mm/memcontrol.c            | 290 +++++++++++++++++++++++++++----------------
>  mm/vmscan.c                |   4 +-
>  mm/workingset.c            |   7 +-
>  4 files changed, 234 insertions(+), 163 deletions(-)
> 
> 

For the series:
Reviewed-by: Roman Gushchin <guro@fb.com>

Thanks!