Re: [PATCH 00/11] mm: fix page aging across multiple cgroups

[Shakeel Butt <shakeelb@google.com>]

On Mon, Jun 3, 2019 at 2:59 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> When applications are put into unconfigured cgroups for memory
> accounting purposes, the cgrouping itself should not change the
> behavior of the page reclaim code. We expect the VM to reclaim the
> coldest pages in the system. But right now the VM can reclaim hot
> pages in one cgroup while there is eligible cold cache in others.
>
> This is because one part of the reclaim algorithm isn't truly cgroup
> hierarchy aware: the inactive/active list balancing. That is the part
> that is supposed to protect hot cache data from one-off streaming IO.
>
> The recursive cgroup reclaim scheme will scan and rotate the physical
> LRU lists of each eligible cgroup at the same rate in a round-robin
> fashion, thereby establishing a relative order among the pages of all
> those cgroups. However, the inactive/active balancing decisions are
> made locally within each cgroup, so when a cgroup is running low on
> cold pages, its hot pages will get reclaimed - even when sibling
> cgroups have plenty of cold cache eligible in the same reclaim run.
>
> For example:
>
>    [root@ham ~]# head -n1 /proc/meminfo
>    MemTotal:        1016336 kB
>
>    [root@ham ~]# ./reclaimtest2.sh
>    Establishing 50M active files in cgroup A...
>    Hot pages cached: 12800/12800 workingset-a
>    Linearly scanning through 18G of file data in cgroup B:
>    real    0m4.269s
>    user    0m0.051s
>    sys     0m4.182s
>    Hot pages cached: 134/12800 workingset-a
>

Can you share reclaimtest2.sh as well? Maybe a selftest to
monitor/test future changes.


> The streaming IO in B, which doesn't benefit from caching at all,
> pushes out most of the workingset in A.
>
> Solution
>
> This series fixes the problem by elevating inactive/active balancing
> decisions to the toplevel of the reclaim run. This is either a cgroup
> that hit its limit, or straight-up global reclaim if there is physical
> memory pressure. From there, it takes a recursive view of the cgroup
> subtree to decide whether page deactivation is necessary.
>
> In the test above, the VM will then recognize that cgroup B has plenty
> of eligible cold cache, and that thet hot pages in A can be spared:
>
>    [root@ham ~]# ./reclaimtest2.sh
>    Establishing 50M active files in cgroup A...
>    Hot pages cached: 12800/12800 workingset-a
>    Linearly scanning through 18G of file data in cgroup B:
>    real    0m4.244s
>    user    0m0.064s
>    sys     0m4.177s
>    Hot pages cached: 12800/12800 workingset-a
>
> Implementation
>
> Whether active pages can be deactivated or not is influenced by two
> factors: the inactive list dropping below a minimum size relative to
> the active list, and the occurence of refaults.
>
> After some cleanups and preparations, this patch series first moves
> refault detection to the reclaim root, then enforces the minimum
> inactive size based on a recursive view of the cgroup tree's LRUs.
>
> History
>
> Note that this actually never worked correctly in Linux cgroups. In
> the past it worked for global reclaim and leaf limit reclaim only (we
> used to have two physical LRU linkages per page), but it never worked
> for intermediate limit reclaim over multiple leaf cgroups.
>
> We're noticing this now because 1) we're putting everything into
> cgroups for accounting, not just the things we want to control and 2)
> we're moving away from leaf limits that invoke reclaim on individual
> cgroups, toward large tree reclaim, triggered by high-level limits or
> physical memory pressure, that is influenced by local protections such
> as memory.low and memory.min instead.
>
> Requirements
>
> These changes are based on the fast recursive memcg stats merged in
> 5.2-rc1. The patches are against v5.2-rc2-mmots-2019-05-29-20-56-12
> plus the page cache fix in https://lkml.org/lkml/2019/5/24/813.
>
>  include/linux/memcontrol.h |  37 +--
>  include/linux/mmzone.h     |  30 +-
>  include/linux/swap.h       |   2 +-
>  mm/memcontrol.c            |   6 +-
>  mm/page_alloc.c            |   2 +-
>  mm/vmscan.c                | 667 ++++++++++++++++++++++---------------------
>  mm/workingset.c            |  74 +++--
>  7 files changed, 437 insertions(+), 381 deletions(-)
>
>