Re: [PATCH v7 0/6] workload-specific and memory pressure-driven zswap writeback

[Bagas Sanjaya <bagasdotme@gmail.com>]

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On Mon, Nov 27, 2023 at 03:45:54PM -0800, Nhat Pham wrote:
> Changelog:
> v7:
>    * Added the mem_cgroup_iter_online() function to the API for the new
>      behavior (suggested by Andrew Morton) (patch 2)
>    * Fixed a missing list_lru_del -> list_lru_del_obj (patch 1)
> v6:
>    * Rebase on top of latest mm-unstable.
>    * Fix/improve the in-code documentation of the new list_lru
>      manipulation functions (patch 1)
> v5:
>    * Replace reference getting with an rcu_read_lock() section for
>      zswap lru modifications (suggested by Yosry)
>    * Add a new prep patch that allows mem_cgroup_iter() to return
>      online cgroup.
>    * Add a callback that updates pool->next_shrink when the cgroup is
>      offlined (suggested by Yosry Ahmed, Johannes Weiner)
> v4:
>    * Rename list_lru_add to list_lru_add_obj and __list_lru_add to
>      list_lru_add (patch 1) (suggested by Johannes Weiner and
> 	 Yosry Ahmed)
>    * Some cleanups on the memcg aware LRU patch (patch 2)
>      (suggested by Yosry Ahmed)
>    * Use event interface for the new per-cgroup writeback counters.
>      (patch 3) (suggested by Yosry Ahmed)
>    * Abstract zswap's lruvec states and handling into 
>      zswap_lruvec_state (patch 5) (suggested by Yosry Ahmed)
> v3:
>    * Add a patch to export per-cgroup zswap writeback counters
>    * Add a patch to update zswap's kselftest
>    * Separate the new list_lru functions into its own prep patch
>    * Do not start from the top of the hierarchy when encounter a memcg
>      that is not online for the global limit zswap writeback (patch 2)
>      (suggested by Yosry Ahmed)
>    * Do not remove the swap entry from list_lru in
>      __read_swapcache_async() (patch 2) (suggested by Yosry Ahmed)
>    * Removed a redundant zswap pool getting (patch 2)
>      (reported by Ryan Roberts)
>    * Use atomic for the nr_zswap_protected (instead of lruvec's lock)
>      (patch 5) (suggested by Yosry Ahmed)
>    * Remove the per-cgroup zswap shrinker knob (patch 5)
>      (suggested by Yosry Ahmed)
> v2:
>    * Fix loongarch compiler errors
>    * Use pool stats instead of memcg stats when !CONFIG_MEMCG_KEM
> 
> There are currently several issues with zswap writeback:
> 
> 1. There is only a single global LRU for zswap, making it impossible to
>    perform worload-specific shrinking - an memcg under memory pressure
>    cannot determine which pages in the pool it owns, and often ends up
>    writing pages from other memcgs. This issue has been previously
>    observed in practice and mitigated by simply disabling
>    memcg-initiated shrinking:
> 
>    https://lore.kernel.org/all/20230530232435.3097106-1-nphamcs@gmail.com/T/#u
> 
>    But this solution leaves a lot to be desired, as we still do not
>    have an avenue for an memcg to free up its own memory locked up in
>    the zswap pool.
> 
> 2. We only shrink the zswap pool when the user-defined limit is hit.
>    This means that if we set the limit too high, cold data that are
>    unlikely to be used again will reside in the pool, wasting precious
>    memory. It is hard to predict how much zswap space will be needed
>    ahead of time, as this depends on the workload (specifically, on
>    factors such as memory access patterns and compressibility of the
>    memory pages).
> 
> This patch series solves these issues by separating the global zswap
> LRU into per-memcg and per-NUMA LRUs, and performs workload-specific
> (i.e memcg- and NUMA-aware) zswap writeback under memory pressure. The
> new shrinker does not have any parameter that must be tuned by the
> user, and can be opted in or out on a per-memcg basis.
> 
> As a proof of concept, we ran the following synthetic benchmark:
> build the linux kernel in a memory-limited cgroup, and allocate some
> cold data in tmpfs to see if the shrinker could write them out and
> improved the overall performance. Depending on the amount of cold data
> generated, we observe from 14% to 35% reduction in kernel CPU time used
> in the kernel builds.
> 
> Domenico Cerasuolo (3):
>   zswap: make shrinking memcg-aware
>   mm: memcg: add per-memcg zswap writeback stat
>   selftests: cgroup: update per-memcg zswap writeback selftest
> 
> Nhat Pham (3):
>   list_lru: allows explicit memcg and NUMA node selection
>   memcontrol: add a new function to traverse online-only memcg hierarchy
>   zswap: shrinks zswap pool based on memory pressure
> 
>  Documentation/admin-guide/mm/zswap.rst      |   7 +
>  drivers/android/binder_alloc.c              |   7 +-
>  fs/dcache.c                                 |   8 +-
>  fs/gfs2/quota.c                             |   6 +-
>  fs/inode.c                                  |   4 +-
>  fs/nfs/nfs42xattr.c                         |   8 +-
>  fs/nfsd/filecache.c                         |   4 +-
>  fs/xfs/xfs_buf.c                            |   6 +-
>  fs/xfs/xfs_dquot.c                          |   2 +-
>  fs/xfs/xfs_qm.c                             |   2 +-
>  include/linux/list_lru.h                    |  54 ++-
>  include/linux/memcontrol.h                  |  18 +
>  include/linux/mmzone.h                      |   2 +
>  include/linux/vm_event_item.h               |   1 +
>  include/linux/zswap.h                       |  27 +-
>  mm/list_lru.c                               |  48 ++-
>  mm/memcontrol.c                             |  32 +-
>  mm/mmzone.c                                 |   1 +
>  mm/swap.h                                   |   3 +-
>  mm/swap_state.c                             |  26 +-
>  mm/vmstat.c                                 |   1 +
>  mm/workingset.c                             |   4 +-
>  mm/zswap.c                                  | 426 +++++++++++++++++---
>  tools/testing/selftests/cgroup/test_zswap.c |  74 ++--
>  24 files changed, 641 insertions(+), 130 deletions(-)
> 
> 
> base-commit: 5cdba94229e58a39ca389ad99763af29e6b0c5a5

No regressions when booting kernel with series applied.

Tested-by: Bagas Sanjaya <bagasdotme@gmail.com>

-- 
An old man doll... just what I always wanted! - Clara

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