[PATCH v6 0/6] workload-specific and memory pressure-driven zswap writeback

Nhat Pham
2023-11-27T19:37:04Z

Changelog:
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: allows mem_cgroup_iter() to check for onlineness
zswap: shrinks zswap pool based on memory pressure

Documentation/admin-guide/mm/zswap.rst | 7 +
drivers/android/binder_alloc.c | 5 +-
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 | 9 +-
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 | 20 +-
mm/mmzone.c | 1 +
mm/shrinker.c | 4 +-
mm/swap.h | 3 +-
mm/swap_state.c | 26 +-
mm/vmscan.c | 26 +-
mm/vmstat.c | 1 +
mm/workingset.c | 4 +-
mm/zswap.c | 426 +++++++++++++++++---
tools/testing/selftests/cgroup/test_zswap.c | 74 ++--
26 files changed, 629 insertions(+), 149 deletions(-)

base-commit: 40b487ae2620fc9187fee68b09d2cb275de0d60e
--
2.34.1