[PATCH v5 00/12] fold per-CPU vmstats remotely

Marcelo Tosatti
2023-03-13T16:29:18Z

This patch series addresses the following two problems:

1. A customer provided some evidence which indicates that
the idle tick was stopped; albeit, CPU-specific vmstat
counters still remained populated.

Thus one can only assume quiet_vmstat() was not
invoked on return to the idle loop. If I understand
correctly, I suspect this divergence might erroneously
prevent a reclaim attempt by kswapd. If the number of
zone specific free pages are below their per-cpu drift
value then zone_page_state_snapshot() is used to
compute a more accurate view of the aforementioned
statistic. Thus any task blocked on the NUMA node
specific pfmemalloc_wait queue will be unable to make
significant progress via direct reclaim unless it is
killed after being woken up by kswapd
(see throttle_direct_reclaim())

2. With a SCHED_FIFO task that busy loops on a given CPU,
and kworker for that CPU at SCHED_OTHER priority,
queuing work to sync per-vmstats will either cause that
work to never execute, or stalld (i.e. stall daemon)
boosts kworker priority which causes a latency
violation

By having vmstat_shepherd flush the per-CPU counters to the
global counters from remote CPUs.

This is done using cmpxchg to manipulate the counters,
both CPU locally (via the account functions),
and remotely (via cpu_vm_stats_fold).

Thanks to Aaron Tomlin for diagnosing issue 1 and writing
the initial patch series.

v5:
- Drop "mm/vmstat: remove remote node draining" (Vlastimil Babka)
- Implement remote node draining for cpu_vm_stats_fold (Vlastimil Babka)

v4:
- Switch per-CPU vmstat counters to s32, required
by RISC-V, ARC architectures

v3:
- Removed unused drain_zone_pages and changes variable (David Hildenbrand)
- Use xchg instead of cmpxchg in refresh_cpu_vm_stats (Peter Xu)
- Add drain_all_pages to vmstat_refresh to make
stats more accurate (Peter Xu)
- Improve changelog of
"mm/vmstat: switch counter modification to cmpxchg" (Peter Xu / David)
- Improve changelog of
"mm/vmstat: remove remote node draining" (David Hildenbrand)

v2:
- actually use LOCK CMPXCHG on counter mod/inc/dec functions
(Christoph Lameter)
- use try_cmpxchg for cmpxchg loops
(Uros Bizjak / Matthew Wilcox)

arch/arm64/include/asm/percpu.h | 16 ++
arch/loongarch/include/asm/percpu.h | 23 +++-
arch/s390/include/asm/percpu.h | 5
arch/x86/include/asm/percpu.h | 39 +++----
include/asm-generic/percpu.h | 17 +++
include/linux/mmzone.h | 3
include/linux/percpu-defs.h | 2
kernel/fork.c | 2
kernel/scs.c | 2
mm/page_alloc.c | 23 ----
mm/vmstat.c | 424 +++++++++++++++++++++++++++++++++++++++++------------------------------------
11 files changed, 307 insertions(+), 249 deletions(-)