This patch series addresses the following two problems:
1. A customer provided 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()). The evidence is:
- The process was trapped in throttle_direct_reclaim().
The function wait_event_killable() was called to wait condition
allow_direct_reclaim(pgdat) for current node to be true.
The allow_direct_reclaim(pgdat) examined the number of free pages
on the node by zone_page_state() which just returns value in
zone->vm_stat[NR_FREE_PAGES].
- On node #1, zone->vm_stat[NR_FREE_PAGES] was 0.
However, the freelist on this node was not empty.
- This inconsistent of vmstat value was caused by percpu vmstat on
nohz_full cpus. Every increment/decrement of vmstat is performed
on percpu vmstat counter at first, then pooled diffs are cumulated
to the zone's vmstat counter in timely manner. However, on nohz_full
cpus (in case of this customer's system, 48 of 52 cpus) these pooled
diffs were not cumulated once the cpu had no event on it so that
the cpu started sleeping infinitely.
I checked percpu vmstat and found there were total 69 counts not
cumulated to the zone's vmstat counter yet.
- In this situation, kswapd did not help the trapped process.
In pgdat_balanced(), zone_wakermark_ok_safe() examined the number
of free pages on the node by zone_page_state_snapshot() which
checks pending counts on percpu vmstat.
Therefore kswapd could know there were 69 free pages correctly.
Since zone->_watermark = {8, 20, 32}, kswapd did not work because
69 was greater than 32 as high watermark.
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.
v6:
- Add more information on throttle_direct_reclaim problem
to commit logs (Michal Hocko)
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(-)
