[PATCH v3 0/2] mm/vmscan: mitigate spurious kswapd_failures reset and add tracepoints

[Jiayuan Chen <jiayuan.chen@linux.dev>]

== Problem ==

We observed an issue in production on a multi-NUMA system where kswapd
runs endlessly, causing sustained heavy IO READ pressure across the
entire system.

The root cause is that direct reclaim triggered by cgroup memory.high
keeps resetting kswapd_failures to 0, even when the node cannot be
balanced. This prevents kswapd from ever stopping after reaching
MAX_RECLAIM_RETRIES.

```bash
bpftrace -e '
 #include <linux/mmzone.h>
 #include <linux/shrinker.h>
kprobe:balance_pgdat {
	$pgdat = (struct pglist_data *)arg0;
	if ($pgdat->kswapd_failures > 0) {
		printf("[node %d] [%lu] kswapd end, kswapd_failures %d\n",
                       $pgdat->node_id, jiffies, $pgdat->kswapd_failures);
	}
}
tracepoint:vmscan:mm_vmscan_direct_reclaim_end {
	printf("[cpu %d] [%ul] reset kswapd_failures %d \n", cpu, jiffies,
               args.nr_reclaimed)
}
'
```

The trace results showed that when kswapd_failures reaches 15, continuous
direct reclaim keeps resetting it to 0. This was accompanied by a flood of
kswapd_failures log entries, and shortly after, we observed massive
refaults occurring.

== Solution ==

Patch 1 fixes the issue by only resetting kswapd_failures when the node
is actually balanced. This introduces pgdat_try_reset_kswapd_failures()
as a wrapper that checks pgdat_balanced() before resetting.

Patch 2 extends the wrapper to track why kswapd_failures was reset,
adding tracepoints for better observability:
  - mm_vmscan_reset_kswapd_failures: traces each reset with reason
  - mm_vmscan_kswapd_reclaim_fail: traces each kswapd reclaim failure

---
v2 -> v3: https://lore.kernel.org/all/20251226080042.291657-1-jiayuan.chen@linux.dev/
  - Add tracepoints for kswapd_failures reset and reclaim failure
  - Expand commit message with test results

v1 -> v2: https://lore.kernel.org/all/20251222122022.254268-1-jiayuan.chen@linux.dev/

Jiayuan Chen (2):
  mm/vmscan: mitigate spurious kswapd_failures reset from direct reclaim
  mm/vmscan: add tracepoint and reason for kswapd_failures reset

 include/linux/mmzone.h        |  9 +++++++
 include/trace/events/vmscan.h | 51 +++++++++++++++++++++++++++++++++++
 mm/memory-tiers.c             |  2 +-
 mm/page_alloc.c               |  2 +-
 mm/vmscan.c                   | 33 ++++++++++++++++++++---
 5 files changed, 91 insertions(+), 6 deletions(-)

-- 
2.43.0