Byungchul Park <byungchul@xxxxxx> writes: > Changes from v1: > 1. Don't allow to resume kswapd if the system is under memory > pressure that might affect direct reclaim by any chance, like > if NR_FREE_PAGES is less than (low wmark + min wmark)/2. > > --->8--- > From 6c73fc16b75907f5da9e6b33aff86bf7d7c9dd64 Mon Sep 17 00:00:00 2001 > From: Byungchul Park <byungchul@xxxxxx> > Date: Tue, 4 Jun 2024 15:27:56 +0900 > Subject: [PATCH v2] mm: let kswapd work again for node that used to be hopeless but may not now > > A system should run with kswapd running in background when under memory > pressure, such as when the available memory level is below the low water > mark and there are reclaimable folios. > > However, the current code let the system run with kswapd stopped if > kswapd has been stopped due to more than MAX_RECLAIM_RETRIES failures > until direct reclaim will do for that, even if there are reclaimable > folios that can be reclaimed by kswapd. This case was observed in the > following scenario: > > CONFIG_NUMA_BALANCING enabled > sysctl_numa_balancing_mode set to NUMA_BALANCING_MEMORY_TIERING > numa node0 (500GB local DRAM, 128 CPUs) > numa node1 (100GB CXL memory, no CPUs) > swap off > > 1) Run a workload with big anon pages e.g. mmap(200GB). > 2) Continue adding the same workload to the system. > 3) The anon pages are placed in node0 by promotion/demotion. > 4) kswapd0 stops because of the unreclaimable anon pages in node0. > 5) Kill the memory hoggers to restore the system. > > After restoring the system at 5), the system starts to run without > kswapd. Even worse, tiering mechanism is no longer able to work since > the mechanism relies on kswapd for demotion. We have run into the situation that kswapd is kept in failure state for long in a multiple tiers system. I think that your solution is too limited, because OOM killing may not happen, while the access pattern of the workloads may change. We have a preliminary and simple solution for this as follows, https://git.kernel.org/pub/scm/linux/kernel/git/vishal/tiering.git/commit/?h=tiering-0.8&id=17a24a354e12d4d4675d78481b358f668d5a6866 where we will try to wake up kswapd to check every 10 seconds if kswapd is in failure state. This is another possible solution. > However, the node0 has pages newly allocated after 5), that might or > might not be reclaimable. Since those are potentially reclaimable, it's > worth hopefully trying reclaim by allowing kswapd to work again. > [snip] -- Best Regards, Huang, Ying
