On Tue, Jun 04, 2024 at 03:57:54PM +0800, Huang, Ying wrote: > 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 My solution just gives a chance for kswapd to work again even if kswapd_failures >= MAX_RECLAIM_RETRIES, if there are potential reclaimable folios. That's it. > limited, because OOM killing may not happen, while the access pattern of I don't get this. OOM will happen as is, through direct reclaim. > 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 Whether tiering is involved or not, the same problem can arise if kswapd gets stopped due to kswapd_failures >= MAX_RECLAIM_RETRIES. Byungchul > 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
