On Tue, Jun 04, 2024 at 04:57:17PM +0800, Huang, Ying wrote: > Byungchul Park <byungchul@xxxxxx> writes: > > > 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. > > A system that fails to reclaim via kswapd may succeed to reclaim via > direct reclaim, because more CPUs are used to scanning the page tables. Honestly, I don't think so with this description. The fact that the system hit MAX_RECLAIM_RETRIES means the system is currently hopeless unless reclaiming folios in a stronger way by *direct reclaim*. The solution for this situation should not be about letting more CPUs particiated in reclaiming, again, *at least in this situation*. What you described here is true only in a normal state where the more CPUs work on reclaiming, the more reclaimable folios can be reclaimed. kswapd can be a helper *only* when there are kswapd-reclaimable folios. Byungchul > In a system with NUMA balancing based page promotion and page demotion > enabled, page promotion will wake up kswapd, but kswapd may fail in some > situations. But page promotion will no trigger direct reclaim or OOM. > > >> 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. > > Your description is about tiering too. Can you describe a situation > without tiering? > > -- > Best Regards, > Huang, Ying > > > 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
