Byungchul Park <byungchul@xxxxxx> writes: > 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. Sometimes, we cannot reclaim just because we doesn't scan fast enough so the Accessed-bit is set again during scanning. With more CPUs, we can scan faster, so make some progress. But, yes, this only cover one situation, there are other situations too. -- Best Regards, Huang, Ying > 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
