On Wed, Jun 05, 2024 at 11:19:02AM +0900, Byungchul Park wrote: > On Wed, Jun 05, 2024 at 10:02:07AM +0800, Huang, Ying wrote: > > 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. > > What I mean is *the issue we try to solve* is not the situation that > can be solved by letting more CPUs participate in reclaiming. Again, in the situation where kswapd has failed more than MAX_RECLAIM_RETRIES, say, holeless, I don't think it makes sense to wake up kswapd every 10 seconds. It'd be more sensible to wake up kwapd only if there are *at least potentially* reclaimable folios. As Ying said, there's no way to precisely track if reclaimable, but it's worth trying when the possibility becomes positive and looks more reasonable. Thoughts? Byungchul > Byungchul > > > -- > > 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
