Date: Wed, 5 Jun 2024 11:19:02 +0900
From: Byungchul Park <byungchul@...com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, kernel_team@...ynix.com, hannes@...xchg.org,
	iamjoonsoo.kim@....com, rientjes@...gle.com
Subject: Re: [PATCH v2] mm: let kswapd work again for node that used to be
 hopeless but may not now

On Wed, Jun 05, 2024 at 10:02:07AM +0800, Huang, Ying wrote:
> Byungchul Park <byungchul@...com> writes:
> 
> > On Tue, Jun 04, 2024 at 04:57:17PM +0800, Huang, Ying wrote:
> >> Byungchul Park <byungchul@...com> writes:
> >> 
> >> > On Tue, Jun 04, 2024 at 03:57:54PM +0800, Huang, Ying wrote:
> >> >> Byungchul Park <byungchul@...com> 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@...com>
> >> >> > 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.

	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

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