linux-kernel - Re: [PATCH v2] mm: let kswapd work again for node that used to be hopeless but may not now

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Date: Tue, 4 Jun 2024 19:25:16 +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 Tue, Jun 04, 2024 at 06:12:22PM +0900, Byungchul Park wrote:
> 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.
> > 
> > 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
> 
> I mentioned "tiering" while I described how to reproduce because I ran
> into the situation while testing with tiering system but I don't think
> it's the necessary condition.
> 
> Let me ask you back, why the logic to stop kswapd was considered in the
> first place?  That's because the problem was already observed anyway

To be clear..

The problem, kswapd_failures >= MAX_RECLAIM_RETRIES, can happen whether
tiering is involved not not.  Once kswapd stops, the system should run
without kswapd even after recovered e.g. by killing the hoggers.  *Even
worse*, tiering mechanism doesn't work in this situation.

I hope what I meant has been delivered.

	Byungchul

> whether tiering is involved or not.  The same problem will arise once
> kswapd stops.
> 
> 	Byungchul
> 
> > 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