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

[Huang, Ying]

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