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, 04 Jun 2024 15:57:54 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Byungchul Park <byungchul@...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

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
limited, because OOM killing may not happen, while the access pattern of
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

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