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Date: Thu, 6 Jun 2024 13:31:56 +0900
From: Sergey Senozhatsky <senozhatsky@chromium.org>
To: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Yosry Ahmed <yosryahmed@google.com>,
	Erhard Furtner <erhard_f@mailbox.org>, Yu Zhao <yuzhao@google.com>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	linuxppc-dev@lists.ozlabs.org, Johannes Weiner <hannes@cmpxchg.org>,
	Nhat Pham <nphamcs@gmail.com>,
	Sergey Senozhatsky <senozhatsky@chromium.org>,
	Minchan Kim <minchan@kernel.org>,
	"Vlastimil Babka (SUSE)" <vbabka@kernel.org>
Subject: Re: kswapd0: page allocation failure: order:0,
 mode:0x820(GFP_ATOMIC), nodemask=(null),cpuset=/,mems_allowed=0 (Kernel
 v6.5.9, 32bit ppc)
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On (24/06/06 10:49), Chengming Zhou wrote:
> > Thanks for trying this out. This is interesting, so even two zpools is
> > too much fragmentation for your use case.
> > 
> > I think there are multiple ways to go forward here:
> > (a) Make the number of zpools a config option, leave the default as
> > 32, but allow special use cases to set it to 1 or similar. This is
> > probably not preferable because it is not clear to users how to set
> > it, but the idea is that no one will have to set it except special use
> > cases such as Erhard's (who will want to set it to 1 in this case).
> > 
> > (b) Make the number of zpools scale linearly with the number of CPUs.
> > Maybe something like nr_cpus/4 or nr_cpus/8. The problem with this
> > approach is that with a large number of CPUs, too many zpools will
> > start having diminishing returns. Fragmentation will keep increasing,
> > while the scalability/concurrency gains will diminish.
> > 
> > (c) Make the number of zpools scale logarithmically with the number of
> > CPUs. Maybe something like 4log2(nr_cpus). This will keep the number
> > of zpools from increasing too much and close to the status quo. The
> > problem is that at a small number of CPUs (e.g. 2), 4log2(nr_cpus)
> > will actually give a nr_zpools > nr_cpus. So we will need to come up
> > with a more fancy magic equation (e.g. 4log2(nr_cpus/4)).
> > 
> > (d) Make the number of zpools scale linearly with memory. This makes
> > more sense than scaling with CPUs because increasing the number of
> > zpools increases fragmentation, so it makes sense to limit it by the
> > available memory. This is also more consistent with other magic
> > numbers we have (e.g. SWAP_ADDRESS_SPACE_SHIFT).
> > 
> > The problem is that unlike zswap trees, the zswap pool is not
> > connected to the swapfile size, so we don't have an indication for how
> > much memory will be in the zswap pool. We can scale the number of
> > zpools with the entire memory on the machine during boot, but this
> > seems like it would be difficult to figure out, and will not take into
> > consideration memory hotplugging and the zswap global limit changing.
> > 
> > (e) A creative mix of the above.
> > 
> > (f) Something else (probably simpler).
> > 
> > I am personally leaning toward (c), but I want to hear the opinions of
> > other people here. Yu, Vlastimil, Johannes, Nhat? Anyone else?
> > 
> > In the long-term, I think we may want to address the lock contention
> > in zsmalloc itself instead of zswap spawning multiple zpools.

Sorry, I'm sure I'm not following this discussion closely enough,
has the lock contention been demonstrated/proved somehow? lock-stats?

> Agree, I think we should try to improve locking scalability of zsmalloc.
> I have some thoughts to share, no code or test data yet:
> 
> 1. First, we can change the pool global lock to per-class lock, which
>    is more fine-grained.

Commit c0547d0b6a4b6 "zsmalloc: consolidate zs_pool's migrate_lock
and size_class's locks" [1] claimed no significant difference
between class->lock and pool->lock.

[1] https://lkml.kernel.org/r/20221128191616.1261026-4-nphamcs@gmail.com