Date: Wed, 5 Jun 2024 16:58:11 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Yu Zhao <yuzhao@...gle.com>
Cc: Erhard Furtner <erhard_f@...lbox.org>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	linuxppc-dev@...ts.ozlabs.org, Johannes Weiner <hannes@...xchg.org>, 
	Nhat Pham <nphamcs@...il.com>, Chengming Zhou <chengming.zhou@...ux.dev>, 
	Sergey Senozhatsky <senozhatsky@...omium.org>, Minchan Kim <minchan@...nel.org>, 
	"Vlastimil Babka (SUSE)" <vbabka@...nel.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)

On Wed, Jun 5, 2024 at 4:53 PM Yu Zhao <yuzhao@...gle.com> wrote:
>
> On Wed, Jun 5, 2024 at 5:42 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
> >
> > On Wed, Jun 5, 2024 at 4:04 PM Erhard Furtner <erhard_f@...lbox.org> wrote:
> > >
> > > On Tue, 4 Jun 2024 20:03:27 -0700
> > > Yosry Ahmed <yosryahmed@...gle.com> wrote:
> > >
> > > > Could you check if the attached patch helps? It basically changes the
> > > > number of zpools from 32 to min(32, nr_cpus).
> > >
> > > Thanks! The patch does not fix the issue but it helps.
> > >
> > > Means I still get to see the 'kswapd0: page allocation failure' in the dmesg, a 'stress-ng-vm: page allocation failure' later on, another kswapd0 error later on, etc. _but_ the machine keeps running the workload, stays usable via VNC and I get no hard crash any longer.
> > >
> > > Without patch kswapd0 error and hard crash (need to power-cycle) <3min. With patch several kswapd0 errors but running for 2 hrs now. I double checked this to be sure.
> >
> > Thanks for trying this out. This is interesting, so even two zpools is
> > too much fragmentation for your use case.
>
> Now I'm a little bit skeptical that the problem is due to fragmentation.
>
> > 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?
>
> I double checked that commit and didn't find anything wrong. If we are
> all in the mood of getting to the bottom, can we try using only 1
> zpool while there are 2 available? I.e.,

Erhard, do you mind checking if Yu's diff below to use a single zpool
fixes the problem completely? There is also an attached patch that
does the same thing if this is easier to apply for you.

>
> static struct zpool *zswap_find_zpool(struct zswap_entry *entry)
> {
>  - return entry->pool->zpools[hash_ptr(entry, ilog2(ZSWAP_NR_ZPOOLS))];
>  + return entry->pool->zpools[0];
> }
>
> > In the long-term, I think we may want to address the lock contention
> > in zsmalloc itself instead of zswap spawning multiple zpools.
> >
> > >
> > > The patch did not apply cleanly on v6.9.3 so I applied it on v6.10-rc2. dmesg of the current v6.10-rc2 run attached.
> > >
> > > Regards,
> > > Erhard

Download attachment "0001-mm-zswap-set-ZSWAP_NR_ZPOOLS-to-1.patch" of type "application/octet-stream" (824 bytes)

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