Re: kswapd0: page allocation failure: order:0, mode:0x820(GFP_ATOMIC), nodemask=(null),cpuset=/,mems_allowed=0 (Kernel v6.5.9, 32bit ppc)

[Yu Zhao <yuzhao@google.com>]

On Wed, Jun 5, 2024 at 5:42 PM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> On Wed, Jun 5, 2024 at 4:04 PM Erhard Furtner <erhard_f@mailbox.org> wrote:
> >
> > On Tue, 4 Jun 2024 20:03:27 -0700
> > Yosry Ahmed <yosryahmed@google.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.,

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