Hi Dennis,

On Tue, Dec 14, 2021 at 9:50 PM Dennis Zhou [off-list ref] wrote:

On Tue, Dec 14, 2021 at 09:12:06PM +0100, Geert Uytterhoeven wrote:

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On Tue, Dec 14, 2021 at 8:18 PM Dennis Zhou [off-list ref] wrote:

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On Tue, Dec 14, 2021 at 08:02:58PM +0100, Geert Uytterhoeven wrote:

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On Tue, Dec 14, 2021 at 6:26 PM Dennis Zhou [off-list ref] wrote:

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On Tue, Dec 14, 2021 at 05:29:22PM +0100, Geert Uytterhoeven wrote:

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On Wed, Dec 1, 2021 at 12:53 PM Vladimir Murzin [off-list ref] wrote:

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On 11/30/21 5:41 PM, Dennis Zhou wrote:

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On Tue, Nov 30, 2021 at 05:29:54PM +0000, Vladimir Murzin wrote:

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Currently, NOMMU pull km allocator via !SMP dependency because most of
them are UP, yet for SMP+NOMMU vm allocator gets pulled which:

* may lead to broken build [1]
* ...or not working runtime due to [2]

It looks like SMP+NOMMU case was overlooked in bbddff054587 ("percpu:
use percpu allocator on UP too") so restore that.

[1]
For ARM SMP+NOMMU (R-class cores)

arm-none-linux-gnueabihf-ld: mm/percpu.o: in function `pcpu_post_unmap_tlb_flush':
mm/percpu-vm.c:188: undefined reference to `flush_tlb_kernel_range'

[2]
static inline
int vmap_pages_range_noflush(unsigned long addr, unsigned long end,
                pgprot_t prot, struct page **pages, unsigned int page_shift)
{
       return -EINVAL;
}

Signed-off-by: Vladimir Murzin <redacted>

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IIRC, RISC-V also have SMP+NOMMU, so adding them as well.

I had seen the j-Core thread, but completely forgot about
Canaan K210 (RV64 SMP+NOMMU).

This became commit 3583521aabac76e5 ("percpu: km: ensure it is used
with NOMMU (either UP or SMP)").  And now booting K210 prints:

    percpu: wasting 10 pages per chunk

a) Is this bad?

It's not great.. Can you share the line on boot with the following
prefix: pcpu-alloc [1].

There are no such lines.
"make mm/percpu.i mm/percpu.s" and inspecting the generated files,
and vmlinux, proves the code is there. But apparently it's not called.

So there may be no issue on my system?

I might be missing something, but that can't be right. Percpu calls
pcpu_dump_alloc_info() from pcpu_setup_first_chunk() which is called by
both embed/page first chunk code.

Ummm. That can't be right. Percpu call pcpu_dump_alloc_info() from
pcpu_setup_first_chunk() which everyone should call. On my machine:

$ dmesg | grep "pcpu-alloc"
[    0.065118] pcpu-alloc: s184320 r8192 d28672 u262144 alloc=1*2097152

Doh, it wasn't printed to the console, due to KERN_DEBUG. Dmesg
does have it:

<7>[    0.000000] pcpu-alloc: s15520 r0 d29536 u45056 alloc=11*4096
<7>[    0.000000] pcpu-alloc: [0] 0 [0] 1

I see, so what's happening is we're allocating 11 pages * 2, and due to
percpu-km we round up to a contiguous 32 pages for backing pages. This
results in the warning of wasting 10 pages. Given the size of the static
region, I'm not too worried for now. I can't imagine the config would
use that much percpu memory.

We can massage the discrepancy for-v5.17. Basically in percpu-km, we
align to 4k even though our allocation gets rounded up to the next power
of 2. I don't have a lot of bandwidth right now, but I might be able to
think about it over the next few weeks.

Note that K210 has only 8 MiB of SRAM, so wasting 10 pages means
wasting 0.5% of RAM.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds