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

Hi Dennis,

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

quoted

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

quoted

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

quoted

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

quoted

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

quoted

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>
---
 mm/Kconfig | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/mm/Kconfig b/mm/Kconfig
index d16ba92..66331e0 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig

@@ -425,9 +425,8 @@ config THP_SWAP
 # UP and nommu archs use km based percpu allocator
 #
 config NEED_PER_CPU_KM
-    depends on !SMP
     bool
-    default y
+    default !SMP || !MMU

Should this be `depends on !SMP || !MMU` with default yes? Because with
SMP && MMU, it shouldn't be an option to run with percpu-km.

IIUC these are equivalent, truth table would not change if is under "depends"
or "default"

SMP    MMU   NEED_PER_CPU_KM
 y      y    !y || !y => n || n => n
 y      n    !y || !n => n || y => y
 n      y    !n || !y => y || n => y
 n      n    !n || !n => y || y => y

quoted

quoted

 config CLEANCACHE
     bool "Enable cleancache driver to cache clean pages if tmem is present"
--
2.7.4

It's interesting to me that this is all coming up at once. Earlier this
month I had the same conversation with people involved with sh [1].

[1] https://lore.kernel.org/linux-sh/YY7tp5attRyK42Zk@fedora/ (local)

I can pull this shortly once I see whatever happened to linux-sh.

Ahh, good to know! Adding SH folks here (start of discussion [0]). I see you came
to the same conclusion, right?

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

Thanks,
Dennis