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SubjectRE: [PATCH 1/2] percpu: km: remove SMP check
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Hi Dennis

> -----Original Message-----
> From: Dennis Zhou [mailto:dennis@kernel.org]
> Sent: 2019年2月27日 1:04
> To: Christopher Lameter <cl@linux.com>
> Cc: Peng Fan <peng.fan@nxp.com>; tj@kernel.org; linux-mm@kvack.org;
> linux-kernel@vger.kernel.org; van.freenix@gmail.com
> Subject: Re: [PATCH 1/2] percpu: km: remove SMP check
>
> On Tue, Feb 26, 2019 at 03:16:44PM +0000, Christopher Lameter wrote:
> > On Mon, 25 Feb 2019, Dennis Zhou wrote:
> >
> > > > @@ -27,7 +27,7 @@
> > > > * chunk size is not aligned. percpu-km code will whine about it.
> > > > */
> > > >
> > > > -#if defined(CONFIG_SMP) &&
> > > > defined(CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK)
> > > > +#if defined(CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK)
> > > > #error "contiguous percpu allocation is incompatible with paged first
> chunk"
> > > > #endif
> > > >
> > > > --
> > > > 2.16.4
> > > >
> > >
> > > Hi,
> > >
> > > I think keeping CONFIG_SMP makes this easier to remember
> > > dependencies rather than having to dig into the config. So this is a NACK
> from me.
> >
> > But it simplifies the code and makes it easier to read.
> >
> >
>
> I think the check isn't quite right after looking at it a little longer.
> Looking at x86, I believe you can compile it with !SMP and
> CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK will still be set. This should
> still work because x86 has an MMU.

You are right, x86 could boots up with NEED_PER_CPU_PAGE_FIRST_CHUNK
=y and SMP=n. Tested with qemu, info as below:

/ # zcat /proc/config.gz | grep NEED_PER_CPU_KM
CONFIG_NEED_PER_CPU_KM=y
/ # zcat /proc/config.gz | grep SMP
CONFIG_BROKEN_ON_SMP=y
# CONFIG_SMP is not set
CONFIG_GENERIC_SMP_IDLE_THREAD=y
/ # zcat /proc/config.gz | grep NEED_PER_CPU_PAGE_FIRST_CHUNK
CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK=y
/ # cat /proc/cpuinfo
processor : 0
vendor_id : AuthenticAMD
cpu family : 6
model : 6
model name : QEMU Virtual CPU version 2.5+
stepping : 3
cpu MHz : 3192.613
cache size : 512 KB
fpu : yes
fpu_exception : yes
cpuid level : 13
wp : yes
flags : fpu de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 syscall nx lm nopl cpuid pni cx16 hypervisor lahf_lm svm 3dnowprefetl
bugs : fxsave_leak sysret_ss_attrs spectre_v1 spectre_v2 spec_store_bypass
bogomips : 6385.22
TLB size : 1024 4K pages
clflush size : 64
cache_alignment : 64
address sizes : 42 bits physical, 48 bits virtual
power management:


But from the comments in this file:
"
* - CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK must not be defined. It's
* not compatible with PER_CPU_KM. EMBED_FIRST_CHUNK should work
* fine.
"

I did not read into details why it is not allowed, but x86 could still work with KM
and NEED_PER_CPU_PAGE_FIRST_CHUNK.

>
> I think more correctly it would be something like below, but I don't have the
> time to fully verify it right now.
>
> Thanks,
> Dennis
>
> ---
> diff --git a/mm/percpu-km.c b/mm/percpu-km.c index
> 0f643dc2dc65..69ccad7d9807 100644
> --- a/mm/percpu-km.c
> +++ b/mm/percpu-km.c
> @@ -27,7 +27,7 @@
> * chunk size is not aligned. percpu-km code will whine about it.
> */
>
> -#if defined(CONFIG_SMP) &&
> defined(CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK)
> +#if !defined(CONFIG_MMU) &&
> +defined(CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK)
> #error "contiguous percpu allocation is incompatible with paged first chunk"
> #endif
>

Acked-by: Peng Fan <peng.fan@nxp.com>

Thanks,
Peng
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