RE: [PATCH 1/2] percpu: km: remove SMP check

[Peng Fan]

> -----Original Message-----
> From: Dennis Zhou [mailto:dennis@xxxxxxxxxx]
> Sent: 2019å2æ28æ 0:41
> To: Peng Fan <peng.fan@xxxxxxx>
> Cc: Dennis Zhou <dennis@xxxxxxxxxx>; Christopher Lameter <cl@xxxxxxxxx>;
> tj@xxxxxxxxxx; linux-mm@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> van.freenix@xxxxxxxxx
> Subject: Re: [PATCH 1/2] percpu: km: remove SMP check
> 
> On Wed, Feb 27, 2019 at 01:02:16PM +0000, Peng Fan wrote:
> > Hi Dennis
> >
> > > -----Original Message-----
> > > From: Dennis Zhou [mailto:dennis@xxxxxxxxxx]
> > > Sent: 2019å2æ27æ 1:04
> > > To: Christopher Lameter <cl@xxxxxxxxx>
> > > Cc: Peng Fan <peng.fan@xxxxxxx>; tj@xxxxxxxxxx; linux-mm@xxxxxxxxx;
> > > linux-kernel@xxxxxxxxxxxxxxx; van.freenix@xxxxxxxxx
> > > 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.
> >
> 
> The first chunk requires special handling on SMP to bring the static variables
> into the percpu address space. On UP, identity mapping makes static variables
> indistinguishable by aligning the percpu address space and the virtual adress
> space. The percpu-km allocator allocates full chunks at a time to deal with
> NOMMU archs. So the difference is if the virtual address space is the same as
> the physical.

Thanks for clarification.

> 
> > >
> > > 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@xxxxxxx>
> >
> > Thanks,
> > Peng
> 
> While this change may seem right to me. Verification would be to double
> check other architectures too. x86 just happened to be a counter example I
> had in mind. Unless someone reports this as being an issue or someone takes
> the time to validate this more thoroughly than my cursory look.
> I think the risk of this outweighs the benefit. This may be something I fix in the
> future when I have more time. This would also involve making sure the
> comments are consistent.

I am not able to check other architectures now. So just leave the code as is.

Thanks,
Peng.

> 
> Thanks,
> Dennis