linux-kernel - Re: [PATCH v2 5/6] percpu: Repurpose __percpu tag as a named address space qualifier

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Message-ID: <CAFULd4ardZ1GT0a9E_Bnu+8VAbxXTcFfvU7QrC14p+XbwdgC2A@mail.gmail.com>
Date: Fri, 6 Dec 2024 21:13:03 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: Dennis Zhou <dennis@...nel.org>
Cc: x86@...nel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	linux-bcachefs@...r.kernel.org, linux-arch@...r.kernel.org, 
	netdev@...r.kernel.org, Nadav Amit <nadav.amit@...il.com>, 
	Arnd Bergmann <arnd@...db.de>, Thomas Gleixner <tglx@...utronix.de>, Tejun Heo <tj@...nel.org>, 
	Christoph Lameter <cl@...ux.com>, Linus Torvalds <torvalds@...ux-foundation.org>, 
	Andy Lutomirski <luto@...nel.org>, Ingo Molnar <mingo@...nel.org>, Brian Gerst <brgerst@...il.com>, 
	"H. Peter Anvin" <hpa@...or.com>, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2 5/6] percpu: Repurpose __percpu tag as a named address
 space qualifier

On Fri, Dec 6, 2024 at 8:35 AM Dennis Zhou <dennis@...nel.org> wrote:
>
> Hi Uros,
>
> On Thu, Dec 05, 2024 at 04:40:55PM +0100, Uros Bizjak wrote:
> > The patch introduces per_cpu_qual define and repurposes __percpu
> > tag as a named address space qualifier using the new define.
> >
> > Arches can now conditionally define __per_cpu_qual as their
> > named address space qualifier for percpu variables.
> >
> > Signed-off-by: Uros Bizjak <ubizjak@...il.com>
> > Acked-by: Nadav Amit <nadav.amit@...il.com>
> > Cc: Arnd Bergmann <arnd@...db.de>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Dennis Zhou <dennis@...nel.org>
> > Cc: Tejun Heo <tj@...nel.org>
> > Cc: Christoph Lameter <cl@...ux.com>
> > Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> > Cc: Andy Lutomirski <luto@...nel.org>
> > Cc: Ingo Molnar <mingo@...nel.org>
> > Cc: Brian Gerst <brgerst@...il.com>
> > Cc: "H. Peter Anvin" <hpa@...or.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > ---
> >  include/asm-generic/percpu.h   | 15 +++++++++++++++
> >  include/linux/compiler_types.h |  2 +-
> >  2 files changed, 16 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/asm-generic/percpu.h b/include/asm-generic/percpu.h
> > index 50597b975a49..3b93b168faa1 100644
> > --- a/include/asm-generic/percpu.h
> > +++ b/include/asm-generic/percpu.h
> > @@ -6,6 +6,21 @@
> >  #include <linux/threads.h>
> >  #include <linux/percpu-defs.h>
> >
> > +/*
> > + * per_cpu_qual is the qualifier for the percpu named address space.
> > + *
> > + * Most arches use generic named address space for percpu variables but
> > + * some arches define percpu variables in different named address space
> > + * (on the x86 arch, percpu variable may be declared as being relative
> > + * to the %fs or %gs segments using __seg_fs or __seg_gs named address
> > + * space qualifier).
> > + */
> > +#ifdef __per_cpu_qual
>
> I read through the series and I think my only nit would be here. Can we
> name this __percpu_qual? My thoughts are that it keeps it consistent
> with the old address space identifier and largely most of the core
> percpu stuff is defined with percpu as the naming scheme.

I based the approach on the definition of per_cpu_offset() a few lines
bellow in include/asm-generic/percpu.h:

--q--
#ifndef __per_cpu_offset
extern unsigned long __per_cpu_offset[NR_CPUS];

#define per_cpu_offset(x) (__per_cpu_offset[x])
#endif
--/q--

Sure, we can call this __percpu_qual. So, the definition in
arch/x86/include/asm/percpu.h would read as:

# define __percpu_qual      __percpu_seg_override

> > +# define per_cpu_qual __per_cpu_qual
> > +#else
> > +# define per_cpu_qual
> > +#endif

The above part could be recoded as:

#ifndef __percpu_qual
# define __percpu_qual
#endif

while the line below would become:

# define __percpu    __percpu_qual BTF_TYPE_TAG(percpu)

> > +
> >  #ifdef CONFIG_SMP
> >
> >  /*
> > diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> > index 981cc3d7e3aa..877fe0c43c5d 100644
> > --- a/include/linux/compiler_types.h
> > +++ b/include/linux/compiler_types.h
> > @@ -57,7 +57,7 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { }
> >  #  define __user     BTF_TYPE_TAG(user)
> >  # endif
> >  # define __iomem
> > -# define __percpu    BTF_TYPE_TAG(percpu)
> > +# define __percpu    per_cpu_qual BTF_TYPE_TAG(percpu)
> >  # define __rcu               BTF_TYPE_TAG(rcu)
> >
> >  # define __chk_user_ptr(x)   (void)0

Let me test these changes a bit, I'll send a v3 in a couple of days.

Thanks,
Uros.