linux-kernel - Re: [PATCH v5] powerpc32: provide VIRT_CPU

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Date:	Mon, 22 Feb 2016 20:15:18 -0600
From:	Scott Wood <oss@...error.net>
To:	Michael Ellerman <mpe@...erman.id.au>,
	Christophe Leroy <christophe.leroy@....fr>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>
Cc:	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v5] powerpc32: provide VIRT_CPU_ACCOUNTING

On Tue, 2016-02-23 at 13:04 +1100, Michael Ellerman wrote:
> On Tue, 2016-02-16 at 15:21 -0600, Scott Wood wrote:
> 
> > On Thu, 2016-02-11 at 17:16 +0100, Christophe Leroy wrote:
> 
> > > This patch provides VIRT_CPU_ACCOUTING to PPC32 architecture.
> > > PPC32 doesn't have the PACA structure, so we use the task_info
> > > structure to store the accounting data.
> > > 
> > > In order to reuse on PPC32 the PPC64 functions, all u64 data has
> > > been replaced by 'unsigned long' so that it is u32 on PPC32 and
> > > u64 on PPC64
> > > 
> > > Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
> > > ---
> > > Changes in v3: unlike previous version of the patch that was inspired
> > > from IA64 architecture, this new version tries to reuse as much as
> > > possible the PPC64 implementation.
> > > 
> > > PPC32 doesn't have PACA and past discusion on v2 version has shown
> > > that it is not worth implementing a PACA in PPC32 architecture
> > > (see below benh opinion)
> > > 
> > > benh: PACA is actually a data structure and you really really don't want
> > > it
> > > on ppc32 :-) Having a register point to current works, having a register
> > > point to per-cpu data instead works too (ie, change what we do today),
> > > but don't introduce a PACA *please* :-)
> > 
> > And Ben never replied to my reply at the time:
> > 
> > "What is special about 64-bit that warrants doing things differently from
> > 32
> > -bit?
> 
> Nothing. It's just historical cruft. But we're not realistically going to
> get
> rid of it anytime soon on 64-bit.

I wasn't suggesting getting rid of it on 64-bit, but rather adding it on 32
-bit, to hold things that are used by both.  I was confused by the vehemence
of Ben's objection.

> > What is the difference between PACA and "per-cpu data", other than the
> > obscure name?"
> 
> Not much. The pacas are allocated differently to per-cpu data, they're
> available earlier in boot etc.

Ah, I was thinking of the general concept of per-cpu data, not the specific
mechanism that Linux implements in percpu.h etc.

>  What we'd like is to have r13 point to the
> per-cpu data area, and then the contents of the paca could just be regular
> per-cpu data. But like I said above that's a big change.

That change seems orthogonal to the question of making the mechanism available
on 32-bit to ease unification of code which uses it.

-Scott