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

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Message-ID: <1456193080.16667.1.camel@ellerman.id.au>
Date:	Tue, 23 Feb 2016 13:04:40 +1100
From:	Michael Ellerman <mpe@...erman.id.au>
To:	Scott Wood <oss@...error.net>,
	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, 2016-02-16 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 been replaced by 'unsigned long' so that it is u32 on PPC32 and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.

> 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. 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.

cheers