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

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Message-ID: <1456190564.2463.123.camel@buserror.net>
Date:	Mon, 22 Feb 2016 19:22:44 -0600
From:	Scott Wood <oss@...error.net>
To:	Christophe Leroy <christophe.leroy@....fr>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v5] powerpc32: provide VIRT_CPU_ACCOUNTING

On Wed, 2016-02-17 at 17:29 +0100, Christophe Leroy wrote:
> 
> Le 16/02/2016 22:21, Scott Wood a écrit :
> > 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 been replaced by 'unsigned long' so that it is u32 on PPC32 and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?  What is the difference between PACA and "per-cpu data", other than
> > the
> > obscure name?"
> > 
> > I can understand wanting to avoid churn, but other than that, doing things
> > differently on 64-bit versus 32-bit sucks.
> > 
> 
> What I can see is that PACA is always available via register r13. Do we 
> have anything equivalent on PPC32 ?

Just current in r2, which is the task_struct, not a task-independent per-cpu
area.

> If we define a per-cpu data for accounting, what will be the quick way 
> to get access to it in entry_32.S ?
> Something like a table of accounting data for each CPU, that we index 
> with thread_info->cpu ?
> This would allow a quite quick access, is it the good way to proceed in 
> order to have something closer to PPC64 ?

Possibly.

-Scott