linux-kernel - Re: [PATCH 0/4] cputime: Virtual cputime accounting small cleanups and consolidation

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Date:	Thu, 21 Jun 2012 14:04:24 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Tony Luck <tony.luck@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Venkatesh Pallipadi <venki@...gle.com>
Subject: Re: [PATCH 0/4] cputime: Virtual cputime accounting small cleanups
 and consolidation

On Thu, 2012-06-21 at 09:58 +0200, Martin Schwidefsky wrote:
> On Thu, 21 Jun 2012 02:46:29 +0200
> Frederic Weisbecker <fweisbec@...il.com> wrote:
> 
> > 2012/6/21 Peter Zijlstra <peterz@...radead.org>:
> > > On Tue, 2012-06-19 at 15:43 +0200, Frederic Weisbecker wrote:
> > >>
> > >> I wish we could do more vtime cputime accounting consolidation
> > >> but archs do the things pretty differently although I bet the
> > >> behaviour could be more unified.
> > >>
> > > Yes.. so s390,ia64 use thread_info, ppc uses their paca (arch private
> > > precursor to per-cpu data).
> 
> s390 uses the prefix page / lowcore to accumulate some accounting information.
> Which basically is per-cpu data with the advantage that it is accessible with
> at address 0-8191 for each cpu. The entry code does not have to load a pointer
> to get to that page, I would prefer NOT to use per-cpu data here.

Yeah, same for ppc and their paca, I just meant to put the data in
per-cpu (your lowcore and ppc's paca qualify) storage instead of
per-task.

But seeing as I completely overlooked the per-task accounting this
doesn't matter anyway.

There being the per-task accounting also completely wrecks the proposal
I outlined. That only works if its only per-cpu accounting.

The alternative is going full 64bit ns and having the tick fallback do
TICK_NSEC increments. 32bit args that don't do VIRT_TIME or IRQ_TIME
won't like that though :/

So yeah, I did miss something obvious.. no cookies for me.
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