linux-kernel - Re: irqsafe_cpu_...() 64-bit operations on 32-bit platform

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Date:	Wed, 04 Jan 2012 10:40:18 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Christoph Lameter" <cl@...ux.com>
Cc:	<mingo@...e.hu>, "Jeremy Fitzhardinge" <jeremy@...p.org>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: irqsafe_cpu_...() 64-bit operations on 32-bit platform

>>> On 03.01.12 at 17:37, Christoph Lameter <cl@...ux.com> wrote:
> On Mon, 2 Jan 2012, Jan Beulich wrote:
> 
>> > However, if you are using a loop to sum up all counters from every
>> > processor then you cannot use the this_cpu ops but would have to use
>> > something like get_64().
>>
>> I understand that, but again statistics aren't my focus here.
> 
> Ok then we need to understand better what you are trying to accomplish.
> You may be outside of the realm of percpu data and be just trying to read
> and write 64 bit variables reliably on 32 bit?

The get_64bit() use case is quite obvious: arch/x86/xen/time.c has
get64(), which really should be required to get defined here.

The same file has potential for using the per-CPU variant: In particular
after the "blocked time" accounting gets removed from
do_stolen_accounting() (for which there is a patch pending), obtaining
a full runstate snapshot (via get_runstate_snapshot()) becomes sort
of overkill - reading the RUNSTATE_runnable and RUNSTATE_offline
array entries individually (and atomically) would be sufficient and faster,
and as xen_runstate is a per-CPU variable doing this through a
per-CPU accessor would be the obviously preferred method. Except
that "irqsafe_...()" isn't really the right thing (as emulating it through
disabling interrupts is not a correct alternative - it has to be
implemented as a single instruction, but since this is arch-specific
code, using it would be safe; alternatively some accessor could be 
introduced that doesn't have fallbacks when there's no single-
instruction implementation, causing the improper use to be noticed
at build time).

Jan

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