linux-kernel - Re: [RFD 1/9] Change cpustat fields to an array.

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Date:	Wed, 28 Sep 2011 21:09:17 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Glauber Costa <glommer@...allels.com>
Cc:	linux-kernel@...r.kernel.org, paul@...lmenage.org,
	lizf@...fujitsu.com, daniel.lezcano@...e.fr,
	jbottomley@...allels.com
Subject: Re: [RFD 1/9] Change cpustat fields to an array.

On Wed, 2011-09-28 at 15:19 -0300, Glauber Costa wrote:
> On 09/27/2011 06:00 PM, Peter Zijlstra wrote:
> > On Fri, 2011-09-23 at 19:20 -0300, Glauber Costa wrote:
> >>   /* Must have preemption disabled for this to be meaningful. */
> >> -#define kstat_this_cpu __get_cpu_var(kstat)
> >> +#define kstat_this_cpu this_cpu_ptr(task_group_kstat(current))
> >
> > This just lost you a debug check, the former would whinge when called
> > without preemption, the new one wont. Its part of the this_cpu feature
> > set to make debugging impossible.
> >
> >> +#else
> >> +#define kstat_cpu(cpu) per_cpu(kstat, cpu)
> >> +#define kstat_this_cpu (&__get_cpu_var(kstat))
> >> +#endif
> >>
> >>   extern unsigned long long nr_context_switches(void);
> >>
> >> @@ -52,8 +62,8 @@ struct irq_desc;
> >>   static inline void kstat_incr_irqs_this_cpu(unsigned int irq,
> >>                                              struct irq_desc *desc)
> >>   {
> >> -       __this_cpu_inc(kstat.irqs[irq]);
> >> -       __this_cpu_inc(kstat.irqs_sum);
> >> +       kstat_this_cpu->irqs[irq]++;
> >> +       kstat_this_cpu->irqs_sum++;
> >
> > It might be worth looking at the asm output of that, I think you made it
> > worse, but I'm not quite sure how smart gcc is, it might just figure out
> > what you meant.
> 
> I'd say leave it alone.
> The biggest difference is that we don't have access to task_group(), or 
> any of the fields in struct task_group. Because of that, we end up 
> having to export a function to do the job of dealing with it.
> 
> Users inside sched.c won't have this problem. Outside of it, we'll add a 
> call to some paths. True, mostly handle_irq paths, but I don't think 
> that's what's going to kill us.
> 
> Now if we really really want to save it, we'd have to move struct 
> task_group and its friends to a more visible location like a header...

I'm not quite getting how task_group is relevant here.

The above will do something like:

	mov gs:$per-cpu-offset-of-kstat, reg
	inc reg + idx*8

whereas __this_cpu_inc() could end up like:

	inc gs:$per-cpu-offset-of-kstat + idx*8

or whatnot. Now clearly gcc could be smart and optimize the temporary
reg thing away in the earlier case, or it might not, I really don't know
how smart that thing is.
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