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

[Glauber Costa]

On 09/28/2011 04:09 PM, Peter Zijlstra wrote:
> 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

except offset of kstat is not fixed. It is dynamic allocated, so you have
to calculate it. You can't really do it without knowing at least the 
address of task_group, and then, the offset of kstat inside it.

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