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    SubjectRe: [RFD 1/9] Change cpustat fields to an array.
    On 09/28/2011 11:09 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:$per-cpu-offset-of-kstat, reg
    > inc reg + idx*8
    >
    > whereas __this_cpu_inc() could end up like:
    >
    > inc gs:$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.
    Btw, asm output with CGROUP_SCHED disabled seem to be no worse than
    what is in now.


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