Messages in this thread | | | Subject | Re: [RFD 1/9] Change cpustat fields to an array. | From | Peter Zijlstra <> | Date | Wed, 28 Sep 2011 21:09:17 +0200 |
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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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