Re: [PATCH] percpu data: only iterate over possible CPUs

[Eric Dumazet]

Andrew Morton a écrit :
> Andrew Morton <akpm@xxxxxxxx> wrote:
> > Users of __GENERIC_PER_CPU definitely need cpu_possible_map to be initialised
> >  by the time setup_per_cpu_areas() is called,
>
> err, they'll need it once Eric's
> dont-waste-percpu-memory-on-not-possible-CPUs patch is merged..
>
> > so I think it makes sense to
> >  say "thou shalt initialise cpu_possible_map in setup_arch()".
> >
> >  I guess Xen isn't doing that.  Can it be made to?
>
> Lame fix:  cpu_possible_map = (1<<NR_CPUS)-1 in setup_arch().

I dont understand why this HOTPLUG stuff is problematic for Xen (or other 
arch) : If CONFIG_HOTPLUG_CPU is configured, then the map should be preset to 
CPU_MASK_ALL. Its even documented in line 332 of include/linux/cpumask.h

 *  #ifdef CONFIG_HOTPLUG_CPU
 *     cpu_possible_map - all NR_CPUS bits set

arch/i386/kernel/smpboot.c is doing the only sane stuff about it :

#ifdef CONFIG_HOTPLUG_CPU
cpumask_t cpu_possible_map = CPU_MASK_ALL;
#else
cpumask_t cpu_possible_map;
#endif

Some remarks :

1) These cpu_possible_map could have __read_mostly attribute.
2) cpu_possible(cpu) macro could be defined to 1 if CONFIG_HOTPLUG_CPU, or a 
test against NR_CPUS

#ifdef CONFIG_HOTPLUG_CPU
#define cpu_possible(cpu)   cpu_isset((cpu), cpu_possible_map)
#else
#define cpu_possible(cpu)   ((unsigned)(cpu) < NR_CPUS)
#endif

Eric
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