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

[Andrew Morton]

Eric Dumazet <dada1@xxxxxxxxxxxxx> wrote:
>
> 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.

Presumably not all architectures are doing that.   And some of the problems
we've had are with CONFIG_HOTPLUG_CPU=n.

> Its even documented in line 332 of include/linux/cpumask.h
> 
>   *  #ifdef CONFIG_HOTPLUG_CPU
>   *     cpu_possible_map - all NR_CPUS bits set

That seems a quite bad idea.  If we know which CPUs are possible we should
populate cpu_possible_map correctly, whether or not CONFIG_HOTPLUG_CPU is
set.  Setting all the bits like that wastes memory and wastes CPU cycles.

<greps>

That comment came from the tender pinkies of pj@xxxxxxx, although I suspect
it was just a transliteration of then-current practice.

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