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

[Nathan Lynch <ntl@pobox.com>]

Andrew Morton wrote:
> Eric Dumazet <dada1@cosmosbay.com> wrote:
> >
> > Andrew Morton a écrit :
> > > Andrew Morton <akpm@osdl.org> 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.

powerpc/ppc64, for instance, determines the number of possible cpus
from information exported by firmware (and I'm mystified as to why
other platforms don't do this).  So it's typical to have a kernel an a
pSeries partition with NR_CPUS=128, but cpu_possible_map = 0xff.


> > 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.

Yes, that comment is wrong or outdated.