Re: [PATCH] percpu data: only iterate over possible CPUs
From: Nathan Lynch
Date: Thu Feb 09 2006 - 11:06:23 EST
Andrew Morton wrote:
> 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.
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Yes, that comment is wrong or outdated.
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