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

From: Eric Dumazet
Date: Wed Feb 08 2006 - 23:34:56 EST

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

* cpu_possible_map - all NR_CPUS bits set

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

cpumask_t cpu_possible_map = CPU_MASK_ALL;
cpumask_t cpu_possible_map;

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

#define cpu_possible(cpu) cpu_isset((cpu), cpu_possible_map)
#define cpu_possible(cpu) ((unsigned)(cpu) < NR_CPUS)

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