Re: [PATCH] Avoid mask based num_possible_cpus and num_online_cpus

[KOSAKI Motohiro]

(1/18/12 1:52 PM), Venki Pallipadi wrote:
> On Tue, Jan 17, 2012 at 9:55 PM, KOSAKI Motohiro
> <kosaki.motohiro@xxxxxxxxx>  wrote:
> > (1/17/12 9:07 PM), Venkatesh Pallipadi wrote:
> > > Kernel's notion of possible cpus (from include/linux/cpumask.h)
> > >    *     cpu_possible_mask- has bit 'cpu' set iff cpu is populatable
> > >
> > >    *  The cpu_possible_mask is fixed at boot time, as the set of CPU id's
> > >    *  that it is possible might ever be plugged in at anytime during the
> > >    *  life of that system boot.
> > >
> > >    #define num_possible_cpus()     cpumask_weight(cpu_possible_mask)
> > >
> > > and on x86 cpumask_weight() calls hweight64 and hweight64 (on older kernels
> > > and systems with !X86_FEATURE_POPCNT) or a popcnt based alternative.
> > >
> > > i.e, We needlessly go through this mask based calculation everytime
> > > num_possible_cpus() is called.
> > >
> > > The problem is there with cpu_online_mask() as well, which is fixed value at
> > > boot time in !CONFIG_HOTPLUG_CPU case and should not change that often even
> > > in HOTPLUG case.
> > >
> > > Though most of the callers of these two routines are init time (with few
> > > exceptions of runtime calls), it is cleaner to use variables
> > > and not go through this repeated mask based calculation.
> > >
> > > Signed-off-by: Venkatesh Pallipadi<venki@xxxxxxxxxx>
> > > ---
> > >    include/linux/cpumask.h |    8 ++++++--
> > >    kernel/cpu.c            |    9 +++++++++
> > >    2 files changed, 15 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
> > > index 4f7a632..2eb04dd 100644
> > > --- a/include/linux/cpumask.h
> > > +++ b/include/linux/cpumask.h
> > > @@ -80,9 +80,13 @@ extern const struct cpumask *const cpu_online_mask;
> > >    extern const struct cpumask *const cpu_present_mask;
> > >    extern const struct cpumask *const cpu_active_mask;
> > >
> > > +extern int nr_online_cpus;
> > > +
> > >    #if NR_CPUS>    1
> > > -#define num_online_cpus()    cpumask_weight(cpu_online_mask)
> > > -#define num_possible_cpus()  cpumask_weight(cpu_possible_mask)
> > > +
> > > +#define num_online_cpus()    (nr_online_cpus)
> > > +#define num_possible_cpus()  (nr_cpu_ids)
> >
> > nr_cpu_ids mean maximum cpu id of cpus. if cpu id are sparse, maximum id
> > doesn't match number of cpus.
>
> Yes. But will it be sparse in any arch? I saw some of the users of
> num_possible_cpus() doing things like allocating a buffer for that
> size and then indexing it using get_cpu(). So, I thought it would be
> better to use the existing nr_cpu_ids instead of inventing another
> variable. If indeed any arch is depending on this being sparse, we can
> have a new variable similar to num_possible_cpus and also audit all
> users of num_possible_cpus to see whether they should be using
> nr_cpu_ids instead.

If my remember is correct, sparc does.


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