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SubjectRe: [NFS] [PATCH 010 of 11] knfsd: make rpc threads pools numa aware
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On Mon, 2006-07-31 at 14:36, Neil Brown wrote:
> On Sunday July 30, akpm@osdl.org wrote:
> > On Mon, 31 Jul 2006 10:42:34 +1000
> > NeilBrown <neilb@suse.de> wrote:
> >
> > > +static int
> > > +svc_pool_map_init_percpu(struct svc_pool_map *m)
> > > +{
> > > + unsigned int maxpools = num_possible_cpus();
> > > + unsigned int pidx = 0;
> > > + unsigned int cpu;
> > > + int err;
> > > +
> > > + err = svc_pool_map_alloc_arrays(m, maxpools);
> > > + if (err)
> > > + return err;
> > > +
> > > + for_each_online_cpu(cpu) {
> > > + BUG_ON(pidx > maxpools);
> > > + m->to_pool[cpu] = pidx;
> > > + m->pool_to[pidx] = cpu;
> > > + pidx++;
> > > + }
> >
> > That isn't right - it assumes that cpu_possible_map is not sparse. If it
> > is sparse, we allocate undersized pools and then overindex them.
>
> I don't think so.
>
> At this point we are largely counting the number of online cpus
> (in pidx (pool index) - this is returned). The two-way mapping
> to_pool and pool_to provides a mapping between the possible-sparse cpu
> list and a dense list of pool indexes.
>
> If further cpus come on line they will be automatically included in
> pool-0. (as to_pool[n] will still be zero).
>
> Does that make it at all clearer?

Umm, I think Andrew's right, num_possible_cpus() should be NR_CPUS.
If there's a value of `cpu' > num_possible_cpus(), which would happen
if the cpu numbers weren't contiguous, then m->to_pool[] will be
overflowed. My bad, I didn't even consider the case of non-contiguous
CPU numbers and none of the machines available for testing had that
property.


Greg.
--
Greg Banks, R&D Software Engineer, SGI Australian Software Group.
I don't speak for SGI.


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