linux-kernel - Re: [PATCH 010 of 11] knfsd: make rpc threads pools numa aware

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Date:	Mon, 31 Jul 2006 14:36:09 +1000
From:	Neil Brown <neilb@...e.de>
To:	Andrew Morton <akpm@...l.org>
Cc:	nfs@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 010 of 11] knfsd: make rpc threads pools numa aware

On Sunday July 30, akpm@...l.org wrote:
> On Mon, 31 Jul 2006 10:42:34 +1000
> NeilBrown <neilb@...e.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?

Thanks,
NeilBrown
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