Re: [PATCH 7/7] x86/resctrl: Determine if Sub-NUMA Cluster is enabled and initialize.

[Tony Luck]

On Mon, Feb 27, 2023 at 02:30:38PM +0100, Peter Newman wrote:
> Hi Tony,
> 
> On Thu, Jan 26, 2023 at 7:42 PM Tony Luck <tony.luck@xxxxxxxxx> wrote:
> > +static __init int find_snc_ways(void)
> > +{
> > +       unsigned long *node_caches;
> > +       int cpu, node, ret;
> > +
> > +       node_caches = kcalloc(BITS_TO_LONGS(nr_node_ids), sizeof(*node_caches), GFP_KERNEL);
> > +       if (!node_caches)
> > +               return 1;
> > +
> > +       cpus_read_lock();
> > +       for_each_node(node) {
> 
> Someone tried this patch on a machine with a CPU-less node...
> 
> We need to check for this:
> 
> +               if (cpumask_empty(cpumask_of_node(node)))
> +                       continue;
> 
> > +               cpu = cpumask_first(cpumask_of_node(node));
> > +               set_bit(get_cpu_cacheinfo_id(cpu, 3), node_caches);
> > +       }
> > +       cpus_read_unlock();

Peter,

Tell me more about your CPU-less nodes.  Your fix avoids a bad
pointer reference (because cpumask_first() returns cpu >= nr_cpu_ids
for an empty bitmask).

But now I'm worried about whether I have the right values in the
formula:

	nr_node_ids / bitmap_weight(node_caches, nr_node_ids);

This fix avoids counting the L3 from a non-existent CPU, but still
counts the node in the numerator.

Is your CPU-less node a full (non-SNC) node? Like this:

          Socket 0                 Socket 1
    +--------------------+  +--------------------+
    |          .         |  |          .         |
    |  SNC 0.0 . SNC 0.1 |  |  zero    . zero    |
    |          .         |  |  CPUs    . CPUs    |
    |          .         |  |          .         |
    |          .         |  |          .         |
    +--------------------+  +--------------------+
    |      L3 Cache      |  |      L3 Cache      |
    +--------------------+  +--------------------+

I could fix this case by counting how many CPU-less
nodes I find, and reducing the numerator (the denominator
didn't count the L3 cache from socket 1 because there
are no CPUs there)

	(nr_node_ids - n_empty_nodes) / bitmap_weight(node_caches, nr_node_ids);

	=> 2 / 1

But that won't work if your CPU-less node is an SNC node
and the other SNC node in the same socket does have some
CPUs:

          Socket 0                 Socket 1
    +--------------------+  +--------------------+
    |          .         |  |          .         |
    |  SNC 0.0 . SNC 0.1 |  |  zero    . SNC 1.1 |
    |          .         |  |  CPUs    .         |
    |          .         |  |          .         |
    |          .         |  |          .         |
    +--------------------+  +--------------------+
    |      L3 Cache      |  |      L3 Cache      |
    +--------------------+  +--------------------+

This would get 3 / 2 ... i.e. I should still count the
empty node because its cache was counted by its SNC
buddy.

-Tony