Re: [RFC][PATCH 05/13] mm/numa: automatically generate node migration order

[Yang Shi]

On Mon, Feb 1, 2021 at 11:13 AM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
>
> On 1/29/21 12:46 PM, Yang Shi wrote:
> ...
> >>  int next_demotion_node(int node)
> >>  {
> >> -       return node_demotion[node];
> >> +       /*
> >> +        * node_demotion[] is updated without excluding
> >> +        * this function from running.  READ_ONCE() avoids
> >> +        * reading multiple, inconsistent 'node' values
> >> +        * during an update.
> >> +        */
> >
> > Don't we need a smp_rmb() here? The single write barrier might be not
> > enough in migration target set. Typically a write barrier should be
> > used in pairs with a read barrier.
>
> I don't think we need one, practically.
>
> Since there is no locking against node_demotion[] updates, although a
> smp_rmb() would ensure that this read is up-to-date, it could change
> freely after the smp_rmb().

Yes, but this should be able to guarantee we see "disable + after"
state. Isn't it more preferred?

>
> In other words, smp_rmb() would shrink the window where a "stale" read
> could occur but would not eliminate it.
>
> >> +       return READ_ONCE(node_demotion[node]);
> >
> > Why not consolidate the patch #4 in this patch? The patch #4 just add
> > the definition of node_demotion and the function, then the function is
> > changed in this patch, and the function is not used by anyone between
> > the adding and changing.
>
> I really wanted to highlight that the locking scheme and the READ_ONCE()
> (or lack thereof) was specifically due to how node_demotion[] was being
> updated.
>
> The READ_ONCE() is not, for instance, inherent to the data structure.
>
> ...
> >> +/*
> >> + * When memory fills up on a node, memory contents can be
> >> + * automatically migrated to another node instead of
> >> + * discarded at reclaim.
> >> + *
> >> + * Establish a "migration path" which will start at nodes
> >> + * with CPUs and will follow the priorities used to build the
> >> + * page allocator zonelists.
> >> + *
> >> + * The difference here is that cycles must be avoided.  If
> >> + * node0 migrates to node1, then neither node1, nor anything
> >> + * node1 migrates to can migrate to node0.
> >> + *
> >> + * This function can run simultaneously with readers of
> >> + * node_demotion[].  However, it can not run simultaneously
> >> + * with itself.  Exclusion is provided by memory hotplug events
> >> + * being single-threaded.
> >
> > Maybe an example diagram for the physical topology and how the
> > migration target is generated in the comment seems helpful to
> > understand the code.
>
> Sure.  Were you thinking of a code comment, or enhanced changelog?

I'd prefer a code comment.

>
> Let's say there's a system with two sockets each with the same three
> classes of memory: fast, medium and slow.  Each memory class is placed
> in its own NUMA node and the CPUs are attached to the fast memory.  That
> leaves 6 NUMA nodes (0-5):
>
>         Socket A: 0, 1, 2
>         Socket B: 3, 4, 5
>
> The migration path for this configuration path would start on the nodes
> with the processors and fast memory, progress through medium and end
> with the slow memory:
>
>         0 -> 1 -> 2 -> stop
>         3 -> 4 -> 5 -> stop
>
> This is represented in the node_demotion[] like this:
>
>         {  1, // Node 0 migrates to 1
>            2, // Node 1 migrates to 2
>           -1, // Node 2 does not migrate
>            4, // Node 3 migrates to 1
>            5, // Node 4 migrates to 2
>           -1} // Node 5 does not migrate
>
> Is that what you were thinking of?

Perfect.

>
> ...
> >> +again:
> >> +       this_pass = next_pass;
> >> +       next_pass = NODE_MASK_NONE;
> >> +       /*
> >> +        * To avoid cycles in the migration "graph", ensure
> >> +        * that migration sources are not future targets by
> >> +        * setting them in 'used_targets'.  Do this only
> >> +        * once per pass so that multiple source nodes can
> >> +        * share a target node.
> >> +        *
> >> +        * 'used_targets' will become unavailable in future
> >> +        * passes.  This limits some opportunities for
> >> +        * multiple source nodes to share a desintation.
> >
> > s/desination/destination
>
> Fixed, thanks.
>
> >> +        */
> >> +       nodes_or(used_targets, used_targets, this_pass);
> >> +       for_each_node_mask(node, this_pass) {
> >> +               int target_node = establish_migrate_target(node, &used_targets);
> >> +
> >> +               if (target_node == NUMA_NO_NODE)
> >> +                       continue;
> >> +
> >> +               /* Visit targets from this pass in the next pass: */
> >> +               node_set(target_node, next_pass);
> >> +       }
> >> +       /* Is another pass necessary? */
> >> +       if (!nodes_empty(next_pass))
> >> +               goto again;
> >> +}
> >> +
> >> +void set_migration_target_nodes(void)
> >
> > It seems this function is not called outside migrate.c, so it should be static.
>
> Fixed, thanks.