On 4/8/21 1:26 AM, Oscar Salvador wrote:
> On Thu, Apr 01, 2021 at 11:32:19AM -0700, Dave Hansen wrote:
>> The protocol for node_demotion[] access and writing is not
>> standard. It has no specific locking and is intended to be read
>> locklessly. Readers must take care to avoid observing changes
>> that appear incoherent. This was done so that node_demotion[]
>
> It might be just me being dense here, but that reads odd.
>
> "Readers must take care to avoid observing changes that appear
> incoherent" - I am not sure what is that supposed to mean.
>
> I guess you mean readers of next_demotion_node()?
> And if so, how do they have to take care? And what would apply for
> "incoherent" terminology here?
I've fleshed out the description a bit. I hope this helps?
> Readers of node_demotion[] (such as next_demotion_node() callers)
> must take care to avoid observing changes that appear incoherent.
> For instance, even though no demotion cycles are allowed, it's
> possible that a cycle could be observed.
>
> Let's say that there are three nodes, A, B and C. node_demotion[]
> is set up to have this path:
>
> A -> B -> C
>
> Suppose it was modified to instead represent this path:
>
> A -> C -> B
>
> There is nothing to stop a reader from seeing B->C and then a
> moment later seeting C->B. That *appears* to be a cycle. This
> can be avoided with RCU and will be implemented in a later patch.
...
>> +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 destination.
>> + */
>> + 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))
>
> When I read this I was about puzzled and it took me a while to figure
> out how the passes were made.
> I think this could benefit from a better explanation on how the passes
> are being performed e.g: why next_pass should be empty before leaving.
>
> Other than that looks good to me.
I've tried to flesh out those comments to elaborate on what is going on:
> /*
> * Visit targets from this pass in the next pass.
> * Eventually, every node will have been part of
> * a pass, and will become set in 'used_targets'.
> */
> node_set(target_node, next_pass);
> }
> /*
> * 'next_pass' contains nodes which became migration
> * targets in this pass. Make additional passes until
> * no more migrations targets are available.
> */
> if (!nodes_empty(next_pass))
> goto again;
> }
