Gregory Price [off-list ref] writes:

On Fri, Jan 26, 2024 at 03:40:27PM +0800, Huang, Ying wrote:

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Gregory Price [off-list ref] writes:

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Two special observations:
- if the weight is non-zero, cur_il_weight must *always* have a
  valid node number, e.g. it cannot be NUMA_NO_NODE (-1).

IIUC, we don't need that, "MAX_NUMNODES-1" is used instead.

Correct, I just thought it pertinent to call this out explicitly since
I'm stealing the top byte, but the node value has traditionally been a
full integer.

This may be relevant should anyone try to carry, a random node value
into this field. For example, if someone tried to copy policy->home_node
into cur_il_weight for whatever reason.

It's worth breaking out a function to defend against this - plus to hide
the bit operations directly as you recommend below.

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 	/* Weighted interleave settings */
-	u8 cur_il_weight;
+	atomic_t cur_il_weight;

If we use this field for node and weight, why not change the field name?
For example, cur_wil_node_weight.

ack.

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+			if (cweight & 0xFF)
+				*policy = cweight >> 8;

Please define some helper functions or macros instead of operate on bits
directly.

ack.

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 			else
 				*policy = next_node_in(current->il_prev,
 						       pol->nodes);

If we record current node in pol->cur_il_weight, why do we still need
curren->il_prev.  Can we only use pol->cur_il_weight?  And if so, we can
even make current->il_prev a union.

I just realized that there's a problem here for shared memory policies.

from weighted_interleave_nodes, I do this:

cur_weight = atomic_read(&policy->cur_il_weight);
...
weight--;
...
atomic_set(&policy->cur_il_weight, cur_weight);

On a shared memory policy, this is a race condition.


I don't think we can combine il_prev and cur_wil_node_weight because
the task policy may be different than the current policy.

i.e. it's totally valid to do the following:

1) set_mempolicy(MPOL_INTERLEAVE)
2) mbind(..., MPOL_WEIGHTED_INTERLEAVE)

Using current->il_prev between these two policies, is just plain incorrect,
so I will need to rethink this, and the existing code will need to be
updated such that weighted_interleave does not use current->il_prev.

IIUC, weighted_interleave_nodes() is only used for mempolicy of tasks
(set_mempolicy()), as in the following code.

+		*nid = (ilx == NO_INTERLEAVE_INDEX) ?
+			weighted_interleave_nodes(pol) :
+			weighted_interleave_nid(pol, ilx);

But, in contrast, it's bad to put task-local "current weight" in
mempolicy.  So, I think that it's better to move cur_il_weight to
task_struct.  And maybe combine it with current->il_prev.

--
Best Regards,
Huang, Ying