cpumask_set_cpu_local_first => cpumask_local_spread, lament

da91309e0a7e (cpumask: Utility function to set n'th cpu...) created a
genuinely weird function.  I never saw it before, it went through DaveM.
(He only does this to make us other maintainers feel better about our own
mistakes.)

cpumask_set_cpu_local_first's purpose is say "I need to spread things
across N online cpus, choose the ones on this numa node first"; you call
it in a loop.

It can fail.  One of the two callers ignores this, the other aborts and
fails the device open.

It can fail in two ways: allocating the off-stack cpumask, or through a
convoluted codepath which AFAICT can only occur if cpu_online_mask
changes.  Which shouldn't happen, because if cpu_online_mask can change
while you call this, it could return a now-offline cpu anyway.

It contains a nonsensical test "!cpumask_of_node(numa_node)".  This was
drawn to my attention by Geert, who said this causes a warning on Sparc.
It sets a single bit in a cpumask instead of returning a cpu number,
because that's what the callers want.

It could be made more efficient by passing the previous cpu rather than
an index, but that would be more invasive to the callers.

Fixes: da91309e0a7e8966d916a74cce42ed170fde06bf
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (then rebased)
Tested-by: Amir Vadai <amirv@mellanox.com>
Acked-by: Amir Vadai <amirv@mellanox.com>
Acked-by: David S. Miller <davem@davemloft.net>

1 files changed, 26 insertions, 48 deletions

diff --git a/lib/cpumask.c b/lib/cpumask.c
index 830dd5d..5f62708 100644
--- a/lib/cpumask.c
+++ b/lib/cpumask.c
@@ -139,64 +139,42 @@ void __init free_bootmem_cpumask_var(cpumask_var_t mask)
 #endif
 
 /**
- * cpumask_set_cpu_local_first - set i'th cpu with local numa cpu's first
- *
+ * cpumask_local_spread - select the i'th cpu with local numa cpu's first
  * @i: index number
- * @numa_node: local numa_node
- * @dstp: cpumask with the relevant cpu bit set according to the policy
+ * @node: local numa_node
  *
- * This function sets the cpumask according to a numa aware policy.
- * cpumask could be used as an affinity hint for the IRQ related to a
- * queue. When the policy is to spread queues across cores - local cores
- * first.
+ * This function selects an online CPU according to a numa aware policy;
+ * local cpus are returned first, followed by non-local ones, then it
+ * wraps around.
  *
- * Returns 0 on success, -ENOMEM for no memory, and -EAGAIN when failed to set
- * the cpu bit and need to re-call the function.
+ * It's not very efficient, but useful for setup.
  */
-int cpumask_set_cpu_local_first(int i, int numa_node, cpumask_t *dstp)
+unsigned int cpumask_local_spread(unsigned int i, int node)
 {
-	cpumask_var_t mask;
 	int cpu;
-	int ret = 0;
-
-	if (!zalloc_cpumask_var(&mask, GFP_KERNEL))
-		return -ENOMEM;
 
+	/* Wrap: we always want a cpu. */
 	i %= num_online_cpus();
 
-	if (numa_node == -1 || !cpumask_of_node(numa_node)) {
-		/* Use all online cpu's for non numa aware system */
-		cpumask_copy(mask, cpu_online_mask);
+	if (node == -1) {
+		for_each_cpu(cpu, cpu_online_mask)
+			if (i-- == 0)
+				return cpu;
 	} else {
-		int n;
-
-		cpumask_and(mask,
-			    cpumask_of_node(numa_node), cpu_online_mask);
-
-		n = cpumask_weight(mask);
-		if (i >= n) {
-			i -= n;
-
-			/* If index > number of local cpu's, mask out local
-			 * cpu's
-			 */
-			cpumask_andnot(mask, cpu_online_mask, mask);
+		/* NUMA first. */
+		for_each_cpu_and(cpu, cpumask_of_node(node), cpu_online_mask)
+			if (i-- == 0)
+				return cpu;
+
+		for_each_cpu(cpu, cpu_online_mask) {
+			/* Skip NUMA nodes, done above. */
+			if (cpumask_test_cpu(cpu, cpumask_of_node(node)))
+				continue;
+
+			if (i-- == 0)
+				return cpu;
 		}
 	}
-
-	for_each_cpu(cpu, mask) {
-		if (--i < 0)
-			goto out;
-	}
-
-	ret = -EAGAIN;
-
-out:
-	free_cpumask_var(mask);
-
-	if (!ret)
-		cpumask_set_cpu(cpu, dstp);
-
-	return ret;
+	BUG();
 }
-EXPORT_SYMBOL(cpumask_set_cpu_local_first);
+EXPORT_SYMBOL(cpumask_local_spread);