cpuset: fix regression when failed to generate sched domains

Impact: properly rebuild sched-domains on kmalloc() failure

When cpuset failed to generate sched domains due to kmalloc()
failure, the scheduler should fallback to the single partition
'fallback_doms' and rebuild sched domains, but now it only
destroys but not rebuilds sched domains.

The regression was introduced by:

| commit dfb512ec48
| Author: Max Krasnyansky <maxk@qualcomm.com>
| Date:   Fri Aug 29 13:11:41 2008 -0700
|
|    sched: arch_reinit_sched_domains() must destroy domains to force rebuild

After the above commit, partition_sched_domains(0, NULL, NULL) will
only destroy sched domains and partition_sched_domains(1, NULL, NULL)
will create the default sched domain.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Max Krasnyansky <maxk@qualcomm.com>
Cc: <stable@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/cpuset.c

@@ -587,7 +587,6 @@ static int generate_sched_domains(cpumask_t **domains,
 	int ndoms;		/* number of sched domains in result */
 	int nslot;		/* next empty doms[] cpumask_t slot */
 
-	ndoms = 0;
 	doms = NULL;
 	dattr = NULL;
 	csa = NULL;
@@ -674,10 +673,8 @@ restart:
 	 * Convert <csn, csa> to <ndoms, doms> and populate cpu masks.
 	 */
 	doms = kmalloc(ndoms * sizeof(cpumask_t), GFP_KERNEL);
-	if (!doms) {
-		ndoms = 0;
+	if (!doms)
 		goto done;
-	}
 
 	/*
 	 * The rest of the code, including the scheduler, can deal with
@@ -732,6 +729,13 @@ restart:
 done:
 	kfree(csa);
 
+	/*
+	 * Fallback to the default domain if kmalloc() failed.
+	 * See comments in partition_sched_domains().
+	 */
+	if (doms == NULL)
+		ndoms = 1;
+
 	*domains    = doms;
 	*attributes = dattr;
 	return ndoms;

kernel/sched.c

@@ -7789,13 +7789,14 @@ static int dattrs_equal(struct sched_domain_attr *cur, int idx_cur,
  *
  * The passed in 'doms_new' should be kmalloc'd. This routine takes
  * ownership of it and will kfree it when done with it. If the caller
- * failed the kmalloc call, then it can pass in doms_new == NULL,
- * and partition_sched_domains() will fallback to the single partition
- * 'fallback_doms', it also forces the domains to be rebuilt.
+ * failed the kmalloc call, then it can pass in doms_new == NULL &&
+ * ndoms_new == 1, and partition_sched_domains() will fallback to
+ * the single partition 'fallback_doms', it also forces the domains
+ * to be rebuilt.
  *
- * If doms_new==NULL it will be replaced with cpu_online_map.
- * ndoms_new==0 is a special case for destroying existing domains.
- * It will not create the default domain.
+ * If doms_new == NULL it will be replaced with cpu_online_map.
+ * ndoms_new == 0 is a special case for destroying existing domains,
+ * and it will not create the default domain.
  *
  * Call with hotplug lock held
  */