sched, cpuset: rework sched domains and CPU hotplug handling (v4)

This is an updated version of my previous cpuset patch on top of
the latest mainline git.
The patch fixes CPU hotplug handling issues in the current cpusets code.
Namely circular locking in rebuild_sched_domains() and unsafe access to
the cpu_online_map in the cpuset cpu hotplug handler.

This version includes changes suggested by Paul Jackson (naming, comments,
style, etc). I also got rid of the separate workqueue thread because it is
now safe to call get_online_cpus() from workqueue callbacks.

Here are some more details:

rebuild_sched_domains() is the only way to rebuild sched domains
correctly based on the current cpuset settings. What this means
is that we need to be able to call it from different contexts,
like cpu hotplug for example.
Also latest scheduler code in -tip now calls rebuild_sched_domains()
directly from functions like arch_reinit_sched_domains().

In order to support that properly we need to rework cpuset locking
rules to avoid circular dependencies, which is what this patch does.
New lock nesting rules are explained in the comments.
We can now safely call rebuild_sched_domains() from virtually any
context. The only requirement is that it needs to be called under
get_online_cpus(). This allows cpu hotplug handlers and the scheduler
to call rebuild_sched_domains() directly.
The rest of the cpuset code now offloads sched domains rebuilds to
a workqueue (async_rebuild_sched_domains()).

This version of the patch addresses comments from the previous review.
I fixed all miss-formated comments and trailing spaces.

I also factored out the code that builds domain masks and split up CPU and
memory hotplug handling. This was needed to simplify locking, to avoid unsafe
access to the cpu_online_map from mem hotplug handler, and in general to make
things cleaner.

The patch passes moderate testing (building kernel with -j 16, creating &
removing domains and bringing cpus off/online at the same time) on the
quad-core2 based machine.

It passes lockdep checks, even with preemptable RCU enabled.
This time I also tested in with suspend/resume path and everything is working
as expected.

Signed-off-by: Max Krasnyansky <[e-mail]>
Acked-by: Paul Jackson <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

kernel/cpuset.c

···
  *  2003-10-22 Updates by Stephen Hemminger.
  *  2004 May-July Rework by Paul Jackson.
  *  2006 Rework by Paul Menage to use generic cgroups
+ *  2008 Rework of the scheduler domains and CPU hotplug handling
+ *       by Max Krasnyansky
  *
  *  This file is subject to the terms and conditions of the GNU General Public
  *  License.  See the file COPYING in the main directory of the Linux
···
 
 static DEFINE_MUTEX(callback_mutex);
 
-/* This is ugly, but preserves the userspace API for existing cpuset
+/*
+ * This is ugly, but preserves the userspace API for existing cpuset
  * users. If someone tries to mount the "cpuset" filesystem, we
- * silently switch it to mount "cgroup" instead */
+ * silently switch it to mount "cgroup" instead
+ */
 static int cpuset_get_sb(struct file_system_type *fs_type,
 			 int flags, const char *unused_dev_name,
 			 void *data, struct vfsmount *mnt)
···
 }
 
 /*
- * Helper routine for rebuild_sched_domains().
+ * Helper routine for generate_sched_domains().
  * Do cpusets a, b have overlapping cpus_allowed masks?
  */
-
 static int cpusets_overlap(struct cpuset *a, struct cpuset *b)
 {
 	return cpus_intersects(a->cpus_allowed, b->cpus_allowed);
···
 }
 
 /*
- * rebuild_sched_domains()
+ * generate_sched_domains()
  *
- * This routine will be called to rebuild the scheduler's dynamic
- * sched domains:
- * - if the flag 'sched_load_balance' of any cpuset with non-empty
- *   'cpus' changes,
- * - or if the 'cpus' allowed changes in any cpuset which has that
- *   flag enabled,
- * - or if the 'sched_relax_domain_level' of any cpuset which has
- *   that flag enabled and with non-empty 'cpus' changes,
- * - or if any cpuset with non-empty 'cpus' is removed,
- * - or if a cpu gets offlined.
- *
- * This routine builds a partial partition of the systems CPUs
- * (the set of non-overlappping cpumask_t's in the array 'part'
- * below), and passes that partial partition to the kernel/sched.c
- * partition_sched_domains() routine, which will rebuild the
- * schedulers load balancing domains (sched domains) as specified
- * by that partial partition.  A 'partial partition' is a set of
- * non-overlapping subsets whose union is a subset of that set.
+ * This function builds a partial partition of the systems CPUs
+ * A 'partial partition' is a set of non-overlapping subsets whose
+ * union is a subset of that set.
+ * The output of this function needs to be passed to kernel/sched.c
+ * partition_sched_domains() routine, which will rebuild the scheduler's
+ * load balancing domains (sched domains) as specified by that partial
+ * partition.
  *
  * See "What is sched_load_balance" in Documentation/cpusets.txt
  * for a background explanation of this.
···
  * domains when operating in the severe memory shortage situations
  * that could cause allocation failures below.
  *
- * Call with cgroup_mutex held.  May take callback_mutex during
- * call due to the kfifo_alloc() and kmalloc() calls.  May nest
- * a call to the get_online_cpus()/put_online_cpus() pair.
- * Must not be called holding callback_mutex, because we must not
- * call get_online_cpus() while holding callback_mutex.  Elsewhere
- * the kernel nests callback_mutex inside get_online_cpus() calls.
- * So the reverse nesting would risk an ABBA deadlock.
+ * Must be called with cgroup_lock held.
  *
  * The three key local variables below are:
  *    q  - a linked-list queue of cpuset pointers, used to implement a
···
  *	element of the partition (one sched domain) to be passed to
  *	partition_sched_domains().
  */
-
-void rebuild_sched_domains(void)
+static int generate_sched_domains(cpumask_t **domains,
+			struct sched_domain_attr **attributes)
 {
-	LIST_HEAD(q);		/* queue of cpusets to be scanned*/
+	LIST_HEAD(q);		/* queue of cpusets to be scanned */
 	struct cpuset *cp;	/* scans q */
 	struct cpuset **csa;	/* array of all cpuset ptrs */
 	int csn;		/* how many cpuset ptrs in csa so far */
···
 	int ndoms;		/* number of sched domains in result */
 	int nslot;		/* next empty doms[] cpumask_t slot */
 
-	csa = NULL;
+	ndoms = 0;
 	doms = NULL;
 	dattr = NULL;
+	csa = NULL;
 
 	/* Special case for the 99% of systems with one, full, sched domain */
 	if (is_sched_load_balance(&top_cpuset)) {
-		ndoms = 1;
 		doms = kmalloc(sizeof(cpumask_t), GFP_KERNEL);
 		if (!doms)
-			goto rebuild;
+			goto done;
+
 		dattr = kmalloc(sizeof(struct sched_domain_attr), GFP_KERNEL);
 		if (dattr) {
 			*dattr = SD_ATTR_INIT;
 			update_domain_attr_tree(dattr, &top_cpuset);
 		}
 		*doms = top_cpuset.cpus_allowed;
-		goto rebuild;
+
+		ndoms = 1;
+		goto done;
 	}
 
 	csa = kmalloc(number_of_cpusets * sizeof(cp), GFP_KERNEL);
···
 		}
 	}
 
-	/* Convert <csn, csa> to <ndoms, doms> */
+	/*
+	 * Now we know how many domains to create.
+	 * Convert <csn, csa> to <ndoms, doms> and populate cpu masks.
+	 */
 	doms = kmalloc(ndoms * sizeof(cpumask_t), GFP_KERNEL);
-	if (!doms)
-		goto rebuild;
+	if (!doms) {
+		ndoms = 0;
+		goto done;
+	}
+
+	/*
+	 * The rest of the code, including the scheduler, can deal with
+	 * dattr==NULL case. No need to abort if alloc fails.
+	 */
 	dattr = kmalloc(ndoms * sizeof(struct sched_domain_attr), GFP_KERNEL);
 
 	for (nslot = 0, i = 0; i < csn; i++) {
 		struct cpuset *a = csa[i];
+		cpumask_t *dp;
 		int apn = a->pn;
 
-		if (apn >= 0) {
-			cpumask_t *dp = doms + nslot;
-
-			if (nslot == ndoms) {
-				static int warnings = 10;
-				if (warnings) {
-					printk(KERN_WARNING
-					 "rebuild_sched_domains confused:"
-					  " nslot %d, ndoms %d, csn %d, i %d,"
-					  " apn %d\n",
-					  nslot, ndoms, csn, i, apn);
-					warnings--;
-				}
-				continue;
-			}
-
-			cpus_clear(*dp);
-			if (dattr)
-				*(dattr + nslot) = SD_ATTR_INIT;
-			for (j = i; j < csn; j++) {
-				struct cpuset *b = csa[j];
-
-				if (apn == b->pn) {
-					cpus_or(*dp, *dp, b->cpus_allowed);
-					b->pn = -1;
-					if (dattr)
-						update_domain_attr_tree(dattr
-								   + nslot, b);
-				}
-			}
-			nslot++;
+		if (apn < 0) {
+			/* Skip completed partitions */
+			continue;
 		}
+
+		dp = doms + nslot;
+
+		if (nslot == ndoms) {
+			static int warnings = 10;
+			if (warnings) {
+				printk(KERN_WARNING
+				 "rebuild_sched_domains confused:"
+				  " nslot %d, ndoms %d, csn %d, i %d,"
+				  " apn %d\n",
+				  nslot, ndoms, csn, i, apn);
+				warnings--;
+			}
+			continue;
+		}
+
+		cpus_clear(*dp);
+		if (dattr)
+			*(dattr + nslot) = SD_ATTR_INIT;
+		for (j = i; j < csn; j++) {
+			struct cpuset *b = csa[j];
+
+			if (apn == b->pn) {
+				cpus_or(*dp, *dp, b->cpus_allowed);
+				if (dattr)
+					update_domain_attr_tree(dattr + nslot, b);
+
+				/* Done with this partition */
+				b->pn = -1;
+			}
+		}
+		nslot++;
 	}
 	BUG_ON(nslot != ndoms);
 
-rebuild:
-	/* Have scheduler rebuild sched domains */
-	get_online_cpus();
-	partition_sched_domains(ndoms, doms, dattr);
-	put_online_cpus();
-
 done:
 	kfree(csa);
-	/* Don't kfree(doms) -- partition_sched_domains() does that. */
-	/* Don't kfree(dattr) -- partition_sched_domains() does that. */
+
+	*domains    = doms;
+	*attributes = dattr;
+	return ndoms;
+}
+
+/*
+ * Rebuild scheduler domains.
+ *
+ * Call with neither cgroup_mutex held nor within get_online_cpus().
+ * Takes both cgroup_mutex and get_online_cpus().
+ *
+ * Cannot be directly called from cpuset code handling changes
+ * to the cpuset pseudo-filesystem, because it cannot be called
+ * from code that already holds cgroup_mutex.
+ */
+static void do_rebuild_sched_domains(struct work_struct *unused)
+{
+	struct sched_domain_attr *attr;
+	cpumask_t *doms;
+	int ndoms;
+
+	get_online_cpus();
+
+	/* Generate domain masks and attrs */
+	cgroup_lock();
+	ndoms = generate_sched_domains(&doms, &attr);
+	cgroup_unlock();
+
+	/* Have scheduler rebuild the domains */
+	partition_sched_domains(ndoms, doms, attr);
+
+	put_online_cpus();
+}
+
+static DECLARE_WORK(rebuild_sched_domains_work, do_rebuild_sched_domains);
+
+/*
+ * Rebuild scheduler domains, asynchronously via workqueue.
+ *
+ * If the flag 'sched_load_balance' of any cpuset with non-empty
+ * 'cpus' changes, or if the 'cpus' allowed changes in any cpuset
+ * which has that flag enabled, or if any cpuset with a non-empty
+ * 'cpus' is removed, then call this routine to rebuild the
+ * scheduler's dynamic sched domains.
+ *
+ * The rebuild_sched_domains() and partition_sched_domains()
+ * routines must nest cgroup_lock() inside get_online_cpus(),
+ * but such cpuset changes as these must nest that locking the
+ * other way, holding cgroup_lock() for much of the code.
+ *
+ * So in order to avoid an ABBA deadlock, the cpuset code handling
+ * these user changes delegates the actual sched domain rebuilding
+ * to a separate workqueue thread, which ends up processing the
+ * above do_rebuild_sched_domains() function.
+ */
+static void async_rebuild_sched_domains(void)
+{
+	schedule_work(&rebuild_sched_domains_work);
+}
+
+/*
+ * Accomplishes the same scheduler domain rebuild as the above
+ * async_rebuild_sched_domains(), however it directly calls the
+ * rebuild routine synchronously rather than calling it via an
+ * asynchronous work thread.
+ *
+ * This can only be called from code that is not holding
+ * cgroup_mutex (not nested in a cgroup_lock() call.)
+ */
+void rebuild_sched_domains(void)
+{
+	do_rebuild_sched_domains(NULL);
 }
 
 /**
···
 		return retval;
 
 	if (is_load_balanced)
-		rebuild_sched_domains();
+		async_rebuild_sched_domains();
 	return 0;
 }
 
···
 	if (val != cs->relax_domain_level) {
 		cs->relax_domain_level = val;
 		if (!cpus_empty(cs->cpus_allowed) && is_sched_load_balance(cs))
-			rebuild_sched_domains();
+			async_rebuild_sched_domains();
 	}
 
 	return 0;
···
 	mutex_unlock(&callback_mutex);
 
 	if (cpus_nonempty && balance_flag_changed)
-		rebuild_sched_domains();
+		async_rebuild_sched_domains();
 
 	return 0;
 }
···
 	default:
 		BUG();
 	}
+
+	/* Unreachable but makes gcc happy */
+	return 0;
 }
 
 static s64 cpuset_read_s64(struct cgroup *cont, struct cftype *cft)
···
 	default:
 		BUG();
 	}
+
+	/* Unrechable but makes gcc happy */
+	return 0;
 }
 
 
···
 }
 
 /*
- * Locking note on the strange update_flag() call below:
- *
  * If the cpuset being removed has its flag 'sched_load_balance'
  * enabled, then simulate turning sched_load_balance off, which
- * will call rebuild_sched_domains().  The get_online_cpus()
- * call in rebuild_sched_domains() must not be made while holding
- * callback_mutex.  Elsewhere the kernel nests callback_mutex inside
- * get_online_cpus() calls.  So the reverse nesting would risk an
- * ABBA deadlock.
+ * will call async_rebuild_sched_domains().
  */
 
 static void cpuset_destroy(struct cgroup_subsys *ss, struct cgroup *cont)
···
 struct cgroup_subsys cpuset_subsys = {
 	.name = "cpuset",
 	.create = cpuset_create,
-	.destroy  = cpuset_destroy,
+	.destroy = cpuset_destroy,
 	.can_attach = cpuset_can_attach,
 	.attach = cpuset_attach,
 	.populate = cpuset_populate,
···
 }
 
 /*
- * If common_cpu_mem_hotplug_unplug(), below, unplugs any CPUs
+ * If CPU and/or memory hotplug handlers, below, unplug any CPUs
  * or memory nodes, we need to walk over the cpuset hierarchy,
  * removing that CPU or node from all cpusets.  If this removes the
  * last CPU or node from a cpuset, then move the tasks in the empty
···
 }
 
 /*
- * The cpus_allowed and mems_allowed nodemasks in the top_cpuset track
- * cpu_online_map and node_states[N_HIGH_MEMORY].  Force the top cpuset to
- * track what's online after any CPU or memory node hotplug or unplug event.
- *
- * Since there are two callers of this routine, one for CPU hotplug
- * events and one for memory node hotplug events, we could have coded
- * two separate routines here.  We code it as a single common routine
- * in order to minimize text size.
- */
-
-static void common_cpu_mem_hotplug_unplug(int rebuild_sd)
-{
-	cgroup_lock();
-
-	top_cpuset.cpus_allowed = cpu_online_map;
-	top_cpuset.mems_allowed = node_states[N_HIGH_MEMORY];
-	scan_for_empty_cpusets(&top_cpuset);
-
-	/*
-	 * Scheduler destroys domains on hotplug events.
-	 * Rebuild them based on the current settings.
-	 */
-	if (rebuild_sd)
-		rebuild_sched_domains();
-
-	cgroup_unlock();
-}
-
-/*
  * The top_cpuset tracks what CPUs and Memory Nodes are online,
  * period.  This is necessary in order to make cpusets transparent
  * (of no affect) on systems that are actively using CPU hotplug
···
  *
  * This routine ensures that top_cpuset.cpus_allowed tracks
  * cpu_online_map on each CPU hotplug (cpuhp) event.
+ *
+ * Called within get_online_cpus().  Needs to call cgroup_lock()
+ * before calling generate_sched_domains().
  */
-
-static int cpuset_handle_cpuhp(struct notifier_block *unused_nb,
+static int cpuset_track_online_cpus(struct notifier_block *unused_nb,
 				unsigned long phase, void *unused_cpu)
 {
+	struct sched_domain_attr *attr;
+	cpumask_t *doms;
+	int ndoms;
+
 	switch (phase) {
-	case CPU_UP_CANCELED:
-	case CPU_UP_CANCELED_FROZEN:
-	case CPU_DOWN_FAILED:
-	case CPU_DOWN_FAILED_FROZEN:
 	case CPU_ONLINE:
 	case CPU_ONLINE_FROZEN:
 	case CPU_DEAD:
 	case CPU_DEAD_FROZEN:
-		common_cpu_mem_hotplug_unplug(1);
 		break;
+
 	default:
 		return NOTIFY_DONE;
 	}
+
+	cgroup_lock();
+	top_cpuset.cpus_allowed = cpu_online_map;
+	scan_for_empty_cpusets(&top_cpuset);
+	ndoms = generate_sched_domains(&doms, &attr);
+	cgroup_unlock();
+
+	/* Have scheduler rebuild the domains */
+	partition_sched_domains(ndoms, doms, attr);
 
 	return NOTIFY_OK;
 }
···
 #ifdef CONFIG_MEMORY_HOTPLUG
 /*
  * Keep top_cpuset.mems_allowed tracking node_states[N_HIGH_MEMORY].
- * Call this routine anytime after you change
- * node_states[N_HIGH_MEMORY].
- * See also the previous routine cpuset_handle_cpuhp().
+ * Call this routine anytime after node_states[N_HIGH_MEMORY] changes.
+ * See also the previous routine cpuset_track_online_cpus().
  */
-
 void cpuset_track_online_nodes(void)
 {
-	common_cpu_mem_hotplug_unplug(0);
+	cgroup_lock();
+	top_cpuset.mems_allowed = node_states[N_HIGH_MEMORY];
+	scan_for_empty_cpusets(&top_cpuset);
+	cgroup_unlock();
 }
 #endif
 
···
 	top_cpuset.cpus_allowed = cpu_online_map;
 	top_cpuset.mems_allowed = node_states[N_HIGH_MEMORY];
 
-	hotcpu_notifier(cpuset_handle_cpuhp, 0);
+	hotcpu_notifier(cpuset_track_online_cpus, 0);
 }
 
 /**