[PATCH] per-task delay accounting taskstats interface: control exit data through cpumasks

On systems with a large number of cpus, with even a modest rate of tasks
exiting per cpu, the volume of taskstats data sent on thread exit can
overflow a userspace listener's buffers.

One approach to avoiding overflow is to allow listeners to get data for a
limited and specific set of cpus.  By scaling the number of listeners
and/or the cpus they monitor, userspace can handle the statistical data
overload more gracefully.

In this patch, each listener registers to listen to a specific set of cpus
by specifying a cpumask.  The interest is recorded per-cpu.  When a task
exits on a cpu, its taskstats data is unicast to each listener interested
in that cpu.

Thanks to Andrew Morton for pointing out the various scalability and
general concerns of previous attempts and for suggesting this design.

[akpm@osdl.org: build fix]
Signed-off-by: Shailabh Nagar <nagar@watson.ibm.com>
Signed-off-by: Balbir Singh <balbir@in.ibm.com>
Signed-off-by: Chandra Seetharaman <sekharan@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/taskstats.h

@@ -91,8 +91,6 @@ struct taskstats {
 };
 
 
-#define TASKSTATS_LISTEN_GROUP	0x1
-
 /*
  * Commands sent from userspace
  * Not versioned. New commands should only be inserted at the enum's end
@@ -124,6 +122,8 @@ enum {
 	TASKSTATS_CMD_ATTR_UNSPEC = 0,
 	TASKSTATS_CMD_ATTR_PID,
 	TASKSTATS_CMD_ATTR_TGID,
+	TASKSTATS_CMD_ATTR_REGISTER_CPUMASK,
+	TASKSTATS_CMD_ATTR_DEREGISTER_CPUMASK,
 	__TASKSTATS_CMD_ATTR_MAX,
 };
 

include/linux/taskstats_kern.h

@@ -11,30 +11,10 @@
 #include <linux/sched.h>
 #include <net/genetlink.h>
 
-enum {
-	TASKSTATS_MSG_UNICAST,		/* send data only to requester */
-	TASKSTATS_MSG_MULTICAST,	/* send data to a group */
-};
-
 #ifdef CONFIG_TASKSTATS
 extern kmem_cache_t *taskstats_cache;
 extern struct mutex taskstats_exit_mutex;
 
-static inline int taskstats_has_listeners(void)
-{
-	if (!genl_sock)
-		return 0;
-	return netlink_has_listeners(genl_sock, TASKSTATS_LISTEN_GROUP);
-}
-
-
-static inline void taskstats_exit_alloc(struct taskstats **ptidstats)
-{
-	*ptidstats = NULL;
-	if (taskstats_has_listeners())
-		*ptidstats = kmem_cache_zalloc(taskstats_cache, SLAB_KERNEL);
-}
-
 static inline void taskstats_exit_free(struct taskstats *tidstats)
 {
 	if (tidstats)
@@ -82,17 +62,18 @@ static inline void taskstats_tgid_free(struct signal_struct *sig)
 		kmem_cache_free(taskstats_cache, stats);
 }
 
-extern void taskstats_exit_send(struct task_struct *, struct taskstats *, int);
+extern void taskstats_exit_alloc(struct taskstats **, unsigned int *);
+extern void taskstats_exit_send(struct task_struct *, struct taskstats *, int, unsigned int);
 extern void taskstats_init_early(void);
 extern void taskstats_tgid_alloc(struct signal_struct *);
 #else
-static inline void taskstats_exit_alloc(struct taskstats **ptidstats)
+static inline void taskstats_exit_alloc(struct taskstats **ptidstats, unsigned int *mycpu)
 {}
 static inline void taskstats_exit_free(struct taskstats *ptidstats)
 {}
 static inline void taskstats_exit_send(struct task_struct *tsk,
 				       struct taskstats *tidstats,
-				       int group_dead)
+				       int group_dead, unsigned int cpu)
 {}
 static inline void taskstats_tgid_init(struct signal_struct *sig)
 {}

kernel/exit.c

@@ -847,6 +847,7 @@ fastcall NORET_TYPE void do_exit(long code)
 	struct task_struct *tsk = current;
 	struct taskstats *tidstats;
 	int group_dead;
+	unsigned int mycpu;
 
 	profile_task_exit(tsk);
 
@@ -884,7 +885,7 @@ fastcall NORET_TYPE void do_exit(long code)
 				current->comm, current->pid,
 				preempt_count());
 
-	taskstats_exit_alloc(&tidstats);
+	taskstats_exit_alloc(&tidstats, &mycpu);
 
 	acct_update_integrals(tsk);
 	if (tsk->mm) {
@@ -905,7 +906,7 @@ fastcall NORET_TYPE void do_exit(long code)
 #endif
 	if (unlikely(tsk->audit_context))
 		audit_free(tsk);
-	taskstats_exit_send(tsk, tidstats, group_dead);
+	taskstats_exit_send(tsk, tidstats, group_dead, mycpu);
 	taskstats_exit_free(tidstats);
 	delayacct_tsk_exit(tsk);
 

kernel/taskstats.c

@@ -19,9 +19,17 @@
 #include <linux/kernel.h>
 #include <linux/taskstats_kern.h>
 #include <linux/delayacct.h>
+#include <linux/cpumask.h>
+#include <linux/percpu.h>
 #include <net/genetlink.h>
 #include <asm/atomic.h>
 
+/*
+ * Maximum length of a cpumask that can be specified in
+ * the TASKSTATS_CMD_ATTR_REGISTER/DEREGISTER_CPUMASK attribute
+ */
+#define TASKSTATS_CPUMASK_MAXLEN	(100+6*NR_CPUS)
+
 static DEFINE_PER_CPU(__u32, taskstats_seqnum) = { 0 };
 static int family_registered;
 kmem_cache_t *taskstats_cache;
@@ -37,8 +45,25 @@ static struct nla_policy taskstats_cmd_get_policy[TASKSTATS_CMD_ATTR_MAX+1]
 __read_mostly = {
 	[TASKSTATS_CMD_ATTR_PID]  = { .type = NLA_U32 },
 	[TASKSTATS_CMD_ATTR_TGID] = { .type = NLA_U32 },
+	[TASKSTATS_CMD_ATTR_REGISTER_CPUMASK] = { .type = NLA_STRING },
+	[TASKSTATS_CMD_ATTR_DEREGISTER_CPUMASK] = { .type = NLA_STRING },};
+
+struct listener {
+	struct list_head list;
+	pid_t pid;
 };
 
+struct listener_list {
+	struct rw_semaphore sem;
+	struct list_head list;
+};
+static DEFINE_PER_CPU(struct listener_list, listener_array);
+
+enum actions {
+	REGISTER,
+	DEREGISTER,
+	CPU_DONT_CARE
+};
 
 static int prepare_reply(struct genl_info *info, u8 cmd, struct sk_buff **skbp,
 			void **replyp, size_t size)
@@ -74,25 +99,68 @@ static int prepare_reply(struct genl_info *info, u8 cmd, struct sk_buff **skbp,
 	return 0;
 }
 
-static int send_reply(struct sk_buff *skb, pid_t pid, int event)
+/*
+ * Send taskstats data in @skb to listener with nl_pid @pid
+ */
+static int send_reply(struct sk_buff *skb, pid_t pid)
 {
 	struct genlmsghdr *genlhdr = nlmsg_data((struct nlmsghdr *)skb->data);
-	void *reply;
+	void *reply = genlmsg_data(genlhdr);
 	int rc;
 
-	reply = genlmsg_data(genlhdr);
-
 	rc = genlmsg_end(skb, reply);
 	if (rc < 0) {
 		nlmsg_free(skb);
 		return rc;
 	}
 
-	if (event == TASKSTATS_MSG_MULTICAST)
-		return genlmsg_multicast(skb, pid, TASKSTATS_LISTEN_GROUP);
 	return genlmsg_unicast(skb, pid);
 }
 
+/*
+ * Send taskstats data in @skb to listeners registered for @cpu's exit data
+ */
+static int send_cpu_listeners(struct sk_buff *skb, unsigned int cpu)
+{
+	struct genlmsghdr *genlhdr = nlmsg_data((struct nlmsghdr *)skb->data);
+	struct listener_list *listeners;
+	struct listener *s, *tmp;
+	struct sk_buff *skb_next, *skb_cur = skb;
+	void *reply = genlmsg_data(genlhdr);
+	int rc, ret;
+
+	rc = genlmsg_end(skb, reply);
+	if (rc < 0) {
+		nlmsg_free(skb);
+		return rc;
+	}
+
+	rc = 0;
+	listeners = &per_cpu(listener_array, cpu);
+	down_write(&listeners->sem);
+	list_for_each_entry_safe(s, tmp, &listeners->list, list) {
+		skb_next = NULL;
+		if (!list_is_last(&s->list, &listeners->list)) {
+			skb_next = skb_clone(skb_cur, GFP_KERNEL);
+			if (!skb_next) {
+				nlmsg_free(skb_cur);
+				rc = -ENOMEM;
+				break;
+			}
+		}
+		ret = genlmsg_unicast(skb_cur, s->pid);
+		if (ret == -ECONNREFUSED) {
+			list_del(&s->list);
+			kfree(s);
+			rc = ret;
+		}
+		skb_cur = skb_next;
+	}
+	up_write(&listeners->sem);
+
+	return rc;
+}
+
 static int fill_pid(pid_t pid, struct task_struct *pidtsk,
 		struct taskstats *stats)
 {
@@ -204,8 +272,73 @@ ret:
 	return;
 }
 
+static int add_del_listener(pid_t pid, cpumask_t *maskp, int isadd)
+{
+	struct listener_list *listeners;
+	struct listener *s, *tmp;
+	unsigned int cpu;
+	cpumask_t mask = *maskp;
 
-static int taskstats_send_stats(struct sk_buff *skb, struct genl_info *info)
+	if (!cpus_subset(mask, cpu_possible_map))
+		return -EINVAL;
+
+	if (isadd == REGISTER) {
+		for_each_cpu_mask(cpu, mask) {
+			s = kmalloc_node(sizeof(struct listener), GFP_KERNEL,
+					 cpu_to_node(cpu));
+			if (!s)
+				goto cleanup;
+			s->pid = pid;
+			INIT_LIST_HEAD(&s->list);
+
+			listeners = &per_cpu(listener_array, cpu);
+			down_write(&listeners->sem);
+			list_add(&s->list, &listeners->list);
+			up_write(&listeners->sem);
+		}
+		return 0;
+	}
+
+	/* Deregister or cleanup */
+cleanup:
+	for_each_cpu_mask(cpu, mask) {
+		listeners = &per_cpu(listener_array, cpu);
+		down_write(&listeners->sem);
+		list_for_each_entry_safe(s, tmp, &listeners->list, list) {
+			if (s->pid == pid) {
+				list_del(&s->list);
+				kfree(s);
+				break;
+			}
+		}
+		up_write(&listeners->sem);
+	}
+	return 0;
+}
+
+static int parse(struct nlattr *na, cpumask_t *mask)
+{
+	char *data;
+	int len;
+	int ret;
+
+	if (na == NULL)
+		return 1;
+	len = nla_len(na);
+	if (len > TASKSTATS_CPUMASK_MAXLEN)
+		return -E2BIG;
+	if (len < 1)
+		return -EINVAL;
+	data = kmalloc(len, GFP_KERNEL);
+	if (!data)
+		return -ENOMEM;
+	nla_strlcpy(data, na, len);
+	ret = cpulist_parse(data, *mask);
+	kfree(data);
+	return ret;
+}
+
+static int taskstats_user_cmd(struct sk_buff *skb, struct genl_info *info)
 {
 	int rc = 0;
 	struct sk_buff *rep_skb;
@@ -213,6 +346,19 @@ static int taskstats_send_stats(struct sk_buff *skb, struct genl_info *info)
 	void *reply;
 	size_t size;
 	struct nlattr *na;
+	cpumask_t mask;
+
+	rc = parse(info->attrs[TASKSTATS_CMD_ATTR_REGISTER_CPUMASK], &mask);
+	if (rc < 0)
+		return rc;
+	if (rc == 0)
+		return add_del_listener(info->snd_pid, &mask, REGISTER);
+
+	rc = parse(info->attrs[TASKSTATS_CMD_ATTR_DEREGISTER_CPUMASK], &mask);
+	if (rc < 0)
+		return rc;
+	if (rc == 0)
+		return add_del_listener(info->snd_pid, &mask, DEREGISTER);
 
 	/*
 	 * Size includes space for nested attributes
@@ -252,7 +398,7 @@ static int taskstats_send_stats(struct sk_buff *skb, struct genl_info *info)
 
 	nla_nest_end(rep_skb, na);
 
-	return send_reply(rep_skb, info->snd_pid, TASKSTATS_MSG_UNICAST);
+	return send_reply(rep_skb, info->snd_pid);
 
 nla_put_failure:
 	return genlmsg_cancel(rep_skb, reply);
@@ -261,9 +407,35 @@ err:
 	return rc;
 }
 
+void taskstats_exit_alloc(struct taskstats **ptidstats, unsigned int *mycpu)
+{
+	struct listener_list *listeners;
+	struct taskstats *tmp;
+	/*
+	 * This is the cpu on which the task is exiting currently and will
+	 * be the one for which the exit event is sent, even if the cpu
+	 * on which this function is running changes later.
+	 */
+	*mycpu = raw_smp_processor_id();
+
+	*ptidstats = NULL;
+	tmp = kmem_cache_zalloc(taskstats_cache, SLAB_KERNEL);
+	if (!tmp)
+		return;
+
+	listeners = &per_cpu(listener_array, *mycpu);
+	down_read(&listeners->sem);
+	if (!list_empty(&listeners->list)) {
+		*ptidstats = tmp;
+		tmp = NULL;
+	}
+	up_read(&listeners->sem);
+	kfree(tmp);
+}
+
 /* Send pid data out on exit */
 void taskstats_exit_send(struct task_struct *tsk, struct taskstats *tidstats,
-			int group_dead)
+			int group_dead, unsigned int mycpu)
 {
 	int rc;
 	struct sk_buff *rep_skb;
@@ -324,7 +496,7 @@ void taskstats_exit_send(struct task_struct *tsk, struct taskstats *tidstats,
 	nla_nest_end(rep_skb, na);
 
 send:
-	send_reply(rep_skb, 0, TASKSTATS_MSG_MULTICAST);
+	send_cpu_listeners(rep_skb, mycpu);
 	return;
 
 nla_put_failure:
@@ -338,16 +510,22 @@ ret:
 
 static struct genl_ops taskstats_ops = {
 	.cmd		= TASKSTATS_CMD_GET,
-	.doit		= taskstats_send_stats,
+	.doit		= taskstats_user_cmd,
 	.policy		= taskstats_cmd_get_policy,
 };
 
 /* Needed early in initialization */
 void __init taskstats_init_early(void)
 {
+	unsigned int i;
+
 	taskstats_cache = kmem_cache_create("taskstats_cache",
 						sizeof(struct taskstats),
 						0, SLAB_PANIC, NULL, NULL);
+	for_each_possible_cpu(i) {
+		INIT_LIST_HEAD(&(per_cpu(listener_array, i).list));
+		init_rwsem(&(per_cpu(listener_array, i).sem));
+	}
 }
 
 static int __init taskstats_init(void)