[PATCH] taskstats: add ioctl to fetch pid/tgid status

[Weiping Zhang]

If a program needs monitor lots of process's status, it needs two
syscalls for every process. The first one is telling kernel which
pid/tgid should be monitored by send a command(write socket) to kernel.
The second one is read the statistics by read socket. This patch add
a new interface /dev/taskstats to reduce two syscalls to one ioctl.
The user just set the target pid/tgid to the struct taskstats.ac_pid,
then kernel will collect statistics for that pid/tgid.

Signed-off-by: Weiping Zhang <zhangweiping@xxxxxxxxxxxxxx>
---
 kernel/taskstats.c | 71 +++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 70 insertions(+), 1 deletion(-)

diff --git a/kernel/taskstats.c b/kernel/taskstats.c
index a2802b6ff4bb..68188cc9663d 100644
--- a/kernel/taskstats.c
+++ b/kernel/taskstats.c
@@ -21,6 +21,8 @@
 #include <net/genetlink.h>
 #include <linux/atomic.h>
 #include <linux/sched/cputime.h>
+#include <linux/miscdevice.h>
+#include <linux/uio.h>
 
 /*
  * Maximum length of a cpumask that can be specified in
@@ -666,6 +668,67 @@ static struct genl_family family __ro_after_init = {
 	.n_ops		= ARRAY_SIZE(taskstats_ops),
 };
 
+static long taskstats_ioctl_pid_tgid(unsigned long arg, bool tgid)
+{
+	struct taskstats kstat;
+	struct taskstats *ustat = (struct taskstats *)arg;
+	u32 id;
+	unsigned long offset = offsetof(struct taskstats, ac_pid);
+	long ret;
+
+	/* userspace set monitored pid/tgid to the field "ac_pid" */
+	if (unlikely(copy_from_user(&id, (void *)(arg + offset), sizeof(u32))))
+		return -EFAULT;
+
+	if (tgid)
+		ret = fill_stats_for_tgid(id, &kstat);
+	else
+		ret = fill_stats_for_pid(id, &kstat);
+	if (ret < 0)
+		return ret;
+
+	if (unlikely(copy_to_user(ustat, &kstat, sizeof(struct taskstats))))
+		return -EFAULT;
+
+	return 0;
+}
+
+static long taskstats_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
+{
+	long ret;
+
+	switch (cmd) {
+	case TASKSTATS_CMD_ATTR_PID:
+		ret = taskstats_ioctl_pid_tgid(arg, false);
+		break;
+	case TASKSTATS_CMD_ATTR_TGID:
+		ret = taskstats_ioctl_pid_tgid(arg, true);
+		break;
+	default:
+		ret = -EINVAL;
+		break;
+	}
+
+	return ret;
+}
+
+static const struct file_operations taskstats_ctl_fops = {
+	.open		= nonseekable_open,
+	.unlocked_ioctl	= taskstats_ioctl,
+#ifdef CONFIG_COMPAT
+	.compat_ioctl	= taskstats_ioctl,
+#endif
+	.owner		= THIS_MODULE,
+	.llseek		= noop_llseek,
+};
+
+static struct miscdevice taskstats_misc = {
+	.minor		= MISC_DYNAMIC_MINOR,
+	.name		= "taskstats",
+	.fops		= &taskstats_ctl_fops,
+};
+
+
 /* Needed early in initialization */
 void __init taskstats_init_early(void)
 {
@@ -682,9 +745,15 @@ static int __init taskstats_init(void)
 {
 	int rc;
 
+	rc = misc_register(&taskstats_misc);
+	if (rc < 0)
+		return rc;
+
 	rc = genl_register_family(&family);
-	if (rc)
+	if (rc) {
+		misc_deregister(&taskstats_misc);
 		return rc;
+	}
 
 	family_registered = 1;
 	pr_info("registered taskstats version %d\n", TASKSTATS_GENL_VERSION);
-- 
2.17.2