[RFC PATCH 1/3] x86/resctrl: Add multiple tasks to the resctrl group at once

[Babu Moger <babu.moger@amd.com>]

Right now, the resctrl task assignment for the MONITOR or CONTROL group
needs to be one at a time. For example:
  $mount -t resctrl resctrl /sys/fs/resctrl/
  $mkdir /sys/fs/resctrl/clos1
  $echo 123 > /sys/fs/resctrl/clos1/tasks
  $echo 456 > /sys/fs/resctrl/clos1/tasks
  $echo 789 > /sys/fs/resctrl/clos1/tasks

This is not user-friendly when dealing with hundreds of tasks.

Improve the user experience by supporting the multiple task assignment
in one command with tasks separated by commas. For example:
  $echo 123,456,789 > /sys/fs/resctrl/clos1/tasks

Signed-off-by: Babu Moger <babu.moger@amd.com>
---
 Documentation/x86/resctrl.rst          |   13 ++++++------
 arch/x86/kernel/cpu/resctrl/rdtgroup.c |   35 ++++++++++++++++++++++++++------
 2 files changed, 35 insertions(+), 13 deletions(-)

diff --git a/Documentation/x86/resctrl.rst b/Documentation/x86/resctrl.rst
index 71a531061e4e..f26e16412bcb 100644
--- a/Documentation/x86/resctrl.rst
+++ b/Documentation/x86/resctrl.rst
@@ -208,12 +208,13 @@ All groups contain the following files:
 "tasks":
 	Reading this file shows the list of all tasks that belong to
 	this group. Writing a task id to the file will add a task to the
-	group. If the group is a CTRL_MON group the task is removed from
-	whichever previous CTRL_MON group owned the task and also from
-	any MON group that owned the task. If the group is a MON group,
-	then the task must already belong to the CTRL_MON parent of this
-	group. The task is removed from any previous MON group.
-
+	group. Multiple tasks can be assigned at once with each task
+	separated by commas. If the group is a CTRL_MON group the task
+	is removed from whichever previous CTRL_MON group owned the task
+	and also from any MON group that owned the task. If the group is
+	a MON group, then the task must already belong to the CTRL_MON
+	parent of this group. The task is removed from any previous MON
+	group.
 
 "cpus":
 	Reading this file shows a bitmask of the logical CPUs owned by
diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
index e5a48f05e787..344607853f4c 100644
--- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
+++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
@@ -686,28 +686,49 @@ static ssize_t rdtgroup_tasks_write(struct kernfs_open_file *of,
 				    char *buf, size_t nbytes, loff_t off)
 {
 	struct rdtgroup *rdtgrp;
+	char *pid_str;
 	int ret = 0;
 	pid_t pid;
 
-	if (kstrtoint(strstrip(buf), 0, &pid) || pid < 0)
+	/* Valid input requires a trailing newline */
+	if (nbytes == 0 || buf[nbytes - 1] != '\n')
 		return -EINVAL;
+
+	buf[nbytes - 1] = '\0';
+
+	cpus_read_lock();
 	rdtgrp = rdtgroup_kn_lock_live(of->kn);
 	if (!rdtgrp) {
-		rdtgroup_kn_unlock(of->kn);
-		return -ENOENT;
+		ret = -ENOENT;
+		goto exit;
+	}
+
+next:
+	if (!buf || buf[0] == '\0')
+		goto exit;
+
+	pid_str = strim(strsep(&buf, ","));
+
+	if (kstrtoint(pid_str, 0, &pid) || pid < 0) {
+		ret = -EINVAL;
+		goto exit;
 	}
+
 	rdt_last_cmd_clear();
 
 	if (rdtgrp->mode == RDT_MODE_PSEUDO_LOCKED ||
-	    rdtgrp->mode == RDT_MODE_PSEUDO_LOCKSETUP) {
-		ret = -EINVAL;
+			rdtgrp->mode == RDT_MODE_PSEUDO_LOCKSETUP) {
 		rdt_last_cmd_puts("Pseudo-locking in progress\n");
-		goto unlock;
+		ret = -EINVAL;
+		goto exit;
 	}
 
 	ret = rdtgroup_move_task(pid, rdtgrp, of);
 
-unlock:
+	goto next;
+
+exit:
+	cpus_read_unlock();
 	rdtgroup_kn_unlock(of->kn);
 
 	return ret ?: nbytes;