[PATCH cgroup/for-next v2 3/5] cgroup/cpuset: Replace cpuset_attach_old_cs by a new attach_old_cs field in task_struct

[Waiman Long <longman@redhat.com>]

In cpuset_can_attach(), the source (old) cpuset of the tasks is
stored in an internal cpuset_attach_old_cs variable to be used later
in cpuset_attach(). It is because such task to old cpuset information
is no longer available when cpuset_attach() is called and it is assumed
that there is only one source cpuset.

To support cgroup_taskset containing tasks from multiple source
cpusets, such an approach will no longer work. The easier way to get
the old cpuset information is to temporarily store that information
in the task_struct itself at cpuset_can_attach() and reuse it in
cpuset_attach(). However, that does increase the size of task_struct
by a 8 bytes for 64-bit kernel.

Add a new attach_old_cs field into task_struct for such purpose and
retire the cpuset_attach_old_cs internal variable.

Even though attach_old_cs can be counted as a reference to the
old cpuset, like cpuset_attach_old_cs, it is strictly used only
for communication between cpuset_can_attach() and cpuset_attach()
within the same task migration session where cgroup_mutex will be held
throughout. So no actual reference counting will be performed.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 include/linux/sched.h  |  3 +++
 kernel/cgroup/cpuset.c | 13 ++++++-------
 2 files changed, 9 insertions(+), 7 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 004e6d56a499..9b6bb1603592 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -63,6 +63,7 @@ struct bpf_run_ctx;
 struct bpf_net_context;
 struct capture_control;
 struct cfs_rq;
+struct cpuset;
 struct fs_struct;
 struct futex_pi_state;
 struct io_context;
@@ -1317,6 +1318,8 @@ struct task_struct {
 	/* Sequence number to catch updates: */
 	seqcount_spinlock_t		mems_allowed_seq;
 	int				cpuset_mem_spread_rotor;
+	/* Old cpuset to be used in cpuset_attach() */
+	struct cpuset			*attach_old_cs;
 #endif
 #ifdef CONFIG_CGROUPS
 	/* Control Group info protected by css_set_lock: */
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 0d01b66f464d..fc632370d07c 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -2968,7 +2968,6 @@ static int update_prstate(struct cpuset *cs, int new_prs)
  * cpuset_can_attach() and cpuset_attach() specific internal data
  * Protected by cpuset_mutex
  */
-static struct cpuset *cpuset_attach_old_cs;
 static bool attach_cpus_updated;
 static bool attach_mems_updated;
 
@@ -3052,9 +3051,7 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
 	bool setsched_check;
 	int ret;
 
-	/* used later by cpuset_attach() */
-	cpuset_attach_old_cs = task_cs(cgroup_taskset_first(tset, &css));
-	oldcs = cpuset_attach_old_cs;
+	oldcs = task_cs(cgroup_taskset_first(tset, &css));
 	cs = css_cs(css);
 
 	mutex_lock(&cpuset_mutex);
@@ -3075,6 +3072,8 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
 				goto out_unlock;
 		}
 
+		/* Save a copy of oldcs to be used later in cpuset_attach() */
+		task->attach_old_cs = oldcs;
 		if (dl_task(task)) {
 			/*
 			 * Count all migrating DL tasks for cpuset task accounting.
@@ -3156,11 +3155,11 @@ static void cpuset_attach(struct cgroup_taskset *tset)
 	struct task_struct *task;
 	struct task_struct *leader;
 	struct cgroup_subsys_state *css;
-	struct cpuset *cs;
-	struct cpuset *oldcs = cpuset_attach_old_cs;
+	struct cpuset *cs, *oldcs;
 	bool queue_task_work = false;
 
-	cgroup_taskset_first(tset, &css);
+	task = cgroup_taskset_first(tset, &css);
+	oldcs = task->attach_old_cs;
 	cs = css_cs(css);
 
 	lockdep_assert_cpus_held();	/* see cgroup_attach_lock() */
-- 
2.54.0