Re: [RFC PATCH 2/3] sched/cpuset: Keep track of SCHED_DEADLINE tasks in cpusets

From: Waiman Long
Date: Wed Mar 15 2023 - 14:11:01 EST

On 3/15/23 14:01, Waiman Long wrote:

On 3/15/23 13:14, Juri Lelli wrote:
On 15/03/23 11:46, Waiman Long wrote:
On 3/15/23 08:18, Juri Lelli wrote:
Qais reported that iterating over all tasks when rebuilding root domains
for finding out which ones are DEADLINE and need their bandwidth
correctly restored on such root domains can be a costly operation (10+
ms delays on suspend-resume).

To fix the problem keep track of the number of DEADLINE tasks belonging
to each cpuset and then use this information (followup patch) to only
perform the above iteration if DEADLINE tasks are actually present in
the cpuset for which a corresponding root domain is being rebuilt.

Reported-by: Qais Yousef <qyousef@xxxxxxxxxxx>
Signed-off-by: Juri Lelli <juri.lelli@xxxxxxxxxx>
---
   include/linux/cpuset.h |  4 ++++
   kernel/cgroup/cgroup.c |  4 ++++
   kernel/cgroup/cpuset.c | 25 +++++++++++++++++++++++++
   kernel/sched/core.c    | 10 ++++++++++
   4 files changed, 43 insertions(+)

diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
index 355f796c5f07..0348dba5680e 100644
--- a/include/linux/cpuset.h
+++ b/include/linux/cpuset.h
@@ -71,6 +71,8 @@ extern void cpuset_init_smp(void);
   extern void cpuset_force_rebuild(void);
   extern void cpuset_update_active_cpus(void);
   extern void cpuset_wait_for_hotplug(void);
+extern void inc_dl_tasks_cs(struct task_struct *task);
+extern void dec_dl_tasks_cs(struct task_struct *task);
   extern void cpuset_lock(void);
   extern void cpuset_unlock(void);
   extern void cpuset_cpus_allowed(struct task_struct *p, struct cpumask *mask);
@@ -196,6 +198,8 @@ static inline void cpuset_update_active_cpus(void)
   static inline void cpuset_wait_for_hotplug(void) { }
+static inline void inc_dl_tasks_cs(struct task_struct *task) { }
+static inline void dec_dl_tasks_cs(struct task_struct *task) { }
   static inline void cpuset_lock(void) { }
   static inline void cpuset_unlock(void) { }
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index c099cf3fa02d..357925e1e4af 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -57,6 +57,7 @@
   #include <linux/file.h>
   #include <linux/fs_parser.h>
   #include <linux/sched/cputime.h>
+#include <linux/sched/deadline.h>
   #include <linux/psi.h>
   #include <net/sock.h>
@@ -6673,6 +6674,9 @@ void cgroup_exit(struct task_struct *tsk)
       list_add_tail(&tsk->cg_list, &cset->dying_tasks);
       cset->nr_tasks--;
+    if (dl_task(tsk))
+        dec_dl_tasks_cs(tsk);
+
       WARN_ON_ONCE(cgroup_task_frozen(tsk));
       if (unlikely(!(tsk->flags & PF_KTHREAD) &&
                test_bit(CGRP_FREEZE, &task_dfl_cgroup(tsk)->flags)))
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 8d82d66d432b..57bc60112618 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -193,6 +193,12 @@ struct cpuset {
       int use_parent_ecpus;
       int child_ecpus_count;
+    /*
+     * number of SCHED_DEADLINE tasks attached to this cpuset, so that we
+     * know when to rebuild associated root domain bandwidth information.
+     */
+    int nr_deadline_tasks;
+
       /* Invalid partition error code, not lock protected */
       enum prs_errcode prs_err;
@@ -245,6 +251,20 @@ static inline struct cpuset *parent_cs(struct cpuset *cs)
       return css_cs(cs->css.parent);
   }
+void inc_dl_tasks_cs(struct task_struct *p)
+{
+    struct cpuset *cs = task_cs(p);
+
+    cs->nr_deadline_tasks++;
+}
+
+void dec_dl_tasks_cs(struct task_struct *p)
+{
+    struct cpuset *cs = task_cs(p);
+
+    cs->nr_deadline_tasks--;
+}
+
   /* bits in struct cpuset flags field */
   typedef enum {
       CS_ONLINE,
@@ -2472,6 +2492,11 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
           ret = security_task_setscheduler(task);
           if (ret)
               goto out_unlock;
+
+        if (dl_task(task)) {
+            cs->nr_deadline_tasks++;
+            cpuset_attach_old_cs->nr_deadline_tasks--;
+        }
       }
Any one of the tasks in the cpuset can cause the test to fail and abort the
attachment. I would suggest that you keep a deadline task transfer count in
the loop and then update cs and cpouset_attach_old_cs only after all the
tasks have been iterated successfully.
Right, Dietmar I think commented pointing out something along these
lines. Think though we already have this problem with current
task_can_attach -> dl_cpu_busy which reserves bandwidth for each tasks
in the destination cs. Will need to look into that. Do you know which
sort of operation would move multiple tasks at once?

Actually, what I said previously may not be enough. There can be multiple controllers attached to a cgroup. If any of thier can_attach() calls fails, the whole transaction is aborted and cancel_attach() will be called. My new suggestion is to add a new deadline task transfer count into the cpuset structure and store the information there temporarily. If cpuset_attach() is called, it means all the can_attach calls succeed. You can then update the dl task count accordingly and clear the temporary transfer count.

I guess you may have to do something similar with dl_cpu_busy().

My 2 cents.

Alternatively, you can do the nr_deadline_tasks update in cpuset_attach(). However, there is an optimization to skip the task iteration if the cpu and memory list haven't changed. You will have to skip that optimization if there are DL tasks in the cpuset.

Cheers,
Longman