Re: [External] Re: [PATCH 2/2] sched/core: Avoid double calling update_rq_clock()

[Hao Jia <jiahao.os@bytedance.com>]

On 2023/5/5 Vincent Guittot wrote:

>>>>    rq_attach_root+0xc4/0xd0
>>>>    cpu_attach_domain+0x3dc/0x7f0
>>>>    partition_sched_domains_locked+0x2a5/0x3c0
>>>>    rebuild_sched_domains_locked+0x477/0x830
>>>>    rebuild_sched_domains+0x1b/0x30
>>>>    cpuset_hotplug_workfn+0x2ca/0xc90
>>>>    ? balance_push+0x56/0xf0
>>>>    ? _raw_spin_unlock+0x15/0x30
>>>>    ? finish_task_switch+0x98/0x2f0
>>>>    ? __switch_to+0x291/0x410
>>>>    ? __schedule+0x65e/0x1310
>>>>    process_one_work+0x1bc/0x3d0
>>>>    worker_thread+0x4c/0x380
>>>>    ? preempt_count_add+0x92/0xa0
>>>>    ? rescuer_thread+0x310/0x310
>>>>    kthread+0xe6/0x110
>>>>    ? kthread_complete_and_exit+0x20/0x20
>>>>    ret_from_fork+0x1f/0x30
>>>>
>>>> For the __balance_push_cpu_stop() case, we remove update_rq_clock() from
>>>> the __migrate_task() function to avoid double updating the rq clock.
>>>> And in order to avoid missing rq clock update, add update_rq_clock()
>>>> call before migration_cpu_stop() calls __migrate_task().
> 
> Can we do the opposite ?
> AFAICT, update_rq_clock() in __balance_push_cpu_stop() is only there for
> __migrate_task(). I prefer to keep the update_rq_clock() as close as possible
> to the user

I'm afraid not, the rq clock also needs to be updated before 
select_fallback_rq() is called.


> 
>>>>
>>>> This also works for unthrottle_cfs_rq(), so we also removed
>>>> update_rq_clock() from the unthrottle_cfs_rq() function to avoid
>>>> warnings caused by calling it multiple times, such as
>>>> __cfsb_csd_unthrottle() and unthrottle_offline_cfs_rqs(). and
> 
> This happens with the for loop added by
> commit: 8ad075c2eb1f ("sched: Async unthrottling for cfs bandwidth")
> 

Yes, the warning caused by this commit.

>>>> in order to avoid missing rq clock update, we correspondingly add
>>>> update_rq_clock() calls before unthrottle_cfs_rq() runs.
> 
> These are special cases that happen because of the for_each.
> As said above, I would prefer keeping update_rq_clock close the their user
> 
> could we use something similar to rq_clock_skip_update() for those list ?
> 

I try to do it with the method you provided. Some things maybe like this?

We also need to clear RQCF_ACT_SKIP after calling rq_clock_loop_update() 
to avoid some warnings.


diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index efdab1489113..f48b5d912d8c 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2420,7 +2420,6 @@ static struct rq *__migrate_task(struct rq *rq, 
struct rq_flags *rf,
         if (!is_cpu_allowed(p, dest_cpu))
                 return rq;

-       update_rq_clock(rq);
         rq = move_queued_task(rq, rf, p, dest_cpu);

         return rq;
@@ -2478,10 +2477,12 @@ static int migration_cpu_stop(void *data)
                                 goto out;
                 }

-               if (task_on_rq_queued(p))
+               if (task_on_rq_queued(p)) {
+                       update_rq_clock(rq);
                         rq = __migrate_task(rq, &rf, p, arg->dest_cpu);
-               else
+               } else {
                         p->wake_cpu = arg->dest_cpu;
+               }

                 /*
                  * XXX __migrate_task() can fail, at which point we 
might end
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 373ff5f55884..1dcef273bebe 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5575,6 +5575,13 @@ static void __cfsb_csd_unthrottle(void *arg)
         struct rq_flags rf;

         rq_lock(rq, &rf);
+       /*
+        * Iterating over the list can trigger several call to 
update_rq_clock()
+        * in unthrottle_cfs_rq().
+        * Do it once and skip the potential next ones.
+        */
+       update_rq_clock(rq);
+       rq_clock_loop_update(rq);

         /*
          * Since we hold rq lock we're safe from concurrent manipulation of
@@ -5595,6 +5602,7 @@ static void __cfsb_csd_unthrottle(void *arg)

         rcu_read_unlock();

+       rq_clock_cancel_loop_update(rq);
         rq_unlock(rq, &rf);
  }

@@ -6114,6 +6122,12 @@ static void __maybe_unused 
unthrottle_offline_cfs_rqs(struct rq *rq)
         struct task_group *tg;

         lockdep_assert_rq_held(rq);
+       /*
+        * The rq clock has already been updated before the
+        * set_rq_offline() runs, so we should skip updating
+        * the rq clock again in unthrottle_cfs_rq().
+        */
+       rq_clock_loop_update(rq);

         rcu_read_lock();
         list_for_each_entry_rcu(tg, &task_groups, list) {
@@ -6137,6 +6151,7 @@ static void __maybe_unused 
unthrottle_offline_cfs_rqs(struct rq *rq)
                         unthrottle_cfs_rq(cfs_rq);
         }
         rcu_read_unlock();
+       rq_clock_cancel_loop_update(rq);
  }

  #else /* CONFIG_CFS_BANDWIDTH */
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index ec7b3e0a2b20..3d4981d354a9 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1536,6 +1536,25 @@ static inline void rq_clock_skip_update(struct rq 
*rq)
         rq->clock_update_flags |= RQCF_REQ_SKIP;
  }

+/*
+ * During cpu offlining and rq wide unthrottling, we can trigger
+ * an update_rq_clock() for several cfs and rt runqueues (Typically
+ * when using list_for_each_entry_*)
+ * rq_clock_loop_update() can be called after updating the clock once
+ * and before iterating over the list to prevent multiple update.
+ */
+static inline void rq_clock_loop_update(struct rq *rq)
+{
+       lockdep_assert_rq_held(rq);
+       rq->clock_update_flags |= RQCF_ACT_SKIP;
+}
+
+static inline void rq_clock_cancel_loop_update(struct rq *rq)
+{
+       lockdep_assert_rq_held(rq);
+       rq->clock_update_flags &= ~RQCF_ACT_SKIP;
+}
+
  /*
   * See rt task throttling, which is the only time a skip