Re: [PATCH v4 2/7] sched/fair: Decay task PELT values during wakeup migration

[Vincent Guittot <vincent.guittot@linaro.org>]

Le mardi 19 avril 2022 à 13:23:27 (+0100), Vincent Donnefort a écrit :
> 
> 
> On 19/04/2022 11:08, Vincent Guittot wrote:
> > On Tue, 12 Apr 2022 at 15:42, Vincent Donnefort
> > <vincent.donnefort@arm.com> wrote:
> > > 
> > > Before being migrated to a new CPU, a task sees its PELT values
> > > synchronized with rq last_update_time. Once done, that same task will also
> > > have its sched_avg last_update_time reset. This means the time between
> > > the migration and the last clock update (B) will not be accounted for in
> > > util_avg and a discontinuity will appear. This issue is amplified by the
> > > PELT clock scaling. If the clock hasn't been updated while the CPU is
> > > idle, clock_pelt will not be aligned with clock_task and that time (A)
> > > will be also lost.
> > > 
> > >     ---------|----- A -----|-----------|------- B -----|>
> > >          clock_pelt   clock_task     clock            now
> > > 
> > > This is especially problematic for asymmetric CPU capacity systems which
> > > need stable util_avg signals for task placement and energy estimation.
> > > 
> > > Ideally, this problem would be solved by updating the runqueue clocks
> > > before the migration. But that would require taking the runqueue lock
> > > which is quite expensive [1]. Instead estimate the missing time and update
> > > the task util_avg with that value:
> > > 
> > >    A + B = clock_task - clock_pelt + sched_clock_cpu() - clock
> > > 
> > > Neither clock_task, clock_pelt nor clock can be accessed without the
> > > runqueue lock. The new cfs_rq last_update_lag is therefore created and
> > > contains those three values when the last_update_time value for that very
> > > same cfs_rq is updated.
> > > 
> > >    last_update_lag = clock - clock_task + clock_pelt
> > > 
> > > And we can then write the missing time as follow:
> > > 
> > >    A + B = sched_clock_cpu() - last_update_lag
> > > 
> > > The B. part of the missing time is however an estimation that doesn't take
> > > into account IRQ and Paravirt time.
> > > 
> > > Now we have an estimation for A + B, we can create an estimator for the
> > > PELT value at the time of the migration. We need for this purpose to
> > > inject last_update_time which is a combination of both clock_pelt and
> > > lost_idle_time. The latter is a time value which is completely lost from a
> > > PELT point of view and must be ignored. And finally, we can write:
> > > 
> > >    now = last_update_time + A + B
> > >        = last_update_time + sched_clock_cpu() - last_update_lag
> > > 
> > > This estimation has a cost, mostly due to sched_clock_cpu(). Limit the
> > > usage to the case where the source CPU is idle as we know this is when the
> > > clock is having the biggest risk of being outdated.
> > > 
> > > [1] https://lore.kernel.org/all/20190709115759.10451-1-chris.redpath@arm.com/
> > > 
> > > Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>

[...]

> > 
> > I'm worried that we will call this for each and every
> > update_cfs_rq_load_avg() whereas the content will be used only when
> > idle and not throttled. Can't we use these conditions to save values
> > only when needed and limit the number of useless updates ?
> 
> I don't think we can use idle here as a condition, once it is idle, it is
> too late to get those clock values.

As an example, the patch below should work. It doesn't handle the throttled case yet and still has to
make sure that rq->enter_idle and rq->clock_pelt_idle are coherent in regards to ILB that
update blocked load.

---
 kernel/sched/fair.c  | 30 ++++++++++++++++++++++++++++++
 kernel/sched/pelt.h  | 21 ++++++++++++++-------
 kernel/sched/sched.h |  3 ++-
 3 files changed, 46 insertions(+), 8 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e6ecf530f19f..f00843f9dd01 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7005,6 +7005,35 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
 
 static void detach_entity_cfs_rq(struct sched_entity *se);
 
+#ifdef CONFIG_NO_HZ_COMMON
+static inline void migrate_se_pelt_lag(struct sched_entity *se)
+{
+       u64 now;
+       struct cfs_rq *cfs_rq;
+       struct rq *rq;
+       bool is_idle;
+
+       cfs_rq = cfs_rq_of(se);
+       rq = rq_of(cfs_rq);
+
+       rcu_read_lock();
+       is_idle = is_idle_task(rcu_dereference(rq->curr));
+       rcu_read_unlock();
+
+       if (!is_idle)
+               return;
+
+	/* TODO handle throttled cfs */
+	/* TODO handle update ilb blocked load update */
+	now = READ_ONCE(rq->clock_pelt_idle);
+	now += sched_clock_cpu(cpu_of(rq)) - READ_ONCE(rq->enter_idle);
+
+       __update_load_avg_blocked_se(now, se);
+}
+#else
+static void migrate_se_pelt_lag(struct sched_entity *se) {}
+#endif
+
 /*
  * Called immediately before a task is migrated to a new CPU; task_cpu(p) and
  * cfs_rq_of(p) references at time of call are still valid and identify the
@@ -7056,6 +7085,7 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
 		 * sounds not bad.
 		 */
 		remove_entity_load_avg(&p->se);
+		migrate_se_pelt_lag(&p->se);
 	}
 
 	/* Tell new CPU we are migrated */
diff --git a/kernel/sched/pelt.h b/kernel/sched/pelt.h
index c336f5f481bc..ece4423026e5 100644
--- a/kernel/sched/pelt.h
+++ b/kernel/sched/pelt.h
@@ -61,6 +61,14 @@ static inline void cfs_se_util_change(struct sched_avg *avg)
 	WRITE_ONCE(avg->util_est.enqueued, enqueued);
 }
 
+static inline u64 rq_clock_pelt(struct rq *rq)
+{
+	lockdep_assert_rq_held(rq);
+	assert_clock_updated(rq);
+
+	return rq->clock_pelt - rq->lost_idle_time;
+}
+
 /*
  * The clock_pelt scales the time to reflect the effective amount of
  * computation done during the running delta time but then sync back to
@@ -78,6 +86,8 @@ static inline void update_rq_clock_pelt(struct rq *rq, s64 delta)
 	if (unlikely(is_idle_task(rq->curr))) {
 		/* The rq is idle, we can sync to clock_task */
 		rq->clock_pelt  = rq_clock_task(rq);
+		WRITE_ONCE(rq->enter_idle, rq_clock(rq)); /* this could be factorized with idle_stamp */
+		WRITE_ONCE(rq->clock_pelt_idle, rq_clock_pelt(rq)); /* last pelt clock update when idle */
 		return;
 	}
 
@@ -130,14 +140,11 @@ static inline void update_idle_rq_clock_pelt(struct rq *rq)
 	 */
 	if (util_sum >= divider)
 		rq->lost_idle_time += rq_clock_task(rq) - rq->clock_pelt;
-}
 
-static inline u64 rq_clock_pelt(struct rq *rq)
-{
-	lockdep_assert_rq_held(rq);
-	assert_clock_updated(rq);
-
-	return rq->clock_pelt - rq->lost_idle_time;
+	/* The rq is idle, we can sync with clock_task */
+	rq->clock_pelt  = rq_clock_task(rq);
+	WRITE_ONCE(rq->enter_idle, rq_clock(rq)); /* this could be factorized with idle_stamp */
+	WRITE_ONCE(rq->clock_pelt_idle, rq_clock_pelt(rq)); /* last pelt clock update when idle */
 }
 
 #ifdef CONFIG_CFS_BANDWIDTH
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 6ab77b171656..108698446762 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1007,7 +1007,8 @@ struct rq {
 	u64			clock_task ____cacheline_aligned;
 	u64			clock_pelt;
 	unsigned long		lost_idle_time;
-
+	u64			clock_pelt_idle;
+	u64			enter_idle;
 	atomic_t		nr_iowait;
 
 #ifdef CONFIG_SCHED_DEBUG
-- 
2.17.1

> > 
> > A quick test with hackbench on a 8 cpus system gives
> > around 60k call for a duration 550 msec run a root level
> > and 180k from a 3rd level cgroups
> 
> If you think this is too much to have this store on this path, we could
> though either:
> 
> a. restrict the feature to when we know it is the most important: EAS?
> 
> b. store the updated value only when "decayed" is non 0.
> 
> [...]