Re: [PATCH] sched/fair: Rate limit calls to update_blocked_averages() for NOHZ

Tim Chen
2021-05-10T21:59:46Z

On 4/9/21 10:59 AM, Tim Chen wrote:

>>> 11.602 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb76c jiffies=0x1004fb739
>>> 11.624 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb739
>>> 11.642 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb739
>>> 11.645 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb739
>>> 11.977 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb739
>>> 12.003 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb739
>>> 12.015 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb739
>>> 12.043 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb739
>>> 12.567 ( ): probe:update_blocked_averages:(ffffffff810cf070) cpu=2 jiffies=0x1004fb73a
>>> 13.856 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb76c jiffies=0x1004fb73b
>>> 13.910 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73b
>>> 14.003 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73b
>>> 14.159 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73b
>>> 14.203 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73b
>>> 14.223 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73b
>>> 14.301 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73b
>>> 14.504 ( ): probe:update_blocked_averages:(ffffffff810cf070) cpu=2 jiffies=0x1004fb73c

>> I don't know exactly what you track with "next_balance=" in
>
> It is the rq->next_balance value as we enter the newidle_balance function.
>
>> probe:newidle_balance but it always starts with the same value
>> 0x1004fb76c in the future to finish with a value 0x1004fb731 in the
>> past.
>
> This indeed is odd as the next_balance should move forward and not backward.

Vincent,

I found a bug in newidle_balance() that moved the next balance time
backward. I fixed it in patch 1 below. This corrects the
next_balance time update and we now have proper load balance rate limiting.

After putting in the other two changes previously discussed with you (patch 2 and 3 below),
I see very good improvement (about +5%) in the database workload I was investigating.
The costly update_blocked_averages() function is called much less frequently and consumed
only 0.2% of cpu cycles instead of 2.6% before the changes.

Including all three patches here together for easier review. The patches
apply to the tip tree's sched/core branch.

Thanks.

Tim

--->8---

From 848eb8f45b53b45cacf70022c98f632daabefe77 Mon Sep 17 00:00:00 2001
Message-Id: <848eb8f45b53b45cacf70022c98f632daabefe77.1620677280.git.tim.c.chen@linux.intel.com>
From: Tim Chen <tim.c.chen@linux.intel.com>
Date: Fri, 7 May 2021 14:19:47 -0700
Subject: [PATCH 1/3] sched: Fix rq->next_balance time going backward

In traces on newidle_balance(), this_rq->next_balance
time goes backward from time to time, e.g.

11.602 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb76c jiffies=0x1004fb739
11.624 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb739
13.856 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb76c jiffies=0x1004fb73b
13.910 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73b
14.637 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb76c jiffies=0x1004fb73c
14.666 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73c

This was due to newidle_balance() updated this_rq->next_balance
to an earlier time than its current value. The real intention
was to make sure next_balance move this_rq->next_balance forward
in its update:

out:
/* Move the next balance forward */
if (time_after(this_rq->next_balance, next_balance))
this_rq->next_balance = next_balance;

The actual outcome was moving this_rq->next_balance backward,
in the wrong direction.

Fix the incorrect check on next_balance causing the problem.

Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
---
kernel/sched/fair.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 1d75af1ecfb4..b0b5698b2184 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10681,7 +10681,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)

out:
/* Move the next balance forward */
- if (time_after(this_rq->next_balance, next_balance))
+ if (time_after(next_balance, this_rq->next_balance))
this_rq->next_balance = next_balance;

if (pulled_task)
--
2.20.1

From f2c9af4af6438ad79076e1a664003dc01ad4fdf0 Mon Sep 17 00:00:00 2001
Message-Id: <f2c9af4af6438ad79076e1a664003dc01ad4fdf0.1620677280.git.tim.c.chen@linux.intel.com>
In-Reply-To: <848eb8f45b53b45cacf70022c98f632daabefe77.1620677280.git.tim.c.chen@linux.intel.com>
References: <848eb8f45b53b45cacf70022c98f632daabefe77.1620677280.git.tim.c.chen@linux.intel.com>
From: Vincent Guittot <vincent.guittot@linaro.org>
Date: Fri, 7 May 2021 14:38:10 -0700
Subject: [PATCH 2/3] sched: Skip update_blocked_averages if we are defering
load balance

In newidle_balance(), the scheduler skips load balance to the new idle cpu when sd is this_rq and when

this_rq->avg_idle < sd->max_newidle_lb_cost

Doing a costly call to update_blocked_averages() will
not be useful and simply adds overhead when this condition is true.

Check the condition early in newidle_balance() to skip update_blocked_averages()
when possible.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
---
kernel/sched/fair.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index b0b5698b2184..f828b75488a0 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10612,17 +10612,20 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
*/
rq_unpin_lock(this_rq, rf);

+ rcu_read_lock();
+ sd = rcu_dereference_check_sched_domain(this_rq->sd);
+
if (this_rq->avg_idle < sysctl_sched_migration_cost ||
- !READ_ONCE(this_rq->rd->overload)) {
+ !READ_ONCE(this_rq->rd->overload) ||
+ (sd && this_rq->avg_idle < sd->max_newidle_lb_cost)) {

- rcu_read_lock();
- sd = rcu_dereference_check_sched_domain(this_rq->sd);
if (sd)
update_next_balance(sd, &next_balance);
rcu_read_unlock();

goto out;
}
+ rcu_read_unlock();

raw_spin_unlock(&this_rq->lock);

--
2.20.1

From c45d13c6156c3cdc340ef3ba523b8750642a9c50 Mon Sep 17 00:00:00 2001
Message-Id: <c45d13c6156c3cdc340ef3ba523b8750642a9c50.1620677280.git.tim.c.chen@linux.intel.com>
In-Reply-To: <848eb8f45b53b45cacf70022c98f632daabefe77.1620677280.git.tim.c.chen@linux.intel.com>
References: <848eb8f45b53b45cacf70022c98f632daabefe77.1620677280.git.tim.c.chen@linux.intel.com>
From: Tim Chen <tim.c.chen@linux.intel.com>
Date: Fri, 7 May 2021 14:54:54 -0700
Subject: [PATCH 3/3] sched: Rate limit load balance in newidle_balance()

Currently newidle_balance() could do load balancng even if the cpu is not
due for a load balance. Make newidle_balance() check the next_balance
time on the cpu's runqueue so it defers load balancing if it is not
due for its load balance.

Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
---
kernel/sched/fair.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f828b75488a0..8e00e1fdd6e0 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10617,6 +10617,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)

if (this_rq->avg_idle < sysctl_sched_migration_cost ||
!READ_ONCE(this_rq->rd->overload) ||
+ time_before(jiffies, this_rq->next_balance) ||
(sd && this_rq->avg_idle < sd->max_newidle_lb_cost)) {

if (sd)
--
2.20.1