From: Tim Chen <tim.c.chen@linux.intel.com>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Joel Fernandes <joel@joelfernandes.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
Paul McKenney <paulmck@kernel.org>,
Frederic Weisbecker <fweisbec@gmail.com>,
Dietmar Eggeman <dietmar.eggemann@arm.com>,
Qais Yousef <qais.yousef@arm.com>,
Ben Segall <bsegall@google.com>,
Daniel Bristot de Oliveira <bristot@redhat.com>,
Ingo Molnar <mingo@redhat.com>,
Juri Lelli <juri.lelli@redhat.com>, Mel Gorman <mgorman@suse.de>,
Peter Zijlstra <peterz@infradead.org>,
Steven Rostedt <rostedt@goodmis.org>,
"Uladzislau Rezki (Sony)" <urezki@gmail.com>,
Neeraj upadhyay <neeraj.iitr10@gmail.com>,
Aubrey Li <aubrey.li@linux.intel.com>
Subject: Re: [PATCH] sched/fair: Rate limit calls to update_blocked_averages() for NOHZ
Date: Tue, 11 May 2021 10:25:37 -0700 [thread overview]
Message-ID: <577b0aae-0111-97aa-0c99-c2a2fcfb5e2e@linux.intel.com> (raw)
In-Reply-To: <CAKfTPtA8nr-fgt4Nw6XqQyT_TEx4uL3nK-ba0xGfkONO+BPG3Q@mail.gmail.com>
On 5/11/21 8:25 AM, Vincent Guittot wrote:
> Hi Tim,
>
> Sometimes, we want to set this_rq->next_balance backward compared to
> its current value. When a CPU is busy, the balance interval is
> multiplied by busy_factor which is set to 16 by default. On SMT2 sched
> domain level, it means that the interval will be 32ms when busy
> instead of 2ms. But if a busy load balance happens just before
> becoming idle, the this_rq->next_balance will be set 32ms later
> whereas it should go down to 2ms as the CPU is now idle. And this
> becomes even worse when you have 128 CPUs at die sched_domain level
> because the idle CPU will have to wait 2048ms instead of the correct
> 128ms interval.
>
>>
>> out:
>> /* Move the next balance forward */
>> - if (time_after(this_rq->next_balance, next_balance))
>> + if (time_after(next_balance, this_rq->next_balance))
>
> The current comparison is correct but next_balance should not be in the past.
I understand then the intention is after the update,
this_rq->next_balance should have a minimum value of jiffies+1. So
we will need
out:
/* Move the next balance forward */
+ this_rq->next_balance = max(jiffies+1, this_rq->next_balance);
if (time_after(this_rq->next_balance, next_balance))
this_rq->next_balance = next_balance;
as next_balance computed will be at least jiffies+1 after your fix to
update_next_balance(). We still need to take care of the case when
this_rq->next_balance <= jiffies.
So combining with your suggestion on the fix to update_next_balance(),
the fix will be
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 1d75af1ecfb4..2dc471c1511c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9901,7 +9901,7 @@ update_next_balance(struct sched_domain *sd, unsigned long *next_balance)
/* used by idle balance, so cpu_busy = 0 */
interval = get_sd_balance_interval(sd, 0);
- next = sd->last_balance + interval;
+ next = max(jiffies+1, sd->last_balance + interval);
if (time_after(*next_balance, next))
*next_balance = next;
@@ -10681,6 +10681,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
out:
/* Move the next balance forward */
+ this_rq->next_balance = max(jiffies+1, this_rq->next_balance);
if (time_after(this_rq->next_balance, next_balance))
this_rq->next_balance = next_balance;
>
> update_next_balance() is only used in newidle_balance() so we could
> modify it to have:
>
> next = max(jiffies+1, next = sd->last_balance + interval)
Is the extra assignment "next = sd->last_balance + interval" needed?
This seems more straight forward:
next = max(jiffies+1, sd->last_balance + interval)
I will try to get the benchmark folks to do another run with this change.
Hopefully I'll get some bandwidth from them soon.
Thanks.
Tim
next prev parent reply other threads:[~2021-05-11 17:25 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-22 15:46 [PATCH] sched/fair: Rate limit calls to update_blocked_averages() for NOHZ Joel Fernandes (Google)
2021-01-22 16:56 ` Vincent Guittot
2021-01-22 18:39 ` Qais Yousef
2021-01-22 19:14 ` Joel Fernandes
2021-01-25 13:23 ` Vincent Guittot
2021-01-26 16:36 ` Qais Yousef
2021-01-22 19:10 ` Joel Fernandes
2021-01-25 10:44 ` Dietmar Eggemann
2021-01-25 17:30 ` Vincent Guittot
2021-01-25 17:53 ` Dietmar Eggemann
2021-01-25 14:42 ` Vincent Guittot
2021-01-27 18:43 ` Joel Fernandes
2021-01-28 13:57 ` Vincent Guittot
2021-01-28 15:09 ` Joel Fernandes
2021-01-28 16:57 ` Qais Yousef
[not found] ` <CAKfTPtBvwm9vZb5C=2oTF6N-Ht6Rvip4Lv18yi7O3G8e-_ZWdg@mail.gmail.com>
2021-01-29 17:27 ` Vincent Guittot
2021-02-03 11:54 ` Dietmar Eggemann
2021-02-03 13:12 ` Vincent Guittot
2021-02-04 9:47 ` Dietmar Eggemann
2021-02-03 17:09 ` Qais Yousef
2021-02-03 17:35 ` Vincent Guittot
2021-02-04 10:45 ` Qais Yousef
2021-02-03 19:56 ` Joel Fernandes
2021-03-23 21:37 ` Tim Chen
2021-03-24 13:44 ` Vincent Guittot
2021-03-24 16:05 ` Tim Chen
2021-04-07 14:02 ` Vincent Guittot
2021-04-07 17:19 ` Tim Chen
2021-04-08 14:51 ` Vincent Guittot
2021-04-08 23:05 ` Tim Chen
2021-04-09 15:26 ` Vincent Guittot
2021-04-09 17:59 ` Tim Chen
2021-05-10 21:59 ` Tim Chen
2021-05-11 15:25 ` Vincent Guittot
2021-05-11 17:25 ` Tim Chen [this message]
2021-05-11 17:56 ` Vincent Guittot
2021-05-12 13:59 ` Qais Yousef
2021-05-13 18:45 ` Tim Chen
2021-05-17 16:14 ` Qais Yousef
2021-06-11 20:00 ` Tim Chen
2021-06-18 10:28 ` Vincent Guittot
2021-06-18 16:14 ` Tim Chen
2021-06-25 8:50 ` Vincent Guittot
2021-02-01 15:13 ` Joel Fernandes
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