Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20210122154600.1722680-1-joel@joelfernandes.org>
 <YBG0W5PFGtGRCEuB@google.com> <CAKfTPtBqj5A_7QmxhhmkNTc3+VT6+AqWgw1GDYrgy1V5+PJMmQ@mail.gmail.com>
 <CAEXW_YRrhEfGcLN5yrLJZm6HrB15M_R5xfpMReG2wE2rSmVWdA@mail.gmail.com>
 <CAKfTPtBvwm9vZb5C=2oTF6N-Ht6Rvip4Lv18yi7O3G8e-_ZWdg@mail.gmail.com>
 <20210129172727.GA30719@vingu-book> <274d8ae5-8f4d-7662-0e04-2fbc92b416fc@linux.intel.com>
 <20210324134437.GA17675@vingu-book> <efad4771-c9d1-5103-de9c-0ec5fa78ee24@linux.intel.com>
 <CAKfTPtDsya_zdUB1ARmoxQs5xWS8o-XrrzyNx5R1iSNrchUXtg@mail.gmail.com>
 <fc0efe4e-0a81-03b8-08cb-029468c57782@linux.intel.com> <CAKfTPtCKavGWja42NdTmb+95ppG-WxYzoTJMmtgkCQcA-btfBw@mail.gmail.com>
 <4aa674d9-db49-83d5-356f-a20f9e2a7935@linux.intel.com> <CAKfTPtDJaTr_HR2t=9CQ-9x6keu-qzx6okci92AdW5cJG8J9zg@mail.gmail.com>
 <2d2294ce-f1d1-f827-754b-4541c1b43be8@linux.intel.com> <ade18978-cd67-6215-28f0-4857c66a99fb@linux.intel.com>
 <CAKfTPtA8nr-fgt4Nw6XqQyT_TEx4uL3nK-ba0xGfkONO+BPG3Q@mail.gmail.com> <577b0aae-0111-97aa-0c99-c2a2fcfb5e2e@linux.intel.com>
In-Reply-To: <577b0aae-0111-97aa-0c99-c2a2fcfb5e2e@linux.intel.com>
From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Tue, 11 May 2021 19:56:23 +0200
Message-ID: <CAKfTPtC2Lj1O7-CoZd51cUDaB6EWkYVO=jbe4yaHVQE-wASAEQ@mail.gmail.com>
Subject: Re: [PATCH] sched/fair: Rate limit calls to update_blocked_averages()
 for NOHZ
To:     Tim Chen <tim.c.chen@linux.intel.com>
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>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Tue, 11 May 2021 at 19:25, Tim Chen <tim.c.chen@linux.intel.com> wrote:
>
>
>
> 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?

No it's a typo mistake while copy pasting the line

> 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
>