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From:   Valentin Schneider <valentin.schneider@arm.com>
To:     Vincent Guittot <vincent.guittot@linaro.org>,
        Scott Wood <swood@redhat.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mel Gorman <mgorman@suse.de>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-rt-users <linux-rt-users@vger.kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v2 1/3] sched/fair: Call newidle_balance() from balance_callback on PREEMPT_RT
In-Reply-To: <CAKfTPtBrGF72Zf37vPS6r4wy_jKagznBbiqKWpPyMEB_u4wo1g@mail.gmail.com>
References: <20210428232821.2506201-1-swood@redhat.com> <20210428232821.2506201-2-swood@redhat.com> <CAKfTPtBrGF72Zf37vPS6r4wy_jKagznBbiqKWpPyMEB_u4wo1g@mail.gmail.com>
Date:   Fri, 07 May 2021 16:19:07 +0100
Message-ID: <87v97uinbo.mognet@arm.com>
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On 05/05/21 14:13, Vincent Guittot wrote:
> On Thu, 29 Apr 2021 at 01:28, Scott Wood <swood@redhat.com> wrote:
>>
>> This is required in order to be able to enable interrupts in the next
>> patch.  This is limited to PREEMPT_RT to avoid adding potentially
>> measurable overhead to the non-RT case (requiring a double switch when
>> pulling a task onto a newly idle cpu).
>
> IIUC, only the newidle_balance is a problem and not the idle load
> balance that runs softirq. In this case, why not skipping
> newidle_balance entirely in case of preempt_rt and kick an idle load
> balance instead as you switch to idle thread context anyway
>

So if I follow you, that would be along the lines of having PREEMPT_RT turn
newidle_balance() into:

        rq->idle_balance = CPU_IDLE;
        rq->next_balance = jiffies;
        trigger_load_balance(rq);

which I'm thinking isn't too crazy.