linux-kernel - Re: [RFC/RFT PATCH 1/2] sched/core: Check and schedule ksoftirq

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Message-ID: <Y6ImigCet+RXzds/@hirez.programming.kicks-ass.net>
Date:   Tue, 20 Dec 2022 22:18:02 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     srinivas pandruvada <srinivas.pandruvada@...ux.intel.com>
Cc:     Frederic Weisbecker <frederic@...nel.org>, rafael@...nel.org,
        daniel.lezcano@...aro.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, len.brown@...el.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [RFC/RFT PATCH 1/2] sched/core: Check and schedule ksoftirq

On Tue, Dec 20, 2022 at 09:51:09PM +0100, Peter Zijlstra wrote:
> On Mon, Dec 19, 2022 at 02:54:55PM -0800, srinivas pandruvada wrote:
> 
> > But after ksoftirqd_run_end(), which will renable local irq, this may
> > further cause more soft irq pending. Here RCU soft irq entry continues
> 
> Right you are.. what about if we spell the idle.c thing like so instead?
> 
> Then we repeat the softirq thing every time we drop out of the idle loop
> for a reschedule.

Uff, that obviously can't work because we already have preemption
disabled here, this needs a bit more work. I think it's possible to
re-arrange things a bit.

I'll try and have a look tomorrow, but the kids have their xmas play at
school so who knows what I'll get done.

> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index f26ab2675f7d..6dce49813bcc 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -381,8 +381,13 @@ void play_idle_precise(u64 duration_ns, u64 latency_ns)
>  	hrtimer_start(&it.timer, ns_to_ktime(duration_ns),
>  		      HRTIMER_MODE_REL_PINNED_HARD);
>  
> -	while (!READ_ONCE(it.done))
> +	while (!READ_ONCE(it.done)) {
> +		rt_mutex_lock(&per_cpu(ksoftirq_lock, cpu));
> +		__run_ksoftirqd(smp_processor_id());
> +		rt_mutex_unlock(&per_cpu(ksoftirq_lock, cpu));
> +
>  		do_idle();
> +	}
>  
>  	cpuidle_use_deepest_state(0);
>  	current->flags &= ~PF_IDLE;