linux-kernel - Re: [PATCH] sched/rt: Do not do push/pull when there is only one CPU

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Date:   Sat, 2 Dec 2017 13:53:31 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Daniel Wagner <wagi@...om.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] sched/rt: Do not do push/pull when there is only one CPU

On 2017-12-01 13:32:22 [-0500], Steven Rostedt wrote:
> 
> Daniel Wagner reported a crash on the beaglebone black. This is a
> single CPU architecture, and does not have a functional:
> arch_send_call_function_single_ipi() and can crash if that is called.
> 
> As it only has one CPU, it shouldn't be called, but if the kernel is
> compiled for SMP, the push/pull RT scheduling logic now calls it for
> irq_work if the one CPU is overloaded, it can use that function to call
> itself and crash the kernel.
> 
> There's no reason for the push/pull logic to even be called if there's
> only one CPU online. Have it bail if it sees that's the case.
> 
> Link: http://lkml.kernel.org/r/8c913cc2-b2e3-8c2e-e503-aff1428f8ff5@monom.org
> Fixes: 4bdced5c9 ("sched/rt: Simplify the IPI based RT balancing logic")
> Cc: stable@...r.kernel.org
> Reported-by: Daniel Wagner <wagi@...om.org>
> ---
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 4056c19ca3f0..50d2f8179f70 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1784,6 +1784,10 @@ static int push_rt_task(struct rq *rq)
>  	if (!rq->rt.overloaded)
>  		return 0;
>  
> +	/* If we are the only CPU, don't bother */
> +	if (num_online_cpus() == 1)
> +		return 0;
> +

what about a check next to sched_feat(RT_PUSH_IPI)? I don't know if this
is a hot path or not (due to bitmap_weight). If it is, then I would
suggest something like a jump-label which is enabled if more than one
CPU has been enabled on boot.

>  	next_task = pick_next_pushable_task(rq);
>  	if (!next_task)
>  		return 0;
> @@ -2038,6 +2042,10 @@ static void pull_rt_task(struct rq *this_rq)
>  	if (likely(!rt_overloaded(this_rq)))
>  		return;
>  
> +	/* If we are the only CPU, don't bother */
> +	if (num_online_cpus() == 1)
> +		return;
> +
>  	/*
>  	 * Match the barrier from rt_set_overloaded; this guarantees that if we
>  	 * see overloaded we must also see the rto_mask bit.

Sebastian