From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
linux-rt-users <linux-rt-users@vger.kernel.org>,
Ingo Molnar <mingo@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Daniel Wagner <wagi@monom.org>,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] sched/rt: Do not do push/pull when there is only one CPU
Date: Sat, 2 Dec 2017 13:53:31 +0100 [thread overview]
Message-ID: <20171202125330.xrxspjeedbpr4hk5@linutronix.de> (raw)
In-Reply-To: <20171201133222.66d41219@gandalf.local.home>
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@vger.kernel.org
> Reported-by: Daniel Wagner <wagi@monom.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
next prev parent reply other threads:[~2017-12-02 12:53 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-12-01 18:32 [PATCH] sched/rt: Do not do push/pull when there is only one CPU Steven Rostedt
2017-12-02 12:53 ` Sebastian Andrzej Siewior [this message]
2017-12-02 13:55 ` Peter Zijlstra
2017-12-02 17:12 ` Steven Rostedt
2017-12-02 18:05 ` Steven Rostedt
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