Re: [PATCH] sched/rt: Do not pull from current CPU if only one cpu to pull

From: Daniel Wagner <wagi@monom.org>
To: Steven Rostedt <rostedt@goodmis.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-rt-users <linux-rt-users@vger.kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] sched/rt: Do not pull from current CPU if only one cpu to pull
Date: Mon, 4 Dec 2017 11:29:17 +0100	[thread overview]
Message-ID: <6564b433-f820-67b7-76fc-bdb442979a7d@monom.org> (raw)
In-Reply-To: <20171202130454.4cbbfe8d@vmware.local.home>

Hi Steven,

On 12/02/2017 07:04 PM, 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.
> 
> Ideally, we should disable the SCHED_FEAT(RT_PUSH_IPI) if the system
> only has a single CPU. But SCHED_FEAT is a constant if sched debugging
> is turned off. Another fix can also be used, and this should also help
> with normal SMP machines. That is, do not initiate the pull code if
> there's only one RT overloaded CPU, and that CPU happens to be the
> current CPU that is scheduling in a lower priority task.
> 
> Even on a system with many CPUs, if there's many RT tasks waiting to
> run on a single CPU, and that CPU schedules in another RT task of lower
> priority, it will initiate the PULL logic in case there's a higher
> priority RT task on another CPU that is waiting to run. But if there is
> no other CPU with waiting RT tasks, it will initiate the RT pull logic
> on itself (as it still has RT tasks waiting to run). This is a wasted
> effort.
> 
> Not only does this help with SMP code where the current CPU is the only
> one with RT overloaded tasks, it should also solve the issue that
> Daniel encountered, because it will prevent the PULL logic from
> executing, as there's only one CPU on the system, and the check added
> here will cause it to exit the RT pull code.
> 
> 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>

Tested-by: Daniel Wagner <wagi@monom.org>

Thanks,
Daniel