linux-kernel - Re: [PATCH 1/2] cpuhotplug/nohz: Remove offline cpus from nohz-idle state

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Message-ID: <20130108042750.GA18214@quicinc.com>
Date:	Mon, 7 Jan 2013 20:27:50 -0800
From:	Srivatsa Vaddagiri <vatsa@...eaurora.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	linux-arm-kernel@...ts.infradead.org,
	Mike Frysinger <vapier@...too.org>,
	uclinux-dist-devel@...ckfin.uclinux.org,
	Ralf Baechle <ralf@...ux-mips.org>, linux-mips@...ux-mips.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linuxppc-dev@...ts.ozlabs.org,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	linux-s390@...r.kernel.org, Paul Mundt <lethal@...ux-sh.org>,
	linux-sh@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	sparclinux@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org, mhocko@...e.cz,
	srivatsa.bhat@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH 1/2] cpuhotplug/nohz: Remove offline cpus from
 nohz-idle state

* Russell King - ARM Linux <linux@....linux.org.uk> [2013-01-05 10:36:27]:

> On Thu, Jan 03, 2013 at 06:58:38PM -0800, Srivatsa Vaddagiri wrote:
> > I also think that the
> > wait_for_completion() based wait in ARM's __cpu_die() can be replaced with a
> > busy-loop based one, as the wait there in general should be terminated within
> > few cycles.
> 
> Why open-code this stuff when we have infrastructure already in the kernel
> for waiting for stuff to happen?  I chose to use the standard infrastructure
> because its better tested, and avoids having to think about whether we need
> CPU barriers and such like to ensure that updates are seen in a timely
> manner.

I was primarily thinking of calling as few generic functions as possible on
a dead cpu. I recall several "am I running on a dead cpu?" checks
(cpu_is_offline(this_cpu) that were put in generic routines during early
versions of cpu hotplug [1] to educate code running on dead cpu, the need for
which went away though with introduction of atomic/stop-machine variant. The
need to add a RCU_NONIDLE() wrapper around ARM's cpu_die() [2] is perhaps a more
recent example of educating code running on dead cpu. As quickly we die as
possible after idle thread of dying cpu gains control, the better!

1. http://lwn.net/Articles/69040/
2. http://lists.infradead.org/pipermail/linux-arm-kernel/2012-July/107971.html

- vatsa
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