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

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Message-ID: <50EBC2F9.70303@linux.vnet.ibm.com>
Date:	Tue, 08 Jan 2013 12:25:53 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Srivatsa Vaddagiri <vatsa@...eaurora.org>,
	linux-mips@...ux-mips.org, linux-sh@...r.kernel.org,
	mhocko@...e.cz, "H. Peter Anvin" <hpa@...or.com>,
	sparclinux@...r.kernel.org, linux-s390@...r.kernel.org,
	x86@...nel.org, Ingo Molnar <mingo@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mike Frysinger <vapier@...too.org>,
	linux-arm-msm@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-arm-kernel@...ts.infradead.org,
	Stephen Boyd <sboyd@...eaurora.org>,
	linux-kernel@...r.kernel.org, Ralf Baechle <ralf@...ux-mips.org>,
	Paul Mundt <lethal@...ux-sh.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	uclinux-dist-devel@...ckfin.uclinux.org,
	linuxppc-dev@...ts.ozlabs.org,
	"David S. Miller" <davem@...emloft.net>,
	Nikunj A Dadhania <nikunj@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"rusty@...tcorp.com.au" <rusty@...tcorp.com.au>
Subject: Re: [PATCH 1/2] cpuhotplug/nohz: Remove offline cpus from nohz-idle
 state

On 01/05/2013 04:06 PM, Russell King - ARM Linux wrote:
> 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.
> 
> My stance on a lot of this idle/cpu dying code is that much of it can
> probably be cleaned up and merged into a single common implementation -
> in which case the use of standard infrastructure for things like waiting
> for other CPUs do stuff is even more justified.

On similar lines, Nikunj (in CC) and I had posted a patchset sometime ago to
consolidate some of the CPU hotplug related code in the various architectures
into a common standard implementation [1].

However, we ended up hitting a problem with Xen, because its existing code
was unlike the other arch/ pieces [2]. At that time, we decided that we will
first make the CPU online and offline paths symmetric in the generic code and
then provide a common implementation of the duplicated bits in arch/, for the
new CPU hotplug model [3].

I guess we should probably revisit it sometime, consolidating the code in
incremental steps if not all at a time...

--
[1]. http://lwn.net/Articles/500185/
[2]. http://thread.gmane.org/gmane.linux.kernel.cross-arch/14342/focus=14430
[3]. http://thread.gmane.org/gmane.linux.kernel.cross-arch/14342/focus=15567

Regards,
Srivatsa S. Bhat

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