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

To: Russell King - ARM Linux <linux@arm.linux.org.uk>
Subject: Re: [PATCH 1/2] cpuhotplug/nohz: Remove offline cpus from nohz-idle state
From: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
Date: Tue, 08 Jan 2013 12:25:53 +0530
Cc: Srivatsa Vaddagiri <vatsa@codeaurora.org>, linux-mips@linux-mips.org, linux-sh@vger.kernel.org, mhocko@suse.cz, "H. Peter Anvin" <hpa@zytor.com>, sparclinux@vger.kernel.org, linux-s390@vger.kernel.org, x86@kernel.org, Ingo Molnar <mingo@redhat.com>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, Mike Frysinger <vapier@gentoo.org>, linux-arm-msm@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>, linux-arm-kernel@lists.infradead.org, Stephen Boyd <sboyd@codeaurora.org>, linux-kernel@vger.kernel.org, Ralf Baechle <ralf@linux-mips.org>, Paul Mundt <lethal@linux-sh.org>, Martin Schwidefsky <schwidefsky@de.ibm.com>, uclinux-dist-devel@blackfin.uclinux.org, linuxppc-dev@lists.ozlabs.org, "David S. Miller" <davem@davemloft.net>, Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>, Peter Zijlstra <peterz@infradead.org>, "rusty@rustcorp.com.au" <rusty@rustcorp.com.au>
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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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