linux-kernel - Re: [PATCH] ARM: smp: Remove CPU: shutdown notice

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Date:   Tue, 17 Jan 2017 23:23:14 +0000
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Petr Mladek <pmladek@...e.com>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        open list <linux-kernel@...r.kernel.org>,
        Aaron Tomlin <atomlin@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] ARM: smp: Remove CPU: shutdown notice

On Tue, Jan 17, 2017 at 03:07:12PM -0800, Florian Fainelli wrote:
> This message is not particularly informative, and is not paired with an
> identical message when a CPU is brought online. Finally, it slows the
> CPU hotplug path down, thus allowing less CPU hotplug operations per
> second. Just remove it.

CPU hotplug isn't a fast operation anyway - it's also fairly disruptive
in that it uses stop_machine() to halt activity everywhere while taking
the CPU offline.

If you're worried about a single printk slowing things down, it sounds
like you're doing something wrong here - maybe you should be using
cpuidle for power management rather than trying to make CPU hotplug do
that for you?

A rapidly changing CPU hotplug state has other side effects - such as
reading /proc/interrupts is meaningless, because as soon as you've read
it the CPUs could have changed.  It also makes a farce of spreading
interrupts over the available CPUs.

All in all, I don't think this is a good idea...

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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