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

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Date:   Tue, 17 Jan 2017 15:39:45 -0800
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
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 01/17/2017 03:23 PM, Russell King - ARM Linux wrote:
> 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.

We have a test that consists in shutting down all CPUs as frequently as
we can and do this for about 2 million iterations which takes roughly
24h, and this printk slows thing down by a reasonable amount. Here are
some numbers on 500 hotplug operations:

w/ printk:
real    0m9.997s
user    0m0.725s
sys     0m3.030s
#

w/o printk:
real    0m8.547s
user    0m0.436s
sys     0m1.838s

> 
> 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?

No, this is not about power management considerations here, just how
fast we can bang it.

> 
> 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...

Well, for one it's inconsistent, and it also leaves room for subtle
timing/caching issues with some platforms (OK, maybe not so much).
Improving the speed and consistency was the primary motive.
-- 
Florian