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

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Date:   Wed, 18 Jan 2017 11:56:46 -0800
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Russell King - ARM Linux <linux@...linux.org.uk>,
        linux-arm-kernel@...ts.infradead.org,
        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>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH] ARM: smp: Remove CPU: shutdown notice

On 01/18/2017 01:55 AM, Petr Mladek wrote:
> On Tue 2017-01-17 15:39:45, Florian Fainelli wrote:
>> 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
> 
> I am curious that a single printk() might make such a big difference.

It does, because of how printk() is implemented (there is nothing wrong
with it, just slow by nature and how the UART gets written to as well).

> 
> One reason might be that the messages are pushed to a "slow" console.

115200 UART, yes that's slow, but not unusual.

> 
> Another reason might be that there are many other messages printed
> on the system and there is a contention on logbuf_lock or other
> console related locks.

The other messages being printed are those from the hotplug script that
I run which just checkpoints its running every 50 instances, so it does
not occur that often, the console really is not busy, which really
extracts the overhead of printing "CPU: shutdown".

> 
> There might be also the opposite problem. The messages are also read
> by userspace tools that store them into /var/log/messages or systemd
> logs. If these are the only messages printed to the log and if there
> is no other activity on the system. Then the waken loggers might make
> a difference, especially if all CPUs are getting disabled and only
> one is available at some point.

There is none of that, no systemd, no syslog, just minimal userspace
running.

> 
> Well, I am not sure what other operations are needed to do the
> CPU hotplug operation.
> 
> I cannot judge how the message is useful and if the speed up
> is worth removing it.
> 
> Best Regards,
> Petr
> 


-- 
Florian