linux-kernel - Re: [PATCH 2/2] printk: Make the console flush configurable in hotplug path

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Message-ID: <20200925141657.GA6325@angband.pl>
Date:   Fri, 25 Sep 2020 16:16:57 +0200
From:   Adam Borowski <kilobyte@...band.pl>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Prasad Sodagudi <psodagud@...eaurora.org>, rostedt@...dmis.org,
        pmladek@...e.com, sergey.senozhatsky@...il.com,
        linux-kernel@...r.kernel.org, tkjos@...gle.com,
        Mohammed Khajapasha <mkhaja@...eaurora.org>
Subject: Re: [PATCH 2/2] printk: Make the console flush configurable in
 hotplug path

On Fri, Sep 25, 2020 at 11:27:54AM +0200, Greg KH wrote:
> On Thu, Sep 24, 2020 at 08:21:07PM +0200, Thomas Gleixner wrote:
> > On Thu, Sep 24 2020 at 08:33, Greg KH wrote:
> > > On Wed, Sep 23, 2020 at 05:08:32PM -0700, Prasad Sodagudi wrote:
> > >> +config CONSOLE_FLUSH_ON_HOTPLUG
> > >> +	bool "Enable console flush configurable in hot plug code path"

> > CPU hotplug is not meant to be a high speed operation and if people
> > think they need it to be fast then its pretty much guaranteed that they
> > want it for the completely wrong reasons.
> 
> Odds are, it's the big/little systems that are trying to use cpu hotplug
> for this type of thing :(

Just a bit of info:
My MT6797X (10 core: 4×A53 + 4×A53 + 2×A72), flickers its cores this way:
the right-hand piece is CPUs, one character per core: bars show utilization,
"o" stands for offline; every line is 0.1 second interval.

topline -i 0.1
mmcblk(⠀) (oooo▄▆oo▅o)
mmcblk(⠀) (oooo▅▄oooo)
mmcblk(⠀) (oooo▆▆oooo)
mmcblk(⠀) (oooo▅ooo▆o)
mmcblk(⠀) (oooo▆▆oo▄o)
mmcblk(⠀) (oooo▆▇oooo)
mmcblk(⠀) (oooo▇ooo▅o)
mmcblk(⠀) (oooo▆ooo█o)
mmcblk(⠀) (oooo▆ooo▄o)
mmcblk(⠀) (oooo▅ooo▆o)
mmcblk(⠀) (oooo▆ooo▅o)
mmcblk(⠀) (oooo▄ooo▇o)
mmcblk(⠀) (oooo▇▆oo▆o)
mmcblk(⠀) (oooo▆ooo▅o)
mmcblk(⠀) (oooo▅▆oooo)
mmcblk(⠀) (oooo▆█oooo)
mmcblk(⠀) (oooo▆▇oooo)
mmcblk(⠀) (oooo▆▆oooo)
mmcblk(⠀) (oooo▅▆oooo)
mmcblk(⠀) (oooo▆▅oooo)
mmcblk(⠀) (oooo▆ooo▆o)
mmcblk(⠀) (oooo▆ooo▇o)
mmcblk(⢀) (oooo▆▇oo▆o)
mmcblk(⠀) (oooo▄ooo▆o)
mmcblk(⠀) (oooo▆ooo█o)
mmcblk(⠀) (oooo▄ooo▇o)
mmcblk(⠀) (oooo▄▆oooo)
mmcblk(⠀) (oooo▆▆oooo)

So it's on the order of a few ons/offs per second.

The offline CPUs are "present" and "offline"; not sure if this means hotplug
or not (I'd expect dropping from "present" to "possible", but I don't know
these parts).


Meow!
-- 
⢀⣴⠾⠻⢶⣦⠀
⣾⠁⢠⠒⠀⣿⡁ in the beginning was the boot and root floppies and they were good.
⢿⡄⠘⠷⠚⠋⠀                                       -- <willmore> on #linux-sunxi
⠈⠳⣄⠀⠀⠀⠀