linux-kernel - Re: [patch 00/10] cpu/hotplug: Final cleanup

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Message-ID: <alpine.DEB.2.20.1612271211490.3687@nanos>
Date:   Tue, 27 Dec 2016 12:17:54 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sam Ravnborg <sam@...nborg.org>
cc:     LKML <linux-kernel@...r.kernel.org>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [patch 00/10] cpu/hotplug: Final cleanup

On Thu, 22 Dec 2016, Sam Ravnborg wrote:
> Hi Thomas.
> 
> On Wed, Dec 21, 2016 at 08:19:47PM +0100, Thomas Gleixner wrote:
> > The following series completes the cpuhotplug rework stage ONE.
> 
> Just curious - what is the stage TWO (and onwards) plan?
> lwn had some coverage of this work but I assume the
> article is somewhat outdated.

Stage 2 is to consolidate all the archictecture code, i.e. reduce it to
really arch specific callbacks and keep all the other things
(synchronization, state settings etc.) in the core code.

After that we can do smart things like speed up the boot hotplug process:

prepare CPU1
kick CPU1

prepare CPU2
kick CPU2

....

prepare CPUN
kick CPUN

check CPU1 alive
bringup CPU1

...

check CPUN alive
bringup CPUN

That saves ~200ms per CPU on x86, which we now spend busy waiting for the
CPU to come alive.

There is more stuff which we can do in the long run like synchrounosly
letting CPUs bring up themself after the basic init/sync checks, but that
needs lot of surgery all over the place.

Thanks,

	tglx