linux-kernel - Re: Bulk CPU Hotplug (Was Re: [PATCH] Do not force shutdown/reboot to boot cpu.)

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Date:	Thu, 11 Apr 2013 16:08:55 -0500
From:	Robin Holt <holt@....com>
To:	Russ Anderson <rja@....com>
Cc:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Paul Mackerras <paulus@...ba.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>, Robin Holt <holt@....com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Shawn Guo <shawn.guo@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Tejun Heo <tj@...nel.org>, Oleg Nesterov <oleg@...hat.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Michel Lespinasse <walken@...gle.com>,
	"rusty@...tcorp.com.au" <rusty@...tcorp.com.au>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: Bulk CPU Hotplug (Was Re: [PATCH] Do not force shutdown/reboot
 to boot cpu.)

On Thu, Apr 11, 2013 at 03:08:20PM -0500, Russ Anderson wrote:
> On Thu, Apr 11, 2013 at 08:15:27PM +0530, Srivatsa S. Bhat wrote:
> > On 04/11/2013 07:53 PM, Russ Anderson wrote:
> > > On Thu, Apr 11, 2013 at 06:15:18PM +0530, Srivatsa S. Bhat wrote:
> > >>
> > >> One more thing we have to note is that, there are 4 notifiers for taking a
> > >> CPU offline:
> > >>
> > >> CPU_DOWN_PREPARE
> > >> CPU_DYING
> > >> CPU_DEAD
> > >> CPU_POST_DEAD
> > >>
> > >> The first can be run in parallel as mentioned above. The second is run in
> > >> parallel in the stop_machine() phase as shown in Russ' patch. But the third
> > >> and fourth set of notifications all end up running only on CPU0, which will
> > >> again slow down things.
> > > 
> > > In my testing the third and fourth set were a small part of the overall
> > > time.  Less than 10%, with cpu notifiers 90+% of the time.
> > 
> > *All* of them are cpu notifiers! All of them invoke __cpu_notify() internally.
> > So how did you differentiate between them and find out that the third and
> > fourth sets take less time?
> 
> I reran a test on a 1024 cpu system, using my test patch to only call
> __stop_machine() once.  Added printks to show the kernel timestamp
> at various points.
> 
> When calling disable_nonboot_cpus() and enable_nonboot_cpus() just after
> booting the system:
>  The loop calling __cpu_notify(CPU_DOWN_PREPARE) took 376.6 seconds.
>  The loop calling cpu_notify_nofail(CPU_DEAD) took 8.1 seconds.
> 
> My guess is that notifiers do more work in the CPU_DOWN_PREPARE case.
> 
> I also added a loop calling a new notifier (CPU_TEST) which none of
> notifiers would recognize, to measure the time it took to spin through
> the call chain without the notifiers doing any work.  It took
> 0.0067 seconds.
> 
> On the actual reboot, as the system was shutting down:
>  The loop calling __cpu_notify(CPU_DOWN_PREPARE) took 333.8 seconds.
>  The loop calling cpu_notify_nofail(CPU_DEAD) took 2.7 seconds.

How about if you take the notifier_call_chain function copy it
to kernel/sys.c, and time each notifier_call() callout individually.

Robin
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