linux-kernel - Re: [PATCH] Do not force shutdown/reboot to boot cpu.

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Date:	Thu, 11 Apr 2013 14:14:26 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Robin Holt <holt@....com>
Cc:	Russ Anderson <rja@....com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"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>
Subject: Re: [PATCH] Do not force shutdown/reboot to boot cpu.


* Robin Holt <holt@....com> wrote:

> On Thu, Apr 11, 2013 at 07:03:58AM -0500, Robin Holt wrote:
> > On Thu, Apr 11, 2013 at 02:00:27PM +0200, Ingo Molnar wrote:
> > > 
> > > * Robin Holt <holt@....com> wrote:
> > > 
> > > > > Ok, so it looks profilable.
> > > > > 
> > > > > The result above is not surprising: most CPUs sit in idle and don't do anything, 
> > > > > while the loop goes on, right?
> > > > > 
> > > > > The interesting thing to profile would be the parallel bring-down, with the 
> > > > > simplest global lock solution you mentioned. In that case most CPUs should be 
> > > > > doing 'something' all the time - maybe spinning on the lock, maybe something else, 
> > > > > right?
> > > > 
> > > > Again, mostly looks idle.
> > > 
> > > Forgot to suggest:
> > > 
> > >   perf record -a /sbin/reboot
> > 
> > I used perf record -a /sbin/reboot -f -d -n
> 
> OK.  Looking at Russ' patch, I understand now why it is looking idle.
> We are still serially doing the DOWN_PREPARE, etc.  All those other cpus
> are still sitting idle.
> 
> Can we call the __cpu_down functions from an smp_call_function()?

I think the kthread_park() will generally schedule.

But ... whether it's an IPI or a wakeup should matter little: wakeups are IPI 
based (sometimes faster, mwait based).

So the main overhead is the serial loop - if that's done in parallel, and then all 
CPUs are waited for in a second loop, then much of the work can go on in parallel.

Thanks,

	Ingo
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