linux-kernel - Re: [PATCH] x86: skip delays during SMP initialization similar to Xen

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Message-ID: <20150517050220.GA15791@gmail.com>
Date:	Sun, 17 May 2015 07:02:21 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Len Brown <lenb@...nel.org>
Cc:	Jan H. Schönherr <jschoenh@...zon.de>,
	Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Anthony Liguori <aliguori@...zon.com>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Tim Deegan <tim@....org>,
	Gang Wei <gang.wei@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86: skip delays during SMP initialization similar to Xen


* Len Brown <lenb@...nel.org> wrote:

> On Thu, May 14, 2015 at 2:44 AM, Ingo Molnar <mingo@...nel.org> wrote:
> 
> >
> >> BTW. this time can be reduced by 7% (113 ms) by deleting
> >> announce_cpu():
> >>
> >> [    1.445815] x86: Booted up 4 nodes, 120 CPUs
> >
> > so that kind of info looks pretty useful, especially when there's
> > hangs/failures.
> 
> I think the messages we print on failure are useful.
> I think the success case should be a 1-line summary.

But we usually don't know a failure until it happens, and then people 
often don't know which quirky debug option to turn on before sending a 
capture of the failure.

It also pretty compressed and looks kind of cool, especially with 
larger CPU counts. Would love to see a 6K CPUs system boot up ;-)

> > I'm wondering what takes 113 msecs to print 120 CPUs - that's 
> > about 1 msec per a few chars of printk produced, seems excessive. 
> > Do you have any idea what's going on there? Does your system print 
> > to a serial console perhaps?
> 
> Yes, serial console -- that server is actually much
> closer to you than it is to me, it is in Finland:-)

LOL ;-)

> I should benchmark it, because 115200 should be faster...

So 115200 baud == 14400 bytes/sec == 14.4 bytes/msec == 0.07 msecs/byte

So with 120 CPUs we print about 5-6 chars per CPU, which is 6*120==720 
bytes, which should take about 50 msecs.

So serial explains about half of the observed overhead.

Thanks,

	Ingo
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