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

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Message-ID: <20150514064407.GA9117@gmail.com>
Date:	Thu, 14 May 2015 08:44:07 +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:

> > [    0.404369] x86: Booting SMP configuration:
> ...
> > [    2.737884] x86: Booted up 4 nodes, 120 CPUs
> > [    2.743758] smpboot: Total of 120 processors activated (671097.18 BogoMIPS)
> >
> > (2.743758-0.404369) = 2.339389 for all 119 processors
> > /119 = .01965873109243697478 - lets call it 19ms each
> 
> For the record, the same (bare metal) box running latest tip boots
> 10ms/processor quicker
> than upstream Linux, as expected.   So this 120 processor box now
> boots 1.19 seconds faster, in total.
> 
> [    0.415969] x86: Booting SMP configuration:
> ...
> [    1.553658] x86: Booted up 4 nodes, 120 CPUs
> [    1.559173] smpboot: Total of 120 processors activated (671182.14 BogoMIPS)
> 
> 1.553658-0.415969 = 1.137689 - seconds to bring 119 processors on-line.
> ./119 = .00956041176470588235 -- 9.5ms per processor, down from 19.

Ok. I think we should be able to further speed that up.

> 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'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?

Thanks,

	Ingo
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