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

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Date:	Thu, 14 May 2015 02:36:58 -0400
From:	Len Brown <lenb@...nel.org>
To:	Ingo Molnar <mingo@...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

> [    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.

BTW. this time can be reduced by 7% (113 ms) by deleting announce_cpu():

[    1.445815] x86: Booted up 4 nodes, 120 CPUs

cheers,
Len Brown, Intel Open Source Technology Center
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