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

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Date:	Thu, 7 May 2015 12:23:51 +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@...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:

> > So I really like this, as it nicely side-steps the 'when should we 
> > do the legacy delays' issue by flagging on x2apic support.
> >
> > If anyone has objections, please holler.
> 
> We should have no delays even for many processors that lack x2apic.

Yes, agreed, but this looks like a good (and safe) first step: 
especially systems that have large CPU counts tend to have x2apic 
support - and that is where such a change matters most.

I'm not against doing a zero delay approach either, if it can be 
triggered robustly.

Here's what the boot time looks like on a 120 CPUs system, with the 
patch applied:

[    0.558947] x86: Booting SMP configuration:
[    0.563375] .... node  #0, CPUs:          #1   #2   #3   #4   #5   #6   #7   #8   #9  #10  #11  #12  #13  #14
[    0.644851] .... node  #1, CPUs:    #15  #16  #17  #18  #19  #20  #21  #22  #23  #24  #25  #26  #27  #28  #29
[    0.830474] .... node  #2, CPUs:    #30  #31  #32  #33  #34  #35  #36  #37  #38  #39  #40  #41  #42  #43  #44
[    1.016357] .... node  #3, CPUs:    #45  #46  #47  #48  #49  #50  #51  #52  #53  #54  #55  #56  #57  #58  #59
[    1.202342] .... node  #0, CPUs:    #60  #61  #62  #63  #64  #65  #66  #67  #68  #69  #70  #71  #72  #73  #74
[    1.283864] .... node  #1, CPUs:    #75  #76  #77  #78  #79  #80  #81  #82  #83  #84  #85  #86  #87  #88  #89
[    1.397131] .... node  #2, CPUs:    #90  #91  #92  #93  #94  #95  #96  #97  #98  #99 #100 #101 #102 #103 #104
[    1.510417] .... node  #3, CPUs:   #105 #106 #107 #108 #109 #110 #111 #112 #113 #114 #115 #116 #117 #118 #119
[    1.620967] x86: Booted up 4 nodes, 120 CPUs
[    1.625928] smpboot: Total of 120 processors activated (672866.16 BogoMIPS)

1.1 seconds to boot 120 CPUs, 10.8 seconds to hit init, that's an 
entirely reasonable runtime I think.

It was 20+ seconds before that, 10+ seconds for the SMP bootup 
sequence.

Thanks,

	Ingo
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