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

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Date:	Thu, 28 May 2015 01:31:20 -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

>> >> 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.

I do not believe that the serial console drain-rate is is a measurable
factor in boot speed.

Right before the remote processors were on-lined, I did a bunch of
back-back printk's
on the same box, and they time-stamped in microseconds -- presumably at the rate
that they could be added to the message buffer:

[    0.802350] 12345678901234567890123456789012345678901234567890123456789
[    0.802350] 123456789012345678901234567890123456789012345678901234567890
[    0.802351] 1234567890123456789012345678901234567890123456789012345678901
[    0.802352] 12345678901234567890123456789012345678901234567890123456789012
[    0.802352] 123456789012345678901234567890123456789012345678901234567890123
[    0.802353] 1234567890123456789012345678901234567890123456789012345678901234
[    0.802354] 12345678901234567890123456789012345678901234567890123456789012345
[    0.802354] 123456789012345678901234567890123456789012345678901234567890123456
[    0.802355] 1234567890123456789012345678901234567890123456789012345678901234567
[    0.802356] 12345678901234567890123456789012345678901234567890123456789012345678
[    0.802356] 123456789012345678901234567890123456789012345678901234567890123456789
[    0.802357] 1234567890123456789012345678901234567890123456789012345678901234567890

cheers,
Len Brown, Intel Open Source Technology Center

ps. yes, if you're watching the console drain...

[    0.807246] NMI watchdog: enabled on all CPUs, permanently consumes
one hw-PMU counter.
[    0.817989] x86: Booting SMP configuration:
[    0.822942] .... node  #0, CPUs:          #1   #2   #3   #4   #5
#6   #7   #8   #9  #10  #11  #12  #13  #14
[    0.909214] .... node  #1, CPUs:    #15  #16  #17  #18  #19  #20
#21  #22  #23  #24  #25  #26  #27  #28  #29
[    1.108675] .... node  #2, CPUs:    #30  #31  #32  #33  #34  #35
#36  #37  #38  #39  #40  #41  #42  #43  #44
[    1.308132] .... node  #3, CPUs:    #45  #46  #47  #48  #49  #50
#51  #52  #53  #54  #55  #56  #57  #58  #59
[    1.507550] .... node  #0, CPUs:    #60  #61  #62  #63  #64  #65
#66  #67  #68  #69  #70  #71  #72  #73  #74
[    1.593693] .... node  #1, CPUs:    #75  #76  #77  #78  #79  #80
#81  #82  #83  #84  #85  #86  #87  #88  #89
[    1.715893] .... node  #2, CPUs:    #90  #91  #92  #93  #94  #95
#96  #97  #98  #99 #100 #101 #102 #103 #104
[    1.838143] .... node  #3, CPUs:   #105 #106 #107 #108 #109 #110
#111 #112 #113 #114 #115 #116 #117 #118 #119
[    1.959675] x86: Booted up 4 nodes, 120 CPUs

It is taking 119 CPUs 1.959675-0.817989 =  1.141686 seconds total
(or 1.141686/119 = 9.5ms / CPU)

While the console could drain that output much faster than it printed
into the msgbuf:

112 bytes/line * 8 lines = 896 bytes to dump all the cpu numbers

896 bytes * 10 bits/byte / 115200 bits/sec = 77ms total
(or 77/119 =  647 us per cpu)
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