linux-kernel - Re: [PATCH 3/4] cpumask: use maxcpus=NUM to extend the cpu limit as well as restrict the limit

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Date:	Fri, 12 Dec 2008 11:03:30 +0100
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Mike Travis <travis@....com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] cpumask: use maxcpus=NUM to extend the cpu limit
	as well as restrict the limit

On Thu, Dec 11, 2008 at 10:19:46AM -0800, Mike Travis wrote:
> Heiko Carstens wrote:
> > maxcpus=n    Restrict boot time cpus to n. Say if you have 4 cpus, using
> > 	     maxcpus=2 will only boot 2. You can choose to bring the
> > 	     other cpus later online, read FAQ's for more info.
> > 
> > It used to be (implementation wise) that maxcpus doesn't influence the number
> > of possible cpus but just indicated how many cpus were brought online at startup
> > of the kernel. Which is what cpu-hotplug.txt describes.
> > 
> > Other present cpus would appear offline and could be brought online later.
> > 
> > For s390 I added the possible_cpus kernel parameter back then, since my
> > understanding back then was that maxcpus doesn't and shouldn't influence the
> > number of possible cpus:
> > 
> > possible_cpus=n		[s390 only] use this to set hotpluggable cpus.
> > 			This option sets possible_cpus bits in
> > 			cpu_possible_map. Thus keeping the numbers of bits set
> > 			constant even if the machine gets rebooted.
> 
> Hmm, I hadn't noticed that.  For a while the X86 devel kernel had an
> "additional_cpus=n" parameter, which was also a bit confusing.  Say you
> wanted, 64 total, you had to give the increment over how many you already
> had [e.g., (want)64 - (have)16  = (additional_cpus=)48.]

Yes, we had the additional_cpus parameter as well. But that was too confusing.
Especially if you add a few cpus while the system is running and then reboot
it. The result for (want) would vary for each configuration change and reboot.

That's why I added possible_cpus to s390, then you get (want) == possible_cpus.

> I just figured that re-using the same kernel parameter was better than adding
> another.  But I'm willing to go either way.

Maybe you could go for possible_cpus as well? Having this in sync for several
architectures seems not so bad :)
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