Re: [PATCH v9 1/2] Documentation: kdump: remind user of nr_cpus

From: Baoquan He
Date: Wed Aug 24 2016 - 01:06:55 EST

On 08/22/16 at 09:14am, "Zhou, Wenjian/åæå" wrote:
> On 08/19/2016 11:57 PM, Jonathan Corbet wrote:
> >On Fri, 19 Aug 2016 08:33:21 +0800
> >"Zhou, Wenjian/åæå" <zhouwj-fnst@xxxxxxxxxxxxxx> wrote:
> >
> >>I was also confused by maxcpus and nr_cpus before writing this patch.
> >>I think it is a good choice to describe it in kernel-parameters.txt.
> >>
> >>Then, only two things need to be done I think.
> >>One is move the above description to maxcpus= in kernel-parameters.txt.
> >>And the other is replace maxcpus with maxcpus/nr_cpus in kdump.txt.
> >>
> >>How do you think?
> >
> >That is not quite what I had in mind, sorry. What I would really like to
> >see in kernel-parameters.txt is an explanation of how those two parameters
> >differ - what do they do differently and how should a user choose one over
> >the other? What we have now offers no guidance in that matter.
> >
> I thought about it. I think user may not need this.
> What user really want to know is how to choose.
> And it is also not a hard work. If nr_cpus is not supported by the ARCH, use maxcpus.
> Otherwise, nr_cpus. The reason why maxcpus still exists is nr_cpus can't be supported
> by some ARCHes.

I think Jon is suggesting that a note can be added into
kernel-parameter.txt to tell what's the difference between nr_cpus and
max_cpus. I checked code and discussed within our kdump team, max_cpus
is used to limit how many 'present' cpus are allowed to be brought up
during system bootup, while nr_cpus is used to set the upper limit of
'possible' cpus. E.g on my laptop, there are 4 cpus while 4 hotplug
cpus, altogether 8 possible cpus. Possible cpus slot is for cpu hot
plug, means during bootup you want to bring up 4 present cpus, but
later you could physically hot plug 4 others. Because of attribute of
some static percpu variables, we need pre-allocate memory for all
possible cpus though some of them may not be really used if no extra
cpu physically hot plugged after system bootup.

Hence for kdump kernel, people never want to do a cpu hot plug in there.
That's why we want to use nr_cpus to limit the number of possible cpu to
save memory. E.g still on my laptop, if I want to do a kdump, the number
of possible cpu is still 8, but you may want to use only 1 cpu to dump,
maybe 2 or 3 for parallel dumping. But you absolutely don't want to set
nr_cpus=8 in your kdump kernel cmdline, though it doesn't cause failure,
memory is wasted because of percpu pre-allocation. So specifying nr_cpus=1
is much better. While with specifying max_cpus=1, the number of possible
cpu is still 8. That's the reason. On x86_64 and s390, there's another
kernel para "possible_cpus=xx" which can be used to set possible cpus for
cpu hot plug. Only when "possible_cpus=0" is specified, smp is disabled.
I am not very sure why this is introduced, number of possible cpu is
decided by the min value of nr_cpus= and possible_cpus=.

nr_cpus and maxcpus might not be very clear to people which are
described in Documentation/kernel-parameters.txt.

Hi Jon, do you think change as below is OK to you?