LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: "Jan Beulich" <JBeulich@suse.com>
To: <douly.fnst@cn.fujitsu.com>
Cc: <tglx@linutronix.de>, <linux-kernel@vger.kernel.org>
Subject: Re: recent patch "x86/acpi: Prevent X2APIC id 0xffffffff from being accounted"
Date: Wed, 02 May 2018 00:39:28 -0600	[thread overview]
Message-ID: <5AE95D2002000078001BFEC4@prv1-mh.provo.novell.com> (raw)
In-Reply-To: <769e14c9-298d-3049-4001-2a8fe903d21e@cn.fujitsu.com>

>>> On 02.05.18 at 03:56, <douly.fnst@cn.fujitsu.com> wrote:
> At 04/27/2018 08:09 PM, Jan Beulich wrote:
>> I'm afraid I don't understand: Limiting the number of disabled CPUs is
>> certainly desirable when those can never be used, but why would you
>> want to do this when they might later get hotplugged? I'm not aware
> 
> Let's see the workflow of CPU hotplug:
> 
>    1) get the CPU device info from ACPI namespace
>       - it contains logical processor id
> 
>    2) through the logical processor id, get the LACPI entry in MADT.
> 
>    3) generate the CPU for kernel(will create a CPU id, can see by lscpu)
> 
> Normally, there are no valid CPU devices in 1) which are mapped to
> the LACPI entries(0xff or 0xffffffff).
> 
> The actually number of hotplugged CPUs depends on CPU devices/processors
> in ACPI namespace. The number calculated from MADT is the maximum
> situation which can be cut and doesn't affect CPU hotplug. Don't worry
> about it.
> 
> Now, in ACPI-based system, Linux gets the number of possible CPUs by
> MADT, We are going to use the ACPI namespace to make the number
> accurate. But it is so hard, because it's so late to initialize the ACPI
> namespace.

So are you envisioning a model when the number of disabled CPUs can be
increased once the ACPI interpreter has been enabled? Otherwise the
maximum recorded during early boot may be too low with the changes in
question. (And FTR, I agree this number may also be way too large without
them, but it being too large is a resource efficiency problem, while it being
too low is a functionality one.)

Also, for background, besides wanting to clarify the correctness of these
two changes, I'm also trying to understand whether we want to mirror
them into the Xen hypervisor, which relies on the Dom0 kernel's ACPI
interpreter (i.e. it can and does parse MADT, but can't and doesn't parse
the ACPI name space). Hence late adjustment of the number of
hotpluggable CPUs would be even more problematic in that environment.

Jan

  reply	other threads:[~2018-05-02  6:39 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-27  7:21 Jan Beulich
2018-04-27  8:32 ` Dou Liyang
2018-04-27 12:09   ` Jan Beulich
2018-05-02  1:56     ` Dou Liyang
2018-05-02  6:39       ` Jan Beulich [this message]
2018-05-02  8:10         ` Dou Liyang

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=5AE95D2002000078001BFEC4@prv1-mh.provo.novell.com \
    --to=jbeulich@suse.com \
    --cc=douly.fnst@cn.fujitsu.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=tglx@linutronix.de \
    --subject='Re: recent patch "x86/acpi: Prevent X2APIC id 0xffffffff from being accounted"' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).