linux-kernel - Re: [PATCH 0/2 v3] cpu hotplug: Preserve topology directory after soft remove event

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Date:   Tue, 27 Sep 2016 07:45:56 -0400
From:   Prarit Bhargava <prarit@...hat.com>
To:     Borislav Petkov <bp@...e.de>
CC:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Len Brown <len.brown@...el.com>,
        Andi Kleen <ak@...ux.intel.com>, Jiri Olsa <jolsa@...hat.com>,
        Juergen Gross <jgross@...e.com>
Subject: Re: [PATCH 0/2 v3] cpu hotplug: Preserve topology directory after
 soft remove event



On 09/26/2016 07:57 AM, Borislav Petkov wrote:
> On Mon, Sep 26, 2016 at 07:45:37AM -0400, Prarit Bhargava wrote:
>> When offline, /sys/devices/system/cpuX/cpu/online is 0.  The problem is that
>> when online is 0, topology disappears so there is no way to determine _the
>> location_ of the offline'd thread.
> 
> What does "the location" mean exactly?
> 
>> cpupower should still print out all asterisks for down'd threads.  It does not
>> because the topology directory is incorrectly removed.
>>
>> IOW how does userspace know the _location_ of the thread?  The topology
>> directory no longer exists when the thread is downed, so core_id and
>> physical_package_id (both of which would be effectively static) do not exist.
>> The whole point of this patchset is to know where the offline'd thread actually is.
> 
> What do you mean "where"?

The socket and core location.

Look at it this way (and let's get the terminology straight at the same time).

You have a socket CPU.  That socket has cores on it.  Each core (at least on
Intel) has two threads.

I down a thread (as you did):

> 
> $ echo 0 > /sys/devices/system/cpu/cpu2/online

This results in the topology directory being destroyed.  It shouldn't be -- the
socket and core are still there.  If you could open up your computer you could
touch them.  This is similar to downing a PCI device, or removing !kernel memory
DIMM from a system.  The device is still physically there.

> $ cat /sys/devices/system/cpu/online
> 0-1,3-7
> 
> So core 2 is right between 1 and 3.

Yes.  But *where* is it relative to the cores and socket(s)?

> 
> If you need to show the package id, you still iterate over the core
> numbers in an increasing order and show '*' for the offlined ones.
> 

Explain this in more detail please?

P.