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

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Message-ID: <20160926115728.sdnvjaz3ty6xpsvr@pd.tnic>
Date:   Mon, 26 Sep 2016 13:57:29 +0200
From:   Borislav Petkov <bp@...e.de>
To:     Prarit Bhargava <prarit@...hat.com>
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 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"?

$ echo 0 > /sys/devices/system/cpu/cpu2/online
$ cat /sys/devices/system/cpu/online
0-1,3-7

So core 2 is right between 1 and 3.

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.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--