linux-kernel - Re: [PATCH 2/2] cpu hotplug, add CONFIG_PERMANENT_CPU

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Message-ID: <20160908074542.GY10138@twins.programming.kicks-ass.net>
Date:   Thu, 8 Sep 2016 09:45:42 +0200
From:   Peter Zijlstra <peterz@...radead.org>
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>,
        Len Brown <len.brown@...el.com>, Borislav Petkov <bp@...e.de>,
        Andi Kleen <ak@...ux.intel.com>, Jiri Olsa <jolsa@...hat.com>,
        Juergen Gross <jgross@...e.com>
Subject: Re: [PATCH 2/2] cpu hotplug, add CONFIG_PERMANENT_CPU_TOPOLOGY

On Wed, Sep 07, 2016 at 07:44:17AM -0400, Prarit Bhargava wrote:
> The information in /sys/devices/system/cpu/cpuX/topology
> directory is useful for userspace monitoring applications and in-tree
> utilities like cpupower & turbostat.
> 
> When down'ing a CPU the /sys/devices/system/cpu/cpuX/topology directory is
> removed during the CPU_DEAD hotplug callback in the kernel.  The problem
> with this model is that the CPU has not been physically removed and the
> data in the topology directory is still valid and the cpu's location is
> now lost to userspace.
> 
> This patch adds CONFIG_PERMANENT_CPU_TOPOLOGY, and is Y by default for
> x86, an N for all other arches.  When enabled the kernel is modified so
> that the topology directory is added to the core cpu sysfs files so that
> the topology directory exists while the CPU is physically present.  When
> disabled, the behavior of the current kernel is maintained (that is, the
> topology directory is removed on a soft down and added on an soft up).
> Adding CONFIG_PERMANENT_CPU_TOPOLOGY may require additional architecture
> so that the cpumask data the CPU's topology is not cleared during a CPU
> down.

So how do you physically remove things and how does the information then
get removed?

Also, why is that an x86 only feature?