From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1758109AbcIHHqD (ORCPT <rfc822;w@1wt.eu>);
        Thu, 8 Sep 2016 03:46:03 -0400
Received: from bombadil.infradead.org ([198.137.202.9]:45528 "EHLO
        bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1758066AbcIHHpt (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Thu, 8 Sep 2016 03:45:49 -0400
Date: Thu, 8 Sep 2016 09:45:42 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Prarit Bhargava <prarit@redhat.com>
Cc: linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
        x86@kernel.org, Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Len Brown <len.brown@intel.com>, Borislav Petkov <bp@suse.de>,
        Andi Kleen <ak@linux.intel.com>, Jiri Olsa <jolsa@redhat.com>,
        Juergen Gross <jgross@suse.com>
Subject: Re: [PATCH 2/2] cpu hotplug, add CONFIG_PERMANENT_CPU_TOPOLOGY
Message-ID: <20160908074542.GY10138@twins.programming.kicks-ass.net>
References: <1473248657-17359-1-git-send-email-prarit@redhat.com>
 <1473248657-17359-3-git-send-email-prarit@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1473248657-17359-3-git-send-email-prarit@redhat.com>
User-Agent: Mutt/1.5.23.1 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

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?