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 S1756426Ab3JJROA (ORCPT <rfc822;w@1wt.eu>);
	Thu, 10 Oct 2013 13:14:00 -0400
Received: from e38.co.us.ibm.com ([32.97.110.159]:58087 "EHLO
	e38.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1756154Ab3JJRN7 (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Thu, 10 Oct 2013 13:13:59 -0400
Date: Thu, 10 Oct 2013 10:13:53 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>, Mel Gorman <mgorman@suse.de>,
        Rik van Riel <riel@redhat.com>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2
Message-ID: <20131010171353.GM5790@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
References: <20131008102505.404025673@infradead.org>
 <20131009225006.7101379c.akpm@linux-foundation.org>
 <20131010121908.GB28601@twins.programming.kicks-ass.net>
 <20131010145738.GA5167@gmail.com>
 <20131010152134.GQ3081@twins.programming.kicks-ass.net>
 <20131010165045.GB12998@gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20131010165045.GB12998@gmail.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
X-TM-AS-MML: No
X-Content-Scanned: Fidelis XPS MAILER
x-cbid: 13101017-1344-0000-0000-000002560BEB
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Thu, Oct 10, 2013 at 06:50:46PM +0200, Ingo Molnar wrote:
> 
> * Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > On Thu, Oct 10, 2013 at 04:57:38PM +0200, Ingo Molnar wrote:
> >
> > > So ... why not make it _really_ cheap, i.e. the read lock costing 
> > > nothing, and tie CPU hotplug to freezing all tasks in the system?
> > 
> > Such that we freeze regular tasks in userspace and kernel tasks in their 
> > special freezer callback so as to guarantee minimal state?
> 
> Yes, absolutely.

That does add quite a bit of latency to the hotplug operations, which
IIRC slows down things like boot, suspend, and resume.

							Thanx, Paul