From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752137AbdBEKSv (ORCPT ); Sun, 5 Feb 2017 05:18:51 -0500 Received: from mx2.suse.de ([195.135.220.15]:41077 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751774AbdBEKSu (ORCPT ); Sun, 5 Feb 2017 05:18:50 -0500 Date: Sun, 5 Feb 2017 11:18:45 +0100 From: Michal Hocko To: Hoeun Ryu Cc: Andy Lutomirski , Andrew Morton , Ingo Molnar , Andy Lutomirski , Kees Cook , "Eric W. Biederman" , Mateusz Guzik , "linux-kernel@vger.kernel.org" , "kernel-hardening@lists.openwall.com" Subject: Re: [PATCH 1/3] fork: dynamically allocate cache array for vmapped stacks using cpuhp Message-ID: <20170205101844.GE22713@dhcp22.suse.cz> References: <1486135892-27249-1-git-send-email-hoeun.ryu@gmail.com> <20170203153914.GI19325@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.6.0 (2016-04-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat 04-02-17 11:01:32, Hoeun Ryu wrote: > On Sat, Feb 4, 2017 at 2:52 AM, Andy Lutomirski wrote: > > On Fri, Feb 3, 2017 at 8:42 AM, Hoeun Ryu wrote: > >> On Sat, Feb 4, 2017 at 12:39 AM, Michal Hocko wrote: > >>> On Sat 04-02-17 00:30:05, Hoeun Ryu wrote: > >>>> Using virtually mapped stack, kernel stacks are allocated via vmalloc. > >>>> In the current implementation, two stacks per cpu can be cached when > >>>> tasks are freed and the cached stacks are used again in task duplications. > >>>> but the array for the cached stacks is statically allocated by per-cpu api. > >>>> In this new implementation, the array for the cached stacks are dynamically > >>>> allocted and freed by cpu hotplug callbacks and the cached stacks are freed > >>>> when cpu is down. setup for cpu hotplug is established in fork_init(). > >>> > >>> Why do we want this? I can see that the follow up patch makes the number > >>> configurable but the changelog doesn't describe the motivation for that. > >>> Which workload would benefit from a higher value? > >>> > >> > >> The key difference of this implementation, the cached stacks for a cpu > >> is freed when a cpu is down. > >> so the cached stacks are no longer wasted. > >> In the current implementation, the cached stacks for a cpu still > >> remain on the system when a cpu is down. > >> I think we could imagine what if a machine has many cpus and someone > >> wants to have bigger size of stack caches. > > > > Then how about just registering a simple hotplug hook to free the > > stacks without worrying about freeing the tiny array as well? > > > > Michal, What do you think about it. it sounds fair enough. This is what I've tried to suggest in the other reply. -- Michal Hocko SUSE Labs