From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760738AbYDNORv (ORCPT ); Mon, 14 Apr 2008 10:17:51 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1759990AbYDNORd (ORCPT ); Mon, 14 Apr 2008 10:17:33 -0400 Received: from mailhub.sw.ru ([195.214.232.25]:3470 "EHLO relay.sw.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759272AbYDNORc (ORCPT ); Mon, 14 Apr 2008 10:17:32 -0400 Subject: Re: Does process need to have a kernel-side stack all the time? From: "Denis V. Lunev" To: Denys Vlasenko Cc: Ingo Molnar , linux-kernel@vger.kernel.org In-Reply-To: <200804141547.57719.vda.linux@googlemail.com> References: <200804141547.57719.vda.linux@googlemail.com> Content-Type: text/plain Organization: OpenVz kernel team Date: Mon, 14 Apr 2008 18:17:29 +0400 Message-Id: <1208182649.7688.7.camel@iris.sw.ru> Mime-Version: 1.0 X-Mailer: Evolution 2.12.3 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2008-04-14 at 15:47 +0200, Denys Vlasenko wrote: > Hi Ingo, > > You are one of the experts in processes/threads and scheduling > in Linux kernel, I hope you can answer this question. > > A lot of effort went into minimizing of stack usage. > If I understand it correctly, one of the reasons for this > was to be efficient and not have lots of pages > used for stacks when we have a lot of threads > (tens of thousands). > > A random thought occurred to me: in a system with so many > threads most of them are not executing anyway, even on > that gigantic Altix machines. Do they all need to have > kernel stack, all the time? I mean: the process which > is running in user space is not using kernel stack at all. > Process which is not running on a CPU right now > is not using it either. But they do still consume > at least 4k (or 8k on 64bits) of RAM. > > Process absolutely must have kernel stack only when > it is actively running in kernel code (not sleeping), > right? > > Can we have per-CPU kernel stacks instead, so that process > gets a kernel stack only every time it enters the kernel; > and make it so that the process which is scheduled away > from a CPU does not need to have kernel stack? > > Currently, when process sleeps, we save some > state in stack, and such a change may require > some substantial surgery. > > Can you tell me whether this is possible at all, > and how difficult you estimate it to be? I do not think that this can help. Usually, the process (thread) invokes some syscall like sleep and goes to the waiting state _from the kernel_, i.e. the kernel stack is used at that moment and will be required during wake up. Regards, Den