From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1765503AbXGNWUU (ORCPT ); Sat, 14 Jul 2007 18:20:20 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1763987AbXGNWUH (ORCPT ); Sat, 14 Jul 2007 18:20:07 -0400 Received: from smtpq2.tilbu1.nb.home.nl ([213.51.146.201]:36636 "EHLO smtpq2.tilbu1.nb.home.nl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1762102AbXGNWUG (ORCPT ); Sat, 14 Jul 2007 18:20:06 -0400 Message-ID: <46994BE3.7010608@gmail.com> Date: Sun, 15 Jul 2007 00:19:15 +0200 From: Rene Herman User-Agent: Thunderbird 1.5.0.12 (X11/20070509) MIME-Version: 1.0 To: Matt Mackall CC: Jeremy Fitzhardinge , Jesper Juhl , Ray Lee , Linux Kernel Mailing List , William Lee Irwin III , David Chinner Subject: Re: [PATCH][RFC] 4K stacks default, not a debug thing any more...? References: <200707111916.35036.jesper.juhl@gmail.com> <2c0942db0707112159v3ee2cd83i74759c7138e273f7@mail.gmail.com> <9a8748490707121324q3b3e6e65ye14ab8e7f089d999@mail.gmail.com> <4696C89E.4010002@goop.org> <9a8748490707121925w5fb22c0o61068f06d66d5845@mail.gmail.com> <4696FC43.3000201@goop.org> <46977C36.8010403@gmail.com> <20070714191737.GA11166@waste.org> In-Reply-To: <20070714191737.GA11166@waste.org> Content-Type: text/plain; charset=ISO-8859-15; format=flowed Content-Transfer-Encoding: 7bit X-AtHome-MailScanner-Information: Please contact support@home.nl for more information X-AtHome-MailScanner: Found to be clean Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On 07/14/2007 09:17 PM, Matt Mackall wrote: > On Fri, Jul 13, 2007 at 03:20:54PM +0200, Rene Herman wrote: >> As far as I'm aware, the actual reason for 4K stacks is that after the >> system has been up and running for some time getting "1 physically >> contiguous pages" becomes significantly easier than 2 which wouldn't be >> arbitrary. > > If there are exactly two free pages in the system, the odds of them > being buddies (ie adjacent AND properly aligned) is quite small. The > available page pool has to grow quite a bit before the availability of > order-1 page pairs approaches 100%. > > So if we fail to allocate an 8k stack when we could have allocated a > 4k stack, we're almost certainly failing significantly prematurely. Quite. Ofcourse, saying "our stacks are 1 page" would be the by far easiest solution to that. Personally, I've been running with 4K stacks exclusively on a variety of machines for quite some time now, but I can't say I'm all too adventurous with respect to filesystems (especially) so I'm not sure how many problems remain with 4K stacks. I did recently see Andrew Morton say that problems _do_ still exist. If it's just XFS -- well, heck... Moreover though, rather than 4K, the issue is "single page" stacks meaning a larger (soft-) pagesize would seem to fix things nicely. I've been reading about that on this list off and on for some time -- no idea where that stands though. > As I've pointed out before, it's fairly easy to make our stack > growable with a trampoline in the troublesome paths. Something like: > > int growstack(int headroom, int func, void *data) > { > void *new_stack; > int ret; > > if (likely(available_stack() > headroom)) > return func(data); > > #ifdef CONFIG_GROWSTACK_STATS > /* gather statistics about stack-heavy paths */ > #endif > /* warn/abort if we're recursing too deeply */ > > new_stack = get_free_page(); > switch_to_new_stack(new_stack); > ret = func(data); > cleanup_stack(new_stack); > return ret; > } This would also need something to tell func() where its current_thread_info is now at. Which might not be much of a problem. Can't think of much else either but it's the kind of thing you'd _like_ to be a problem just to have an excuse to shoot down an icky notion like that... Would you intend this just as a "make this path work until we fix it properly" kind of thing? Rene.