Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761138Ab3DDNrX (ORCPT ); Thu, 4 Apr 2013 09:47:23 -0400 Received: from mail-lb0-f182.google.com ([209.85.217.182]:39687 "EHLO mail-lb0-f182.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1761095Ab3DDNrV (ORCPT ); Thu, 4 Apr 2013 09:47:21 -0400 MIME-Version: 1.0 In-Reply-To: <20130404131003.GE1367@redhat.com> References: <1365066635-2959-1-git-send-email-sgruszka@redhat.com> <20130404131003.GE1367@redhat.com> Date: Thu, 4 Apr 2013 15:47:19 +0200 Message-ID: Subject: Re: [PATCH -tip 0/4] do not make cputime scaling in kernel From: Frederic Weisbecker To: Stanislaw Gruszka Cc: Ingo Molnar , Peter Zijlstra , "H. Peter Anvin" , Steven Rostedt , Andrew Morton , Thomas Gleixner , Linus Torvalds , LKML , Paul Turner Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2631 Lines: 54 2013/4/4 Stanislaw Gruszka : > On Thu, Apr 04, 2013 at 02:31:42PM +0200, Frederic Weisbecker wrote: >> I don't know. I'm not convinced userland is the right place to perform >> this kind of check. The kernel perhaps doesn't give guarantee about >> utime/stime precision but now users may have got used to that scaled >> behaviour. It's also a matter of security, a malicous app can hide >> from the tick to make its activity less visible from tools like top. >> >> It's sortof an ABI breakage to remove such an implicit protection. And >> fixing that from userspace with a lib or so won't change that fact. > > I think number of fields in /proc/PID/stat is not part of ABI. For > example commit 5b172087f99189416d5f47fd7ab5e6fb762a9ba3 add various > new fields at the end of the file. What is imported to keep unchanged > ABI is not changing order or meaning of fields we already have. Oh I wasn't considering the layout of the proc file but the semantic change in its utime/stime fields. > Regarding top, I added those additional fields to allow top to detect > those malicious software. Patched top will work well with old and new > (patched) kernel. Problem is old top with new kernel, but I believe > users who care about security update they software regularly. The usual rule is that but you can't remove a feature from the kernel and tell userspace to fix it itself. That's basically an ABI breakage. A semantic one. We do it sometimes for some cases. But the more we are dealing with a central ABI component, the harder it is to change it. And here it is quite a central component. > > Besides for most cases (not counting hostile software), those > statistical stime/utime accounting give good approximation of CPU > time utilizing by each process. Yeah but still the users are expecting that result to be scaled now. > >> How about that 128bits based idea? I'm adding Paul Turner in Cc >> because he seemed to agree with doing it using 128bits maths. > > For problem that I try to solve 128bits math is not necessary, assuming > we can do multiplication in user space. Taking into account how easily > things can be done in user space using floating point math, I prefer not > to add complexity in kernel. This solution make kernel simpler and > faster. I'm always all for making the kernel simpler and faster, as long as we don't break userspace. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/