From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751360AbWFUKDy (ORCPT ); Wed, 21 Jun 2006 06:03:54 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751445AbWFUKDy (ORCPT ); Wed, 21 Jun 2006 06:03:54 -0400 Received: from nf-out-0910.google.com ([64.233.182.189]:48306 "EHLO nf-out-0910.google.com") by vger.kernel.org with ESMTP id S1751360AbWFUKDx (ORCPT ); Wed, 21 Jun 2006 06:03:53 -0400 DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=beta; d=googlemail.com; h=received:date:x-x-sender:to:cc:subject:in-reply-to:message-id:references:mime-version:content-type:from; b=CuX2MRP/f9TJxzdMafd5m6zewlz5lvP1v7+f/C6K7qaUbm9YWsN43zL1AorLsgisHiQdDe5+Vt8IJ/X0Xov1Tlch+sniX3v2MF3bduN08W/B0wkXUGusZAvr6PI+2N67Plf2fVw46rig9McRHCz2KucOChvUZP0YQ7fZ1yKyPpc= Date: Wed, 21 Jun 2006 12:03:57 +0100 (BST) X-X-Sender: simlo@localhost.localdomain To: Steven Rostedt cc: Esben Nielsen , Thomas Gleixner , Ingo Molnar , linux-kernel@vger.kernel.org Subject: Re: Why can't I set the priority of softirq-hrt? (Re: 2.6.17-rt1) In-Reply-To: Message-ID: References: <20060618070641.GA6759@elte.hu> <1150816429.6780.222.camel@localhost.localdomain> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed From: Esben Nielsen Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 21 Jun 2006, Steven Rostedt wrote: > [...] > No, I think fixing the bug you found is better :) > Thomas convinced me. I don't like the design though. Moving the task priorities around isn't very healthy. It is confusing and it complicates stuff a lot. But as Thomas says, with my solution one could make DOS attack from userspace by arranging a lot of timers to timeout simultaniously (or sufficient close to each other). However, this is still possible, but less easy to do: In the interrupt every timer expirering is touched. I.e. by having a lot of timers you push a lot of work into the interrupt routine. In fact the interrupt routine also have to take the timer out of the binary tree, so the worst case interrupt latency is O(N log N), N being the total amount of timer in the system!! But what about the configure option HIGH_RES_RESOLUTION ? It is barely used in the system now. But if timers closer than the resolution were batched together in a plist, then all the interrupt should do was to merge two plists, namely the list of newly timeouts and the list of stuff still pending to be executed by the softirq thread. The worst case would then be max(O(log N), O(), which is both fairly limited as N can never be bigger than 1G on a 32 bit system such that log N <= 30. Esben > -- Steve >