From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1030725AbWFOP4L (ORCPT ); Thu, 15 Jun 2006 11:56:11 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1030731AbWFOP4L (ORCPT ); Thu, 15 Jun 2006 11:56:11 -0400 Received: from mtagate3.uk.ibm.com ([195.212.29.136]:17080 "EHLO mtagate3.uk.ibm.com") by vger.kernel.org with ESMTP id S1030725AbWFOP4J (ORCPT ); Thu, 15 Jun 2006 11:56:09 -0400 Message-ID: <449182FB.6020907@watson.ibm.com> Date: Thu, 15 Jun 2006 11:55:39 -0400 From: Shailabh Nagar User-Agent: Debian Thunderbird 1.0.2 (X11/20051002) X-Accept-Language: en-us, en MIME-Version: 1.0 To: Jay Lan CC: Balbir Singh , LKML , Andrew Morton , Chris Sturtivant Subject: Re: ON/OFF control of taskstats accounting data at do_exit References: <449093D6.6000806@engr.sgi.com> <4490CDC2.3090009@watson.ibm.com> <4490D515.8070308@engr.sgi.com> In-Reply-To: <4490D515.8070308@engr.sgi.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Jay Lan wrote: > > I was talking about turning off system-wise taskstats data preparation and > delivery when every task exits. Sometimes customers like to do some > benchmarking and need to turn off nonessential features. Lets go through the implications of turning on/off collection, assembly and delivery of the per-task accounting data. Collection is defined by different subsystems using taskstats. For atleast one of these (delay accounting), turning on/off dynamically has been tried and deemed to cause a lot of overhead (due to accumalated nature of data) and also be prone to races. Complexity of code added did not justify the value so on/off was restricted to a boot time decision. Assembly and delivery of data is done by the taskstats code, calling subsystem-specific functions to fill the commonly used struct taskstats and relying on genetlink to do the delivery. This can be turned on/off using a dynamic parameter such as /sys/kernel/taskstats_enable which sets some internal variable that is used to do early exits from relevant functions (mainly taskstats_send_stats and taskstats_exit_send) Doing so will have impact on a) queries sent to the kernel by monitoring applications b) task exit data sent by kernel to apps listening on the multicast socket For consistency, I'm assuming both a) and b) will have to be affected when taskstats is turned off. Also, I'm assuming monitoring applications aren't aware of the turn off. What happens to case a) ? Apps will need to get some error message as a reply. Some assembly overhead is saved (since such an error reply can be sent right away as soon as a query command is seen) but no substantial saving on the delivery part. For case b), we can save on assembly and delivery by exiting early from taskstats_exit_send(). But won't we need to send some message (if not periodically, atleast once) to listening apps that they shouldn't expect any exit data ? Semantics of suddenly not seeing any exit data could be misinterpreted ? Its easy enough to implement...just concerned about the semantics of doing so (as far as userspace apps are concerned) and utility in general settings. Utility in case where only CSA is running (delay accounting and other users turned off) is clear. Thoughts ? --Shailabh