Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751418AbXBOAfr (ORCPT ); Wed, 14 Feb 2007 19:35:47 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751414AbXBOAfr (ORCPT ); Wed, 14 Feb 2007 19:35:47 -0500 Received: from x35.xmailserver.org ([64.71.152.41]:3874 "EHLO x35.xmailserver.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751418AbXBOAfq (ORCPT ); Wed, 14 Feb 2007 19:35:46 -0500 X-AuthUser: davidel@xmailserver.org Date: Wed, 14 Feb 2007 16:35:44 -0800 (PST) From: Davide Libenzi X-X-Sender: davide@alien.or.mcafeemobile.com To: Benjamin LaHaise cc: Russell King , Ingo Molnar , Linux Kernel Mailing List , Linus Torvalds , Arjan van de Ven , Christoph Hellwig , Andrew Morton , Alan Cox , Ulrich Drepper , Zach Brown , Evgeniy Polyakov , "David S. Miller" , Suparna Bhattacharya , Thomas Gleixner Subject: Re: [patch 06/11] syslets: core, documentation In-Reply-To: <20070214234030.GO32271@kvack.org> Message-ID: References: <20070214110419.GC4241@flint.arm.linux.org.uk> <20070214180344.GI32271@kvack.org> <20070214200347.GK32271@kvack.org> <20070214203438.GL32271@kvack.org> <20070214214409.GM32271@kvack.org> <20070214234030.GO32271@kvack.org> X-GPG-FINGRPRINT: CFAE 5BEE FD36 F65E E640 56FE 0974 BF23 270F 474E X-GPG-PUBLIC_KEY: http://www.xmailserver.org/davidel.asc MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1536 Lines: 35 On Wed, 14 Feb 2007, Benjamin LaHaise wrote: > On Wed, Feb 14, 2007 at 03:17:59PM -0800, Davide Libenzi wrote: > > > That's an incorrect assumption. Every task/thread in the system has FPU > > > state associated with it, in part due to the fact that glibc has to change > > > some of the rounding mode bits, making them different than the default from > > > a freshly initialized state. > > > > IMO I still belive this is not a huge problem. FPU state propagation/copy > > can be done in a clever way, once we detect the in-async condition. > > Show me. clts() and stts() are expensive hardware operations which there > is no means of avoiding as control register writes impact the CPU in a not > trivial manner. I've spent far too much time staring at profiles of what > goes on in the context switch code in the process of looking for optimizations > on this very issue to be ignored on this point. The trivial case is the cachehit case. Everything flows like usual since we don't swap threads. If we're going to sleep, __async_schedule has to save/copy (depending if TS_USEDFPU is set) the current FPU state to the newly selected service thread (return-to-userspace thread). When a fault eventually happen in the new userspace thread, context is restored. - Davide - 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/