Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755479Ab2BSWo4 (ORCPT ); Sun, 19 Feb 2012 17:44:56 -0500 Received: from terminus.zytor.com ([198.137.202.10]:56676 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755301Ab2BSWoz (ORCPT ); Sun, 19 Feb 2012 17:44:55 -0500 Message-ID: <4F417B4F.3040406@zytor.com> Date: Sun, 19 Feb 2012 14:44:31 -0800 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0) Gecko/20120131 Thunderbird/10.0 MIME-Version: 1.0 To: Linus Torvalds CC: Thomas Gleixner , Ingo Molnar , x86@kernel.org, Linux Kernel Mailing List Subject: Re: [PATCH 2/2] i387: support lazy restore of FPU state References: In-Reply-To: X-Enigmail-Version: 1.3.5 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1760 Lines: 43 On 02/19/2012 02:37 PM, Linus Torvalds wrote: > > - on *every* task switch from task A, we write A->thread.fpu.last_cpu, > whether we owned the FPU or not. And we only write a real CPU number in > the case where we owned it, and the FPU save left the state untouched > in the FPU. > > - so when we switch into task A next time, comparing the current CPU > number with that 'last_cpu' field inarguably says "when I last switched > out, I really saved it on this CPU" > > That, together with verifying that the per-cpu "fpu_owner_task" matches > "task A", guarantees that the state is really valid. Because we will > clear (or set to another task) fpu_owner_task if it ever gets > switched to anything else. > > But somebody should really validate this. Think through all the > kernel_fpu_begin() etc cases. I think it looks pretty obvious, and it > really does seem to work and improve task switching, but... > I think your logic is correct but suboptimal. What would make more sense to me is that we write last_cpu when we *load* the state. After all, if you didn't load the state you couldn't have modified it. In kernel_fpu_begin, *if* we end up flushing the state, we should set last_cpu to -1 indicating that *no* CPU currently owns the state -- after all, even on this CPU we would now have to reload the state from memory. Does that make sense? -hpa -- H. Peter Anvin, Intel Open Source Technology Center I work for Intel. I don't speak on their behalf. -- 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/