From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753815AbaE1Stk (ORCPT ); Wed, 28 May 2014 14:49:40 -0400 Received: from cam-admin0.cambridge.arm.com ([217.140.96.50]:53824 "EHLO cam-admin0.cambridge.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751020AbaE1Stj (ORCPT ); Wed, 28 May 2014 14:49:39 -0400 Date: Wed, 28 May 2014 19:49:10 +0100 From: Will Deacon To: Kevin Hilman Cc: "larry.bassel@linaro.org" , Catalin Marinas , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , "linaro-kernel@lists.linaro.org" Subject: Re: [PATCH v5 2/2] arm64: enable context tracking Message-ID: <20140528184910.GC20523@arm.com> References: <1401130573-7443-1-git-send-email-larry.bassel@linaro.org> <1401130573-7443-3-git-send-email-larry.bassel@linaro.org> <20140528114438.GF15222@arm.com> <7hegzd99tw.fsf@paris.lan> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <7hegzd99tw.fsf@paris.lan> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 28, 2014 at 04:55:39PM +0100, Kevin Hilman wrote: > Hi Will, Hey Kevin, > Will Deacon writes: > > Apologies if we've discussed this before (it rings a bell), but why are we > > penalising the fast syscall path with this? Shouldn't TIF_NOHZ contribute to > > out _TIF_WORK_MASK, then we could do the tracking on the syscall slow path? > > I'll answer here since Larry inherited this design decision from me. > > I considered (and even implemented) forcing the slow syscall path > based on TIF_NOHZ but decided (perhaps wrongly) not to. I guess the > choice is between: > > - forcing the overhead of syscall tracing path on all > TIF_NOHZ processes > > - forcing the (much smaller) ct_user_exit overhead on all syscalls, > (including the fast syscall path) > > I had decided that the former was better, but as I write this, I'm > thinking that the NOHZ tasks should probably eat the extra overhead > since we expect their interactions with the kernel to be minimal anyways > (part of the goal of full NOHZ.) > > Ultimately, I'm OK with either way and have the other version ready. I was just going by the comment in kernel/context_tracking.c: * The context tracking uses the syscall slow path to implement its user-kernel * boundaries probes on syscalls. This way it doesn't impact the syscall fast * path on CPUs that don't do context tracking. which doesn't match what the current patch does. It also makes it sounds like context tracking is really a per-CPU thing, but I've never knowingly used it before. I think putting this on the slowpath is inline with the expectations in the core code. > > I think that would tidy up your mov into x19 too. > > That's correct. If we force the syscall_trace path, the ct_user_enter > wouldn't have to do any context save/restore. That would be nice. > > Also -- how do you track ret_from_fork in the child with these patches? > > Not sure I follow the question, but ret_from_fork calls > ret_to_user, which calls kernel_exit, which calls ct_user_enter. Sorry, I got myself in a muddle. I noticed that x19 is live in ret_from_fork so made a mental note to check that is ok (I think it is) but then concluded incorrectly that you don't trace there. Will