Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753295AbbBSUkp (ORCPT ); Thu, 19 Feb 2015 15:40:45 -0500 Received: from jablonecka.jablonka.cz ([91.219.244.36]:59412 "EHLO jablonecka.jablonka.cz" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752134AbbBSUkn (ORCPT ); Thu, 19 Feb 2015 15:40:43 -0500 Date: Thu, 19 Feb 2015 21:40:36 +0100 From: Vojtech Pavlik To: Josh Poimboeuf Cc: Peter Zijlstra , Andrew Morton , Ingo Molnar , Jiri Kosina , Seth Jennings , linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/3] sched: add sched_task_call() Message-ID: <20150219204036.GA16882@suse.com> References: <20150218152100.GZ5029@twins.programming.kicks-ass.net> <20150218171256.GA28553@treble.hsd1.ky.comcast.net> <20150219002058.GD5029@twins.programming.kicks-ass.net> <20150219041753.GA13423@treble.redhat.com> <20150219101607.GG5029@twins.programming.kicks-ass.net> <20150219162429.GA15980@treble.redhat.com> <20150219163359.GA25438@suse.cz> <20150219170353.GB15980@treble.redhat.com> <20150219171929.GA13178@suse.cz> <20150219173255.GC15980@treble.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150219173255.GC15980@treble.redhat.com> X-Bounce-Cookie: It's a lemon tree, dear Watson! User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2264 Lines: 53 On Thu, Feb 19, 2015 at 11:32:55AM -0600, Josh Poimboeuf wrote: > On Thu, Feb 19, 2015 at 06:19:29PM +0100, Vojtech Pavlik wrote: > > On Thu, Feb 19, 2015 at 11:03:53AM -0600, Josh Poimboeuf wrote: > > > On Thu, Feb 19, 2015 at 05:33:59PM +0100, Vojtech Pavlik wrote: > > > > On Thu, Feb 19, 2015 at 10:24:29AM -0600, Josh Poimboeuf wrote: > > > > > > > > > > No, these tasks will _never_ make syscalls. So you need to guarantee > > > > > > they don't accidentally enter the kernel while you flip them. Something > > > > > > like so should do. > > > > > > > > > > > > You set TIF_ENTER_WAIT on them, check they're still in userspace, flip > > > > > > them then clear TIF_ENTER_WAIT. > > > > > > > > > > Ah, that's a good idea. But how do we check if they're in user space? > > > > > > > > I don't see the benefit in holding them in a loop - you can just as well > > > > flip them from the syscall code as kGraft does. > > > > > > But we were talking specifically about HPC tasks which never make > > > syscalls. > > > > Yes. I'm saying that rather than guaranteeing they don't enter the > > kernel (by having them spin) you can flip them in case they try to do > > that instead. That solves the race condition just as well. > > Ok, gotcha. > > We'd still need a safe way to check if they're in user space though. Having a safe way would be very nice and actually quite useful in other cases, too. For this specific purpose, however, we don't need a very safe way, though. We don't require atomicity in any way, we don't mind even if it creates false negatives, only false positives would be bad. kGraft looks at the stacktrace of CPU hogs and if it finds no kernel addresses there, it assumes userspace. Not very nice, but does the job. > How about with a TIF_IN_USERSPACE thread flag? It could be cleared/set > right at the border. Then for running tasks it's as simple as: > > if (test_tsk_thread_flag(task, TIF_IN_USERSPACE)) > klp_switch_task_universe(task); -- Vojtech Pavlik Director SUSE Labs -- 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/