Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752330AbbBSESa (ORCPT ); Wed, 18 Feb 2015 23:18:30 -0500 Received: from mx1.redhat.com ([209.132.183.28]:57360 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751942AbbBSES3 (ORCPT ); Wed, 18 Feb 2015 23:18:29 -0500 Date: Wed, 18 Feb 2015 22:17:53 -0600 From: Josh Poimboeuf To: Peter Zijlstra Cc: Andrew Morton , Ingo Molnar , Jiri Kosina , Seth Jennings , linux-kernel@vger.kernel.org, Vojtech Pavlik Subject: Re: [PATCH 1/3] sched: add sched_task_call() Message-ID: <20150219041753.GA13423@treble.redhat.com> References: <20150216204436.GH5029@twins.programming.kicks-ass.net> <20150216220505.GB11861@treble.redhat.com> <20150217092450.GI5029@twins.programming.kicks-ass.net> <20150217141211.GC11861@treble.redhat.com> <20150217181541.GP5029@twins.programming.kicks-ass.net> <20150217212532.GJ11861@treble.redhat.com> <20150218152100.GZ5029@twins.programming.kicks-ass.net> <20150218171256.GA28553@treble.hsd1.ky.comcast.net> <20150219002058.GD5029@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20150219002058.GD5029@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.23.1-rc1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2875 Lines: 80 On Thu, Feb 19, 2015 at 01:20:58AM +0100, Peter Zijlstra wrote: > On Wed, Feb 18, 2015 at 11:12:56AM -0600, Josh Poimboeuf wrote: > > > So uhm, what happens if your target task is running? When will you > > > retry? The problem I see is that if you do a sample approach you might > > > never hit an opportune moment. > > > > We attack it from multiple angles. > > > > First we check the stack of all sleeping tasks. That patches the > > majority of tasks immediately. If necessary, we also do that > > periodically in a workqueue to catch any stragglers. > > So not only do you need an 'atomic' stack save, you need to analyze and > flip its state in the same atomic region. The task cannot start running > again after the save and start using old functions while you analyze the > stack and flip it. Yes, exactly. > > The next line of attack is patching tasks when exiting the kernel to > > user space (system calls, interrupts, signals), to catch all CPU-bound > > and some I/O-bound tasks. That's done in patch 9 [1] of the consistency > > model patch set. > > So the HPC people are really into userspace that does for (;;) ; and > isolate that on CPUs and have the tick interrupt stopped and all that. > > You'll not catch those threads on the sysexit path. > > And I'm fairly sure they'll not want to SIGSTOP/CONT their stuff either. > > Now its fairly easy to also handle this; just mark those tasks with a > _TIF_WORK_SYSCALL_ENTRY flag, have that slowpath wait for the flag to > go-away, then flip their state and clear the flag. I guess you mean patch the task when it makes a syscall? I'm doing that already on syscall exit with a bit in _TIF_ALLWORK_MASK and _TIF_DO_NOTIFY_MASK. > > As a last resort, if there are still any tasks which are sleeping on a > > to-be-patched function, the user can send them SIGSTOP and SIGCONT to > > force them to be patched. > > You typically cannot SIGSTOP/SIGCONT kernel threads. Also > TASK_UNINTERRUPTIBLE sleeps are unaffected by signals. > > Bit pesky that.. needs pondering. I did have a scheme for patching kthreads which are sleeping on to-be-patched functions. But now I'm thinking that kthreads will almost never be a problem. Most kthreads are basically this: void thread_fn() { while (1) { /* do some stuff */ schedule(); /* do other stuff */ } } So a kthread would typically only fail the stack check if we're trying to patch either schedule() or the top-level thread_fn. Patching thread_fn wouldn't be possible unless we killed the thread. And I'd guess we can probably live without being able to patch schedule() for now :-) -- Josh -- 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/