Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752052AbcCYJNX (ORCPT ); Fri, 25 Mar 2016 05:13:23 -0400 Received: from mail-wm0-f43.google.com ([74.125.82.43]:35159 "EHLO mail-wm0-f43.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750797AbcCYJNU (ORCPT ); Fri, 25 Mar 2016 05:13:20 -0400 Message-ID: <1458897196.3870.8.camel@gmail.com> Subject: Re: [PATCH RT 4/6] rt/locking: Reenable migration accross schedule From: Mike Galbraith To: Thomas Gleixner Cc: Sebastian Andrzej Siewior , linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org, Steven Rostedt Date: Fri, 25 Mar 2016 10:13:16 +0100 In-Reply-To: References: <1455318168-7125-1-git-send-email-bigeasy@linutronix.de> <1455318168-7125-4-git-send-email-bigeasy@linutronix.de> <1458463425.3908.5.camel@gmail.com> <1458814024.23732.35.camel@gmail.com> <1458817594.3972.16.camel@gmail.com> <1458884330.3836.21.camel@gmail.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.16.5 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 7009 Lines: 158 On Fri, 2016-03-25 at 09:52 +0100, Thomas Gleixner wrote: > On Fri, 25 Mar 2016, Mike Galbraith wrote: > > On Thu, 2016-03-24 at 12:06 +0100, Mike Galbraith wrote: > > > On Thu, 2016-03-24 at 11:44 +0100, Thomas Gleixner wrote: > > > > > > > > > On the bright side, with the busted migrate enable business reverted, > > > > > plus one dinky change from me [1], master-rt.today has completed 100 > > > > > iterations of Steven's hotplug stress script along side endless > > > > > futexstress, and is happily doing another 900 as I write this, so the > > > > > next -rt should finally be hotplug deadlock free. > > > > > > > > > > Thomas's state machinery seems to work wonders. 'course this being > > > > > hotplug, the other shoe will likely apply itself to my backside soon. > > > > > > > > That's a given :) > > > > > > blk-mq applied it shortly after I was satisfied enough to poke xmit. > > > > The other shoe is that notifiers can depend upon RCU grace periods, so > > when pin_current_cpu() snags rcu_sched, the hotplug game is over. > > > > blk_mq_queue_reinit_notify: > > /* > > * We need to freeze and reinit all existing queues. Freezing > > * involves synchronous wait for an RCU grace period and doing it > > * one by one may take a long time. Start freezing all queues in > > * one swoop and then wait for the completions so that freezing can > > * take place in parallel. > > */ > > list_for_each_entry(q, &all_q_list, all_q_node) > > blk_mq_freeze_queue_start(q); > > list_for_each_entry(q, &all_q_list, all_q_node) { > > blk_mq_freeze_queue_wait(q); > > Yeah, I stumbled over that already when analysing all the hotplug notifier > sites. That's definitely a horrible one. > > > Hohum (sharpens rock), next. > > /me recommends frozen sharks With the sharp rock below and the one I'll follow up with, master-rt on my DL980 just passed 3 hours of endless hotplug stress concurrent with endless tbench 8, stockfish and futextest. It has never survived this long with this load by a long shot. hotplug/rt: Do not let pin_current_cpu() block RCU grace periods Notifiers may depend upon grace periods continuing to advance as blk_mq_queue_reinit_notify() below. crash> bt ffff8803aee76400 PID: 1113 TASK: ffff8803aee76400 CPU: 0 COMMAND: "stress-cpu-hotp" #0 [ffff880396fe7ad8] __schedule at ffffffff816b7142 #1 [ffff880396fe7b28] schedule at ffffffff816b797b #2 [ffff880396fe7b48] blk_mq_freeze_queue_wait at ffffffff8135c5ac #3 [ffff880396fe7b80] blk_mq_queue_reinit_notify at ffffffff8135f819 #4 [ffff880396fe7b98] notifier_call_chain at ffffffff8109b8ed #5 [ffff880396fe7bd8] __raw_notifier_call_chain at ffffffff8109b91e #6 [ffff880396fe7be8] __cpu_notify at ffffffff81072825 #7 [ffff880396fe7bf8] cpu_notify_nofail at ffffffff81072b15 #8 [ffff880396fe7c08] notify_dead at ffffffff81072d06 #9 [ffff880396fe7c38] cpuhp_invoke_callback at ffffffff81073718 #10 [ffff880396fe7c78] cpuhp_down_callbacks at ffffffff81073a70 #11 [ffff880396fe7cb8] _cpu_down at ffffffff816afc71 #12 [ffff880396fe7d38] do_cpu_down at ffffffff8107435c #13 [ffff880396fe7d60] cpu_down at ffffffff81074390 #14 [ffff880396fe7d70] cpu_subsys_offline at ffffffff814cd854 #15 [ffff880396fe7d80] device_offline at ffffffff814c7cda #16 [ffff880396fe7da8] online_store at ffffffff814c7dd0 #17 [ffff880396fe7dd0] dev_attr_store at ffffffff814c4fc8 #18 [ffff880396fe7de0] sysfs_kf_write at ffffffff812cfbe4 #19 [ffff880396fe7e08] kernfs_fop_write at ffffffff812cf172 #20 [ffff880396fe7e50] __vfs_write at ffffffff81241428 #21 [ffff880396fe7ed0] vfs_write at ffffffff81242535 #22 [ffff880396fe7f10] sys_write at ffffffff812438f9 #23 [ffff880396fe7f50] entry_SYSCALL_64_fastpath at ffffffff816bb4bc RIP: 00007fafd918acd0 RSP: 00007ffd2ca956e8 RFLAGS: 00000246 RAX: ffffffffffffffda RBX: 000000000226a770 RCX: 00007fafd918acd0 RDX: 0000000000000002 RSI: 00007fafd9cb9000 RDI: 0000000000000001 RBP: 00007ffd2ca95700 R8: 000000000000000a R9: 00007fafd9cb3700 R10: 00000000ffffffff R11: 0000000000000246 R12: 0000000000000007 R13: 0000000000000001 R14: 0000000000000009 R15: 000000000000000a ORIG_RAX: 0000000000000001 CS: 0033 SS: 002b blk_mq_queue_reinit_notify: /* * We need to freeze and reinit all existing queues. Freezing * involves synchronous wait for an RCU grace period and doing it * one by one may take a long time. Start freezing all queues in * one swoop and then wait for the completions so that freezing can * take place in parallel. */ list_for_each_entry(q, &all_q_list, all_q_node) blk_mq_freeze_queue_start(q); list_for_each_entry(q, &all_q_list, all_q_node) { blk_mq_freeze_queue_wait(q); crash> bt ffff880176cc9900 PID: 17 TASK: ffff880176cc9900 CPU: 0 COMMAND: "rcu_sched" #0 [ffff880176cd7ab8] __schedule at ffffffff816b7142 #1 [ffff880176cd7b08] schedule at ffffffff816b797b #2 [ffff880176cd7b28] rt_spin_lock_slowlock at ffffffff816b974d #3 [ffff880176cd7bc8] rt_spin_lock_fastlock at ffffffff811b0f3c #4 [ffff880176cd7be8] rt_spin_lock__no_mg at ffffffff816bac1b #5 [ffff880176cd7c08] pin_current_cpu at ffffffff8107406a #6 [ffff880176cd7c50] migrate_disable at ffffffff810a0e9e #7 [ffff880176cd7c70] rt_spin_lock at ffffffff816bad69 #8 [ffff880176cd7c90] lock_timer_base at ffffffff810fc5e8 #9 [ffff880176cd7cc8] try_to_del_timer_sync at ffffffff810fe290 #10 [ffff880176cd7cf0] del_timer_sync at ffffffff810fe381 #11 [ffff880176cd7d58] schedule_timeout at ffffffff816b9e4b #12 [ffff880176cd7df0] rcu_gp_kthread at ffffffff810f52b4 #13 [ffff880176cd7e70] kthread at ffffffff8109a02f #14 [ffff880176cd7f50] ret_from_fork at ffffffff816bb6f2 Game Over. Signed-off-by: Mike Galbraith --- include/linux/sched.h | 1 + kernel/cpu.c | 2 +- kernel/rcu/tree.c | 3 +++ 3 files changed, 5 insertions(+), 1 deletion(-) --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1492,6 +1492,7 @@ struct task_struct { #ifdef CONFIG_COMPAT_BRK unsigned brk_randomized:1; #endif + unsigned sched_is_rcu:1; /* RT: is a critical RCU thread */ unsigned long atomic_flags; /* Flags needing atomic access. */ --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -156,7 +156,7 @@ void pin_current_cpu(void) hp = this_cpu_ptr(&hotplug_pcp); if (!hp->unplug || hp->refcount || force || preempt_count() > 1 || - hp->unplug == current) { + hp->unplug == current || current->sched_is_rcu) { hp->refcount++; return; } --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -2100,6 +2100,9 @@ static int __noreturn rcu_gp_kthread(voi struct rcu_state *rsp = arg; struct rcu_node *rnp = rcu_get_root(rsp); + /* RT: pin_current_cpu() MUST NOT block RCU grace periods. */ + current->sched_is_rcu = 1; + rcu_bind_gp_kthread(); for (;;) {