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Date: Mon, 22 Aug 2016 17:45:26 -0700 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Wanpeng Li <kernellwp@...il.com> Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...nel.org>, Lai Jiangshan <jiangshanlai@...il.com>, dipankar@...ibm.com, Andrew Morton <akpm@...ux-foundation.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Josh Triplett <josh@...htriplett.org>, Thomas Gleixner <tglx@...utronix.de>, Peter Zijlstra <peterz@...radead.org>, Steven Rostedt <rostedt@...dmis.org>, dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com, Frédéric Weisbecker <fweisbec@...il.com>, oleg@...hat.com, bobby.prani@...il.com Subject: Re: [PATCH tip/core/rcu 3/5] sched: Make wake_up_nohz_cpu() handle CPUs going offline On Tue, Aug 23, 2016 at 06:57:20AM +0800, Wanpeng Li wrote: > 2016-08-22 23:30 GMT+08:00 Paul E. McKenney <paulmck@...ux.vnet.ibm.com>: > > Both timers and hrtimers are maintained on the outgoing CPU until > > CPU_DEAD time, at which point they are migrated to a surviving CPU. If a > > mod_timer() executes between CPU_DYING and CPU_DEAD time, x86 systems > > will splat in native_smp_send_reschedule() when attempting to wake up > > the just-now-offlined CPU, as shown below from a NO_HZ_FULL kernel: > > > > [ 7976.741556] WARNING: CPU: 0 PID: 661 at /home/paulmck/public_git/linux-rcu/arch/x86/kernel/smp.c:125 native_smp_send_reschedule+0x39/0x40 > > [ 7976.741595] Modules linked in: > > [ 7976.741595] CPU: 0 PID: 661 Comm: rcu_torture_rea Not tainted 4.7.0-rc2+ #1 > > [ 7976.741595] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011 > > [ 7976.741595] 0000000000000000 ffff88000002fcc8 ffffffff8138ab2e 0000000000000000 > > [ 7976.741595] 0000000000000000 ffff88000002fd08 ffffffff8105cabc 0000007d1fd0ee18 > > [ 7976.741595] 0000000000000001 ffff88001fd16d40 ffff88001fd0ee00 ffff88001fd0ee00 > > [ 7976.741595] Call Trace: > > [ 7976.741595] [<ffffffff8138ab2e>] dump_stack+0x67/0x99 > > [ 7976.741595] [<ffffffff8105cabc>] __warn+0xcc/0xf0 > > [ 7976.741595] [<ffffffff8105cb98>] warn_slowpath_null+0x18/0x20 > > [ 7976.741595] [<ffffffff8103cba9>] native_smp_send_reschedule+0x39/0x40 > > [ 7976.741595] [<ffffffff81089bc2>] wake_up_nohz_cpu+0x82/0x190 > > [ 7976.741595] [<ffffffff810d275a>] internal_add_timer+0x7a/0x80 > > [ 7976.741595] [<ffffffff810d3ee7>] mod_timer+0x187/0x2b0 > > [ 7976.741595] [<ffffffff810c89dd>] rcu_torture_reader+0x33d/0x380 > > [ 7976.741595] [<ffffffff810c66f0>] ? sched_torture_read_unlock+0x30/0x30 > > [ 7976.741595] [<ffffffff810c86a0>] ? rcu_bh_torture_read_lock+0x80/0x80 > > [ 7976.741595] [<ffffffff8108068f>] kthread+0xdf/0x100 > > [ 7976.741595] [<ffffffff819dd83f>] ret_from_fork+0x1f/0x40 > > [ 7976.741595] [<ffffffff810805b0>] ? kthread_create_on_node+0x200/0x200 > > > > However, in this case, the wakeup is redundant, because the timer > > migration will reprogram timer hardware as needed. Note that the fact > > that preemption is disabled does not avoid the splat, as the offline > > operation has already passed both the synchronize_sched() and the > > stop_machine() that would be blocked by disabled preemption. > > > > This commit therefore modifies wake_up_nohz_cpu() to avoid attempting > > to wake up offline CPUs. It also adds a comment stating that the > > caller must tolerate lost wakeups when the target CPU is going offline, > > and suggesting the CPU_DEAD notifier as a recovery mechanism. > > Interesting, I have a patch which posted several weeks ago fix another > similar issue, https://lkml.org/lkml/2016/8/4/143 Anyway, if my patch > also fixes your bug? I will see your several weeks and raise you more than a month: http://lkml.kernel.org/g/20160630175845.GA10269@linux.vnet.ibm.com So you try mine and then I will try yours. ;-) Especially given that I am not seeing how the code path in my trace above reaches your change in sched_can_stop_tick()... Thanx, Paul
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