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.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>