sched: Stop resched_cpu() from sending IPIs to offline CPUs

The rcutorture test suite occasionally provokes a splat due to invoking
resched_cpu() on an offline CPU:

WARNING: CPU: 2 PID: 8 at /home/paulmck/public_git/linux-rcu/arch/x86/kernel/smp.c:128 native_smp_send_reschedule+0x37/0x40
Modules linked in:
CPU: 2 PID: 8 Comm: rcu_preempt Not tainted 4.14.0-rc4+ #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014
task: ffff902ede9daf00 task.stack: ffff96c50010c000
RIP: 0010:native_smp_send_reschedule+0x37/0x40
RSP: 0018:ffff96c50010fdb8 EFLAGS: 00010096
RAX: 000000000000002e RBX: ffff902edaab4680 RCX: 0000000000000003
RDX: 0000000080000003 RSI: 0000000000000000 RDI: 00000000ffffffff
RBP: ffff96c50010fdb8 R08: 0000000000000000 R09: 0000000000000001
R10: 0000000000000000 R11: 00000000299f36ae R12: 0000000000000001
R13: ffffffff9de64240 R14: 0000000000000001 R15: ffffffff9de64240
FS:  0000000000000000(0000) GS:ffff902edfc80000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000f7d4c642 CR3: 000000001e0e2000 CR4: 00000000000006e0
Call Trace:
 resched_curr+0x8f/0x1c0
 resched_cpu+0x2c/0x40
 rcu_implicit_dynticks_qs+0x152/0x220
 force_qs_rnp+0x147/0x1d0
 ? sync_rcu_exp_select_cpus+0x450/0x450
 rcu_gp_kthread+0x5a9/0x950
 kthread+0x142/0x180
 ? force_qs_rnp+0x1d0/0x1d0
 ? kthread_create_on_node+0x40/0x40
 ret_from_fork+0x27/0x40
Code: 14 01 0f 92 c0 84 c0 74 14 48 8b 05 14 4f f4 00 be fd 00 00 00 ff 90 a0 00 00 00 5d c3 89 fe 48 c7 c7 38 89 ca 9d e8 e5 56 08 00 <0f> ff 5d c3 0f 1f 44 00 00 8b 05 52 9e 37 02 85 c0 75 38 55 48
---[ end trace 26df9e5df4bba4ac ]---

This splat cannot be generated by expedited grace periods because they
always invoke resched_cpu() on the current CPU, which is good because
expedited grace periods require that resched_cpu() unconditionally
succeed.  However, other parts of RCU can tolerate resched_cpu() acting
as a no-op, at least as long as it doesn't happen too often.

This commit therefore makes resched_cpu() invoke resched_curr() only if
the CPU is either online or is the current CPU.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>

kernel/sched/core.c

@@ -508,7 +508,8 @@ void resched_cpu(int cpu)
 	unsigned long flags;
 
 	raw_spin_lock_irqsave(&rq->lock, flags);
-	resched_curr(rq);
+	if (cpu_online(cpu) || cpu == smp_processor_id())
+		resched_curr(rq);
 	raw_spin_unlock_irqrestore(&rq->lock, flags);
 }