powerpc/watchdog: Do not backtrace locked CPUs twice if allcpus backtrace is enabled

If sysctl_hardlockup_all_cpu_backtrace is enabled, there is no need to
IPI stuck CPUs for backtrace before trigger_allbutself_cpu_backtrace(),
which does the same thing again.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/watchdog.c

@@ -133,8 +133,10 @@ static void watchdog_smp_panic(int cpu, u64 tb)
 	pr_emerg("Watchdog CPU:%d detected Hard LOCKUP other CPUS:%*pbl\n",
 			cpu, cpumask_pr_args(&wd_smp_cpus_pending));
 
+	if (!sysctl_hardlockup_all_cpu_backtrace) {
 		/*
-	 * Try to trigger the stuck CPUs.
+		 * Try to trigger the stuck CPUs, unless we are going to
+		 * get a backtrace on all of them anyway.
 		 */
 		for_each_cpu(c, &wd_smp_cpus_pending) {
 			if (c == cpu)
@@ -142,6 +144,7 @@ static void watchdog_smp_panic(int cpu, u64 tb)
 			smp_send_nmi_ipi(c, wd_lockup_ipi, 1000000);
 		}
 		smp_flush_nmi_ipi(1000000);
+	}
 
 	/* Take the stuck CPUs out of the watch group */
 	set_cpumask_stuck(&wd_smp_cpus_pending, tb);