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

@@ -133,15 +133,18 @@ 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));
 
-	/*
-	 * Try to trigger the stuck CPUs.
-	 */
-	for_each_cpu(c, &wd_smp_cpus_pending) {
-		if (c == cpu)
-			continue;
-		smp_send_nmi_ipi(c, wd_lockup_ipi, 1000000);
+	if (!sysctl_hardlockup_all_cpu_backtrace) {
+		/*
+		 * 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)
+				continue;
+			smp_send_nmi_ipi(c, wd_lockup_ipi, 1000000);
+		}
+		smp_flush_nmi_ipi(1000000);
 	}
-	smp_flush_nmi_ipi(1000000);
 
 	/* Take the stuck CPUs out of the watch group */
 	set_cpumask_stuck(&wd_smp_cpus_pending, tb);