[PATCH linux-next][RFC]torture: avoid offline tick_do_timer_cpu

[Zhouyi Zhou]

During CPU-hotplug torture (CONFIG_NO_HZ_FULL=y), if we try to
offline tick_do_timer_cpu, the operation will fail because in
function tick_nohz_cpu_down:
```
if (tick_nohz_full_running && tick_do_timer_cpu == cpu)
      return -EBUSY;
```
Above bug was first discovered in torture tests performed in PPC VM
of Open Source Lab of Oregon State University, and reproducable in RISC-V
and X86-64 (with additional kernel commandline cpu0_hotplug).

In this patch, we avoid offline tick_do_timer_cpu by distribute
the offlining cpu among remaining cpus.

Signed-off-by: Zhouyi Zhou <zhouzhouyi at gmail.com>
---
 include/linux/tick.h        |  1 +
 kernel/time/tick-common.c   |  1 +
 kernel/time/tick-internal.h |  1 -
 kernel/torture.c            | 10 ++++++++++
 4 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/include/linux/tick.h b/include/linux/tick.h
index bfd571f18cfd..23cc0b205853 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -14,6 +14,7 @@
 #include <linux/rcupdate.h>
 
 #ifdef CONFIG_GENERIC_CLOCKEVENTS
+extern int tick_do_timer_cpu __read_mostly;
 extern void __init tick_init(void);
 /* Should be core only, but ARM BL switcher requires it */
 extern void tick_suspend_local(void);
diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
index 46789356f856..87b9b9afa320 100644
--- a/kernel/time/tick-common.c
+++ b/kernel/time/tick-common.c
@@ -48,6 +48,7 @@ ktime_t tick_next_period;
  *    procedure also covers cpu hotplug.
  */
 int tick_do_timer_cpu __read_mostly = TICK_DO_TIMER_BOOT;
+EXPORT_SYMBOL_GPL(tick_do_timer_cpu);
 #ifdef CONFIG_NO_HZ_FULL
 /*
  * tick_do_timer_boot_cpu indicates the boot CPU temporarily owns
diff --git a/kernel/time/tick-internal.h b/kernel/time/tick-internal.h
index 649f2b48e8f0..8953dca10fdd 100644
--- a/kernel/time/tick-internal.h
+++ b/kernel/time/tick-internal.h
@@ -15,7 +15,6 @@
 
 DECLARE_PER_CPU(struct tick_device, tick_cpu_device);
 extern ktime_t tick_next_period;
-extern int tick_do_timer_cpu __read_mostly;
 
 extern void tick_setup_periodic(struct clock_event_device *dev, int broadcast);
 extern void tick_handle_periodic(struct clock_event_device *dev);
diff --git a/kernel/torture.c b/kernel/torture.c
index 789aeb0e1159..bccbdd33dda2 100644
--- a/kernel/torture.c
+++ b/kernel/torture.c
@@ -33,6 +33,7 @@
 #include <linux/delay.h>
 #include <linux/stat.h>
 #include <linux/slab.h>
+#include <linux/tick.h>
 #include <linux/trace_clock.h>
 #include <linux/ktime.h>
 #include <asm/byteorder.h>
@@ -358,7 +359,16 @@ torture_onoff(void *arg)
 			schedule_timeout_interruptible(HZ / 10);
 			continue;
 		}
+#ifdef CONFIG_NO_HZ_FULL
+		/* do not offline tick do timer cpu */
+		if (tick_nohz_full_running) {
+			cpu = (torture_random(&rand) >> 4) % maxcpu;
+			if (cpu >= tick_do_timer_cpu)
+				cpu = (cpu + 1) % (maxcpu + 1);
+		} else
+#else
 		cpu = (torture_random(&rand) >> 4) % (maxcpu + 1);
+#endif
 		if (!torture_offline(cpu,
 				     &n_offline_attempts, &n_offline_successes,
 				     &sum_offline, &min_offline, &max_offline))
-- 
2.34.1