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 <redacted>
---
 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);