Hi Peter,

[FYI, it's a private test report for your RFC patch.]
[auto build test ERROR on tip/sched/core]
[also build test ERROR on tip/auto-latest next-20200526]
[cannot apply to linus/master linux/master v5.7-rc7]
[if your patch is applied to the wrong git tree, please drop us a note to help
improve the system. BTW, we also suggest to use '--base' option to specify the
base tree in git format-patch, please see https://stackoverflow.com/a/37406982]

url:    https://github.com/0day-ci/linux/commits/Peter-Zijlstra/Fix-the-scheduler-IPI-mess/20200527-010828
base:   https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git 2ebb17717550607bcd85fb8cf7d24ac870e9d762
config: i386-tinyconfig (attached as .config)
compiler: gcc-9 (Debian 9.3.0-13) 9.3.0
reproduce (this is a W=1 build):
        # save the attached .config to linux build tree
        make W=1 ARCH=i386 

If you fix the issue, kindly add following tag as appropriate
Reported-by: kbuild test robot <lkp@intel.com>

All errors (new ones prefixed by >>, old ones prefixed by <<):

kernel/sched/idle.c: In function 'do_idle':
>> kernel/sched/idle.c:292:2: error: implicit declaration of function 'flush_smp_call_function_from_idle' [-Werror=implicit-function-declaration]
292 |  flush_smp_call_function_from_idle();
|  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: some warnings being treated as errors

vim +/flush_smp_call_function_from_idle +292 kernel/sched/idle.c

   225	
   226	/*
   227	 * Generic idle loop implementation
   228	 *
   229	 * Called with polling cleared.
   230	 */
   231	static void do_idle(void)
   232	{
   233		int cpu = smp_processor_id();
   234		/*
   235		 * If the arch has a polling bit, we maintain an invariant:
   236		 *
   237		 * Our polling bit is clear if we're not scheduled (i.e. if rq->curr !=
   238		 * rq->idle). This means that, if rq->idle has the polling bit set,
   239		 * then setting need_resched is guaranteed to cause the CPU to
   240		 * reschedule.
   241		 */
   242	
   243		__current_set_polling();
   244		tick_nohz_idle_enter();
   245	
   246		while (!need_resched()) {
   247			rmb();
   248	
   249			local_irq_disable();
   250	
   251			if (cpu_is_offline(cpu)) {
   252				tick_nohz_idle_stop_tick();
   253				cpuhp_report_idle_dead();
   254				arch_cpu_idle_dead();
   255			}
   256	
   257			arch_cpu_idle_enter();
   258	
   259			/*
   260			 * In poll mode we reenable interrupts and spin. Also if we
   261			 * detected in the wakeup from idle path that the tick
   262			 * broadcast device expired for us, we don't want to go deep
   263			 * idle as we know that the IPI is going to arrive right away.
   264			 */
   265			if (cpu_idle_force_poll || tick_check_broadcast_expired()) {
   266				tick_nohz_idle_restart_tick();
   267				cpu_idle_poll();
   268			} else {
   269				cpuidle_idle_call();
   270			}
   271			arch_cpu_idle_exit();
   272		}
   273	
   274		/*
   275		 * Since we fell out of the loop above, we know TIF_NEED_RESCHED must
   276		 * be set, propagate it into PREEMPT_NEED_RESCHED.
   277		 *
   278		 * This is required because for polling idle loops we will not have had
   279		 * an IPI to fold the state for us.
   280		 */
   281		preempt_set_need_resched();
   282		tick_nohz_idle_exit();
   283		__current_clr_polling();
   284	
   285		/*
   286		 * We promise to call sched_ttwu_pending() and reschedule if
   287		 * need_resched() is set while polling is set. That means that clearing
   288		 * polling needs to be visible before doing these things.
   289		 */
   290		smp_mb__after_atomic();
   291	
 > 292		flush_smp_call_function_from_idle();
   293		sched_ttwu_pending();
   294		schedule_idle();
   295	
   296		if (unlikely(klp_patch_pending(current)))
   297			klp_update_patch_state(current);
   298	}
   299	

---
0-DAY CI Kernel Test Service, Intel Corporation
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