[PATCH v2] sched/rt: Skip currently executing CPU in rto_next_cpu()

[Chen Jinghuang <chenjinghuang2@huawei.com>]

CPU0 becomes overloaded when hosting a CPU-bound RT task, a non-CPU-bound
RT task, and a CFS task stuck in kernel space. When other CPUs switch from
RT to non-RT tasks, RT load balancing (LB) is triggered; with
HAVE_RT_PUSH_IPI enabled, they send IPIs to CPU0 to drive the execution
of rto_push_irq_work_func. During push_rt_task on CPU0,
if next_task->prio < rq->donor->prio, resched_curr() sets NEED_RESCHED
and after the push operation completes, CPU0 calls rto_next_cpu().
Since only CPU0 is overloaded in this scenario, rto_next_cpu() should
ideally return -1 (no further IPI needed).

However, multiple CPUs invoking tell_cpu_to_push() during LB increments
rd->rto_loop_next. Even when rd->rto_cpu is set to -1, the mismatch between
rd->rto_loop and rd->rto_loop_next forces rto_next_cpu() to restart its
search from -1. With CPU0 remaining overloaded (satisfying rt_nr_migratory
&& rt_nr_total > 1), it gets reselected, causing CPU0 to queue irq_work to
itself and send self-IPIs repeatedly. As long as CPU0 stays overloaded and
other CPUs run pull_rt_tasks(), it falls into an infinite self-IPI loop,
wasting CPU cycles on unnecessary interrupt handling.

The triggering scenario is as follows:

         cpu0	        	   cpu1               	      cpu2
                   	        pull_rt_task
	                      tell_cpu_to_push
                 <------------irq_work_queue_on
rto_push_irq_work_func
       push_rt_task
    resched_curr(rq)                                      pull_rt_task
    rto_next_cpu                                        tell_cpu_to_push
     			 <-------------------------- atomic_inc(rto_loop_next)
rd->rto_loop != next
     rto_next_cpu
   irq_work_queue_on
rto_push_irq_work_func

Fix redundant self-IPI by filtering the initiating CPU in rto_next_cpu().

Fixes: 4bdced5c9a292 ("sched/rt: Simplify the IPI based RT balancing logic")
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Chen Jinghuang <chenjinghuang2@huawei.com>
---
v1-->v2:
	- Replace the original "check NEED_RESCHED on target CPU"
	  logic with "skip the currently executing CPU"
	- This modification eliminates self-IPIS
---
---
 kernel/sched/rt.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 7936d4333731..dc0d583aa59a 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -2100,6 +2100,7 @@ static void push_rt_tasks(struct rq *rq)
  */
 static int rto_next_cpu(struct root_domain *rd)
 {
+	int this_cpu = smp_processor_id();
 	int next;
 	int cpu;
 
@@ -2118,14 +2119,17 @@ static int rto_next_cpu(struct root_domain *rd)
 	 */
 	for (;;) {
 
-		/* When rto_cpu is -1 this acts like cpumask_first() */
-		cpu = cpumask_next(rd->rto_cpu, rd->rto_mask);
-
-		rd->rto_cpu = cpu;
+		do {
+			/* When rto_cpu is -1 this acts like cpumask_first() */
+			cpu = cpumask_next(rd->rto_cpu, rd->rto_mask);
+			rd->rto_cpu = cpu;
+			/* Do not send IPI to self */
+		} while (cpu == this_cpu);
 
 		if (cpu < nr_cpu_ids)
 			return cpu;
 
+
 		rd->rto_cpu = -1;
 
 		/*
-- 
2.34.1