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

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

[Chen Jinghuang]

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,
which triggers a CPU hardlockup due to continuous self-interrupts.

The trigging 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().
This solution has been verified to effectively eliminate spurious self-IPIs
and prevent CPU hardlockup scenarios.

Fixes: 4bdced5c9a29 ("sched/rt: Simplify the IPI based RT balancing logic")
Suggested-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Chen Jinghuang <chenjinghuang2@huawei.com>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>

---
Changes since v2: https://lore.kernel.org/all/20251125083649.1814558-1-chenjinghuang2@huawei.com/
- Replace the original "check NEED_RESCHED on target CPU"
  logic with "skip the currently executing CPU"
- This modification eliminates self-IPIS

Changes since v1: https://lore.kernel.org/all/20251121014004.564508-1-chenjinghuang2@huawei.com/
- Remove unneeded extra whitespace
- Add Reviewed-by tag from Steven Rostedt
---
 kernel/sched/rt.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index f1867fe8e5c5..ec10ed7bb75d 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;
 
@@ -2118,10 +2119,13 @@ 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);
+		do {
+			/* When rto_cpu is -1 this acts like cpumask_first() */
+			cpu = cpumask_next(rd->rto_cpu, rd->rto_mask);
 
-		rd->rto_cpu = cpu;
+			rd->rto_cpu = cpu;
+			/* Do not send IPI to self */
+		} while (cpu == this_cpu);
 
 		if (cpu < nr_cpu_ids)
 			return cpu;
-- 
2.34.1

Re: [RESEND] sched/rt: Skip currently executing CPU in rto_next_cpu()

[Steven Rostedt]

Peter,

Are you OK with this change? If so, care to take it?

-- Steve


On Mon, 5 Jan 2026 03:40:12 +0000
Chen Jinghuang <chenjinghuang2@huawei.com> wrote:

> 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,
> which triggers a CPU hardlockup due to continuous self-interrupts.
> 
> The trigging 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().
> This solution has been verified to effectively eliminate spurious self-IPIs
> and prevent CPU hardlockup scenarios.
> 
> Fixes: 4bdced5c9a29 ("sched/rt: Simplify the IPI based RT balancing logic")
> Suggested-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> Signed-off-by: Chen Jinghuang <chenjinghuang2@huawei.com>
> Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> 
> ---
> Changes since v2: https://lore.kernel.org/all/20251125083649.1814558-1-chenjinghuang2@huawei.com/
> - Replace the original "check NEED_RESCHED on target CPU"
>   logic with "skip the currently executing CPU"
> - This modification eliminates self-IPIS
> 
> Changes since v1: https://lore.kernel.org/all/20251121014004.564508-1-chenjinghuang2@huawei.com/
> - Remove unneeded extra whitespace
> - Add Reviewed-by tag from Steven Rostedt
> ---
>  kernel/sched/rt.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index f1867fe8e5c5..ec10ed7bb75d 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;
>  
> @@ -2118,10 +2119,13 @@ 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);
> +		do {
> +			/* When rto_cpu is -1 this acts like cpumask_first() */
> +			cpu = cpumask_next(rd->rto_cpu, rd->rto_mask);
>  
> -		rd->rto_cpu = cpu;
> +			rd->rto_cpu = cpu;
> +			/* Do not send IPI to self */
> +		} while (cpu == this_cpu);
>  
>  		if (cpu < nr_cpu_ids)
>  			return cpu;

Re: [RESEND] sched/rt: Skip currently executing CPU in rto_next_cpu()

[K Prateek Nayak]

Hello Chen, Steve,

On 1/5/2026 9:10 AM, Chen Jinghuang wrote:
> @@ -2118,10 +2119,13 @@ 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);
> +		do {
> +			/* When rto_cpu is -1 this acts like cpumask_first() */
> +			cpu = cpumask_next(rd->rto_cpu, rd->rto_mask);
>  
> -		rd->rto_cpu = cpu;
> +			rd->rto_cpu = cpu;
> +			/* Do not send IPI to self */
> +		} while (cpu == this_cpu);

nit.

Since we are already within an infinite for-loop, can't we simply do:

    /* Do not send IPI to self */
    if (cpu == this_cpu)
        continue;

here and go evaluate cpumask_next() again instead of adding another
do-while? Was the nested loop intentional to highlight these bits
explicitly?

>  
>  		if (cpu < nr_cpu_ids)
>  			return cpu;

-- 
Thanks and Regards,
Prateek

Re: [RESEND] sched/rt: Skip currently executing CPU in rto_next_cpu()

[Steven Rostedt]

On Tue, 6 Jan 2026 14:11:57 +0530
K Prateek Nayak <kprateek.nayak@amd.com> wrote:

> Hello Chen, Steve,
> 
> On 1/5/2026 9:10 AM, Chen Jinghuang wrote:
> > @@ -2118,10 +2119,13 @@ 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);
> > +		do {
> > +			/* When rto_cpu is -1 this acts like cpumask_first() */
> > +			cpu = cpumask_next(rd->rto_cpu, rd->rto_mask);
> >  
> > -		rd->rto_cpu = cpu;
> > +			rd->rto_cpu = cpu;
> > +			/* Do not send IPI to self */
> > +		} while (cpu == this_cpu);  
> 
> nit.
> 
> Since we are already within an infinite for-loop, can't we simply do:
> 
>     /* Do not send IPI to self */
>     if (cpu == this_cpu)
>         continue;
> 
> here and go evaluate cpumask_next() again instead of adding another
> do-while? Was the nested loop intentional to highlight these bits
> explicitly?

Hmm, yeah, I think I added the loop to express this issue. But I think your
suggestion could work too. I originally had something like that, but for
some reason I thought it added an extra branch (over the added do { } while).

> 
> >  
> >  		if (cpu < nr_cpu_ids)
> >  			return cpu;  
> 


	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 not send IPI to self */
+		if (cpu == this_cpu)
+			continue;
+
		if (cpu < nr_cpu_ids)
			return cpu;

		rd->rto_cpu = -1;

		/*
		 * ACQUIRE ensures we see the @rto_mask changes
		 * made prior to the @next value observed.
		 *
		 * Matches WMB in rt_set_overload().
		 */
		next = atomic_read_acquire(&rd->rto_loop_next);

		if (rd->rto_loop == next)
			break;

		rd->rto_loop = next;
	}

Looks to be equivalent.

Chen, care to send a new version?

-- Steve

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

[Chen Jinghuang]

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,
which triggers a CPU hardlockup due to continuous self-interrupts.

The trigging 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().
This solution has been verified to effectively eliminate spurious self-IPIs
and prevent CPU hardlockup scenarios.

Fixes: 4bdced5c9a29 ("sched/rt: Simplify the IPI based RT balancing logic")
Suggested-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Suggested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Chen Jinghuang <chenjinghuang2@huawei.com>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
---
Changes since v4: https://lore.kernel.org/all/20260108070355.259530-1-chenjinghuang2@huawei.com/
- Add Reviewed-by tag from Valentin Schneider

Changes since v3: https://lore.kernel.org/all/20260105034012.3196947-1-chenjinghuang2@huawei.com/
- Replace the do-while loop with a continue statement to simplify the loop logic
- Add Suggested-by tag from K Prateek Nayak

Changes since v2: https://lore.kernel.org/all/20251125083649.1814558-1-chenjinghuang2@huawei.com/
- Replace the original "check NEED_RESCHED on target CPU"
  logic with "skip the currently executing CPU"
- This modification eliminates self-IPIS

Changes since v1: https://lore.kernel.org/all/20251121014004.564508-1-chenjinghuang2@huawei.com/
- Remove unneeded extra whitespace
- Add Reviewed-by tag from Steven Rostedt
---
 kernel/sched/rt.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index f1867fe8e5c5..e0ff90905019 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;
 
@@ -2123,6 +2124,10 @@ static int rto_next_cpu(struct root_domain *rd)
 
 		rd->rto_cpu = cpu;
 
+		/* Do not send IPI to self */
+		if (cpu == this_cpu)
+			continue;
+
 		if (cpu < nr_cpu_ids)
 			return cpu;
 
-- 
2.34.1

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

[Chen Jinghuang]

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,
which triggers a CPU hardlockup due to continuous self-interrupts.

The trigging 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().
This solution has been verified to effectively eliminate spurious self-IPIs
and prevent CPU hardlockup scenarios.

Fixes: 4bdced5c9a29 ("sched/rt: Simplify the IPI based RT balancing logic")
Suggested-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Suggested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Chen Jinghuang <chenjinghuang2@huawei.com>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
---
Changes since v4: https://lore.kernel.org/all/20260108070355.259530-1-chenjinghuang2@huawei.com/
- Add Reviewed-by tag from Valentin Schneider

Changes since v3: https://lore.kernel.org/all/20260105034012.3196947-1-chenjinghuang2@huawei.com/
- Replace the do-while loop with a continue statement to simplify the loop logic
- Add Suggested-by tag from K Prateek Nayak

Changes since v2: https://lore.kernel.org/all/20251125083649.1814558-1-chenjinghuang2@huawei.com/
- Replace the original "check NEED_RESCHED on target CPU"
  logic with "skip the currently executing CPU"
- This modification eliminates self-IPIS

Changes since v1: https://lore.kernel.org/all/20251121014004.564508-1-chenjinghuang2@huawei.com/
- Remove unneeded extra whitespace
- Add Reviewed-by tag from Steven Rostedt
---
 kernel/sched/rt.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index f1867fe8e5c5..e0ff90905019 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;
 
@@ -2123,6 +2124,10 @@ static int rto_next_cpu(struct root_domain *rd)
 
 		rd->rto_cpu = cpu;
 
+		/* Do not send IPI to self */
+		if (cpu == this_cpu)
+			continue;
+
 		if (cpu < nr_cpu_ids)
 			return cpu;
 
-- 
2.34.1

Re: [RESEND] sched/rt: Skip currently executing CPU in rto_next_cpu()

[Peter Zijlstra]

On Thu, Jan 22, 2026 at 01:25:33AM +0000, Chen Jinghuang wrote:
> 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,
> which triggers a CPU hardlockup due to continuous self-interrupts.
> 
> The trigging 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().
> This solution has been verified to effectively eliminate spurious self-IPIs
> and prevent CPU hardlockup scenarios.
> 
> Fixes: 4bdced5c9a29 ("sched/rt: Simplify the IPI based RT balancing logic")
> Suggested-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> Suggested-by: K Prateek Nayak <kprateek.nayak@amd.com>
> Signed-off-by: Chen Jinghuang <chenjinghuang2@huawei.com>
> Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> Reviewed-by: Valentin Schneider <vschneid@redhat.com>
> ---

Thanks!

[tip: sched/core] sched/rt: Skip currently executing CPU in rto_next_cpu()

[tip-bot2 for Chen Jinghuang]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     94894c9c477e53bcea052e075c53f89df3d2a33e
Gitweb:        https://git.kernel.org/tip/94894c9c477e53bcea052e075c53f89df3d2a33e
Author:        Chen Jinghuang <chenjinghuang2@huawei.com>
AuthorDate:    Thu, 22 Jan 2026 01:25:33 
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Tue, 03 Feb 2026 12:04:19 +01:00

sched/rt: Skip currently executing CPU in rto_next_cpu()

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,
which triggers a CPU hardlockup due to continuous self-interrupts.

The trigging 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().
This solution has been verified to effectively eliminate spurious self-IPIs
and prevent CPU hardlockup scenarios.

Fixes: 4bdced5c9a29 ("sched/rt: Simplify the IPI based RT balancing logic")
Suggested-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Suggested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Chen Jinghuang <chenjinghuang2@huawei.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Link: https://patch.msgid.link/20260122012533.673768-1-chenjinghuang2@huawei.com
---
 kernel/sched/rt.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 0a9b2cd..a768047 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -2106,6 +2106,7 @@ 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;
 
@@ -2129,6 +2130,10 @@ static int rto_next_cpu(struct root_domain *rd)
 
 		rd->rto_cpu = cpu;
 
+		/* Do not send IPI to self */
+		if (cpu == this_cpu)
+			continue;
+
 		if (cpu < nr_cpu_ids)
 			return cpu;