linux-kernel - [RFC PATCH v3 06/10] sched/rt: Don't select paravirt CPU for wakeup and push/pull rt task

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Message-ID: <20250910174210.1969750-7-sshegde@linux.ibm.com>
Date: Wed, 10 Sep 2025 23:12:06 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, tglx@...utronix.de, yury.norov@...il.com,
        maddy@...ux.ibm.com, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, gregkh@...uxfoundation.org
Cc: sshegde@...ux.ibm.com, vschneid@...hat.com, iii@...ux.ibm.com,
        huschle@...ux.ibm.com, rostedt@...dmis.org, dietmar.eggemann@....com,
        vineeth@...byteword.org, jgross@...e.com, pbonzini@...hat.com,
        seanjc@...gle.com
Subject: [RFC PATCH v3 06/10] sched/rt: Don't select paravirt CPU for wakeup and push/pull rt task

For RT class, 
- During wakeup don't select a paravirt CPU.
- Don't pull a task towards a paravirt CPU.
- Don't push a task to a paravirt CPU.

Signed-off-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
---
 kernel/sched/rt.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 7936d4333731..54bfac66624b 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1552,6 +1552,9 @@ select_task_rq_rt(struct task_struct *p, int cpu, int flags)
 		if (!test && target != -1 && !rt_task_fits_capacity(p, target))
 			goto out_unlock;
 
+		/* Avoid moving to a paravirt CPU */
+		if (is_cpu_paravirt(target))
+			goto out_unlock;
 		/*
 		 * Don't bother moving it if the destination CPU is
 		 * not running a lower priority task.
@@ -1876,7 +1879,7 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
 	for (tries = 0; tries < RT_MAX_TRIES; tries++) {
 		cpu = find_lowest_rq(task);
 
-		if ((cpu == -1) || (cpu == rq->cpu))
+		if ((cpu == -1) || (cpu == rq->cpu) || is_cpu_paravirt(cpu))
 			break;
 
 		lowest_rq = cpu_rq(cpu);
@@ -1974,7 +1977,7 @@ static int push_rt_task(struct rq *rq, bool pull)
 			return 0;
 
 		cpu = find_lowest_rq(rq->curr);
-		if (cpu == -1 || cpu == rq->cpu)
+		if (cpu == -1 || cpu == rq->cpu || is_cpu_paravirt(cpu))
 			return 0;
 
 		/*
@@ -2237,6 +2240,10 @@ static void pull_rt_task(struct rq *this_rq)
 	if (likely(!rt_overload_count))
 		return;
 
+	/* There is no point in pulling the task towards a paravirt cpu */
+	if (is_cpu_paravirt(this_rq->cpu))
+		return;
+
 	/*
 	 * Match the barrier from rt_set_overloaded; this guarantees that if we
 	 * see overloaded we must also see the rto_mask bit.
-- 
2.47.3