linux-kernel - [RFC PATCH v3 04/10] sched/core: Dont allow to use CPU marked as paravirt

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Message-ID: <20250910174210.1969750-5-sshegde@linux.ibm.com>
Date: Wed, 10 Sep 2025 23:12:04 +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 04/10] sched/core: Dont allow to use CPU marked as paravirt

Don't allow a paravirt CPU to be used while looking for a CPU to use.

Push task mechanism uses stopper thread which going to call
select_fallback_rq and use this mechanism to avoid picking a paravirt CPU.

Signed-off-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
---
 kernel/sched/core.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b8a84e4691c8..279b0dd72b5e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2462,8 +2462,13 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
 		return cpu_online(cpu);
 
 	/* Non kernel threads are not allowed during either online or offline. */
-	if (!(p->flags & PF_KTHREAD))
-		return cpu_active(cpu);
+	if (!(p->flags & PF_KTHREAD)) {
+		/* A user thread shouldn't be allowed on a paravirt cpu */
+		if (is_cpu_paravirt(cpu))
+			return false;
+		else
+			return cpu_active(cpu);
+	}
 
 	/* KTHREAD_IS_PER_CPU is always allowed. */
 	if (kthread_is_per_cpu(p))
@@ -2473,6 +2478,10 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
 	if (cpu_dying(cpu))
 		return false;
 
+	/* Non percpu kthreads should stay away from paravirt cpu*/
+	if (is_cpu_paravirt(cpu))
+		return false;
+
 	/* But are allowed during online. */
 	return cpu_online(cpu);
 }
-- 
2.47.3