linux-kernel - [HELPER PATCH 2] helper: disable arch handling if paravirt file being written

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Message-ID: <20251119062100.1112520-18-sshegde@linux.ibm.com>
Date: Wed, 19 Nov 2025 11:51:00 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Cc: sshegde@...ux.ibm.com, 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, srikar@...ux.ibm.com,
        gregkh@...uxfoundation.org, pbonzini@...hat.com, seanjc@...gle.com,
        kprateek.nayak@....com, vschneid@...hat.com, iii@...ux.ibm.com,
        huschle@...ux.ibm.com, rostedt@...dmis.org, dietmar.eggemann@....com,
        christophe.leroy@...roup.eu
Subject: [HELPER PATCH 2] helper: disable arch handling if paravirt file being written

Arch specific code can update the mask based on the steal time. For
debugging it is desired to overwrite the arch logic. Do that with this
helper script.

Signed-off-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
---
 arch/powerpc/platforms/pseries/lpar.c | 3 +++
 drivers/base/cpu.c                    | 2 ++
 include/linux/sched.h                 | 4 ++++
 kernel/sched/core.c                   | 1 +
 4 files changed, 10 insertions(+)

diff --git a/arch/powerpc/platforms/pseries/lpar.c b/arch/powerpc/platforms/pseries/lpar.c
index 090e5c48243b..04bc75e22e7b 100644
--- a/arch/powerpc/platforms/pseries/lpar.c
+++ b/arch/powerpc/platforms/pseries/lpar.c
@@ -681,6 +681,9 @@ static void process_steal(int cpu)
 	unsigned long steal = 0;
 	unsigned int i;
 
+	if (static_branch_unlikely(&disable_arch_paravirt_handling))
+		return;
+
 	if (!should_cpu_process_steal(cpu))
 		return;
 
diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
index 043e4f4ce1a9..fbaddbfe0b01 100644
--- a/drivers/base/cpu.c
+++ b/drivers/base/cpu.c
@@ -402,7 +402,9 @@ static ssize_t store_paravirt_cpus(struct device *dev,
 	/* No more paravirt cpus */
 	if (cpumask_empty(temp_mask)) {
 		cpumask_copy((struct cpumask *)&__cpu_paravirt_mask, temp_mask);
+		static_branch_disable(&disable_arch_paravirt_handling);
 	} else {
+		static_branch_enable(&disable_arch_paravirt_handling);
 		cpumask_copy((struct cpumask *)&__cpu_paravirt_mask, temp_mask);
 
 		/* Enable tick on nohz_full cpu */
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 3628edd1468b..1afa5dd5b0ae 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2427,4 +2427,8 @@ extern void migrate_enable(void);
 
 DEFINE_LOCK_GUARD_0(migrate, migrate_disable(), migrate_enable())
 
+#ifdef CONFIG_PARAVIRT
+DECLARE_STATIC_KEY_FALSE(disable_arch_paravirt_handling);
+#endif
+
 #endif
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 65c247c24191..b65a9898c694 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -10873,6 +10873,7 @@ void sched_change_end(struct sched_change_ctx *ctx)
 #ifdef CONFIG_PARAVIRT
 struct cpumask __cpu_paravirt_mask __read_mostly;
 EXPORT_SYMBOL(__cpu_paravirt_mask);
+DEFINE_STATIC_KEY_FALSE(disable_arch_paravirt_handling);
 
 static DEFINE_PER_CPU(struct cpu_stop_work, pv_push_task_work);
 
-- 
2.47.3