linux-kernel - [PATCH v2 1/2] smp: Introduce a helper function to check for pending IPIs

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Message-ID: <20251020141718.150919-2-ulf.hansson@linaro.org>
Date: Mon, 20 Oct 2025 16:17:11 +0200
From: Ulf Hansson <ulf.hansson@...aro.org>
To: "Rafael J . Wysocki" <rafael@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Cc: Mark Rutland <mark.rutland@....com>,
	Marc Zyngier <maz@...nel.org>,
	Maulik Shah <quic_mkshah@...cinc.com>,
	Sudeep Holla <sudeep.holla@....com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	linux-pm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	Ulf Hansson <ulf.hansson@...aro.org>
Subject: [PATCH v2 1/2] smp: Introduce a helper function to check for pending IPIs

When governors used during cpuidle, tries to find the most optimal
idlestate for a CPU or a group of CPUs, they are known to quite often fail.
One reason for this, is that we are not taking into account whether there
has been an IPI scheduled for any of the CPUs that are affected by the
selected idlestate.

To enable pending IPIs to be taken into account for cpuidle decisions,
let's introduce a new helper function, cpus_may_have_pending_ipi().

Note that, the implementation is intentionally as lightweight as possible,
in favor of always providing the correct information. For cpuidle decisions
this is good enough.

Suggested-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Ulf Hansson <ulf.hansson@...aro.org>
---

Changes in v2:
	- Implemented a common function, rather than making it arch-specific. As
	suggested by Thomas and Marc.
	- Renamed the function to indicate that it doesn't provide correctness.
	- Clarified function description and commit message.

---
 include/linux/smp.h |  5 +++++
 kernel/smp.c        | 24 ++++++++++++++++++++++++
 2 files changed, 29 insertions(+)

diff --git a/include/linux/smp.h b/include/linux/smp.h
index 18e9c918325e..093e5458493e 100644
--- a/include/linux/smp.h
+++ b/include/linux/smp.h
@@ -168,6 +168,7 @@ int smp_call_function_any(const struct cpumask *mask,
 
 void kick_all_cpus_sync(void);
 void wake_up_all_idle_cpus(void);
+bool cpus_may_have_pending_ipi(const struct cpumask *mask);
 
 /*
  * Generic and arch helpers
@@ -216,6 +217,10 @@ smp_call_function_any(const struct cpumask *mask, smp_call_func_t func,
 
 static inline void kick_all_cpus_sync(void) {  }
 static inline void wake_up_all_idle_cpus(void) {  }
+static inline bool cpus_may_have_pending_ipi(const struct cpumask *mask)
+{
+	return false;
+}
 
 #define setup_max_cpus 0
 
diff --git a/kernel/smp.c b/kernel/smp.c
index 02f52291fae4..775f90790935 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -1087,6 +1087,30 @@ void wake_up_all_idle_cpus(void)
 }
 EXPORT_SYMBOL_GPL(wake_up_all_idle_cpus);
 
+/**
+ * cpus_may_have_pending_ipi - Check for pending IPIs for CPUs
+ * @mask: The CPU mask for the CPUs to check.
+ *
+ * This function walks through the @mask to check if there are any pending IPIs
+ * scheduled, for any of the CPUs in the @mask.
+ *
+ * It's important for the caller to know that this function does not guarantee
+ * correctness, as the intent is to be as lightweight as possible.
+ *
+ * Returns true if there is a pending IPI scheduled and false otherwise.
+ */
+bool cpus_may_have_pending_ipi(const struct cpumask *mask)
+{
+	unsigned int cpu;
+
+	for_each_cpu(cpu, mask) {
+		if (!llist_empty(per_cpu_ptr(&call_single_queue, cpu)))
+			return true;
+	}
+
+        return false;
+}
+
 /**
  * struct smp_call_on_cpu_struct - Call a function on a specific CPU
  * @work: &work_struct
-- 
2.43.0