linux-kernel - [PATCH v3 4/5] softirq: Unify should_wakeup

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Message-ID: <20241014090339.2478-5-kprateek.nayak@amd.com>
Date: Mon, 14 Oct 2024 09:03:38 +0000
From: K Prateek Nayak <kprateek.nayak@....com>
To: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot
	<vincent.guittot@...aro.org>, Thomas Gleixner <tglx@...utronix.de>,
	<linux-kernel@...r.kernel.org>, Sebastian Andrzej Siewior
	<bigeasy@...utronix.de>
CC: Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt
	<rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel Gorman
	<mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>, "Paul E.
 McKenney" <paulmck@...nel.org>, Rik van Riel <riel@...riel.com>, "Neeraj
 Upadhyay" <neeraj.upadhyay@...nel.org>, Leonardo Bras <leobras@...hat.com>,
	Thorsten Blum <thorsten.blum@...lux.com>, Zqiang <qiang.zhang1211@...il.com>,
	Tejun Heo <tj@...nel.org>, NeilBrown <neilb@...e.de>, Caleb Sander Mateos
	<csander@...estorage.com>, "Gautham R . Shenoy" <gautham.shenoy@....com>,
	Chen Yu <yu.c.chen@...el.com>, Julia Lawall <Julia.Lawall@...ia.fr>, "K
 Prateek Nayak" <kprateek.nayak@....com>
Subject: [PATCH v3 4/5] softirq: Unify should_wakeup_ksoftirqd()

Define softirq_ctrl::cnt for !PREEMPT_RT kernels too and unify
should_wakeup_ksoftirqd() to return based on softirq_ctrl::cnt.
Since these counts can change quite frequently when running interrupt
heavy benchmark, declare per-cpu softirq_ctrl as cacheline aligned.

No functional changes intended since !PREEMPT_RT kernels do not
increment the softirq_ctrl::cnt (yet) and should always return true
mimicking the current behavior.

Signed-off-by: K Prateek Nayak <kprateek.nayak@....com>
---
v2..v3:

o Removed ifdefs around local_lock_t. (Peter)
---
 kernel/softirq.c | 51 ++++++++++++++++++++++--------------------------
 1 file changed, 23 insertions(+), 28 deletions(-)

diff --git a/kernel/softirq.c b/kernel/softirq.c
index fc8c3a7dbe35..0730c2b43ae4 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -88,23 +88,6 @@ EXPORT_PER_CPU_SYMBOL_GPL(hardirqs_enabled);
 EXPORT_PER_CPU_SYMBOL_GPL(hardirq_context);
 #endif
 
-/*
- * SOFTIRQ_OFFSET usage:
- *
- * On !RT kernels 'count' is the preempt counter, on RT kernels this applies
- * to a per CPU counter and to task::softirqs_disabled_cnt.
- *
- * - count is changed by SOFTIRQ_OFFSET on entering or leaving softirq
- *   processing.
- *
- * - count is changed by SOFTIRQ_DISABLE_OFFSET (= 2 * SOFTIRQ_OFFSET)
- *   on local_bh_disable or local_bh_enable.
- *
- * This lets us distinguish between whether we are currently processing
- * softirq and whether we just have bh disabled.
- */
-#ifdef CONFIG_PREEMPT_RT
-
 /*
  * RT accounts for BH disabled sections in task::softirqs_disabled_cnt and
  * also in per CPU softirq_ctrl::cnt. This is necessary to allow tasks in a
@@ -122,10 +105,32 @@ struct softirq_ctrl {
 	int		cnt;
 };
 
-static DEFINE_PER_CPU(struct softirq_ctrl, softirq_ctrl) = {
+static DEFINE_PER_CPU_ALIGNED(struct softirq_ctrl, softirq_ctrl) = {
 	.lock	= INIT_LOCAL_LOCK(softirq_ctrl.lock),
 };
 
+static inline bool should_wake_ksoftirqd(void)
+{
+	return !this_cpu_read(softirq_ctrl.cnt);
+}
+
+/*
+ * SOFTIRQ_OFFSET usage:
+ *
+ * On !RT kernels 'count' is the preempt counter, on RT kernels this applies
+ * to a per CPU counter and to task::softirqs_disabled_cnt.
+ *
+ * - count is changed by SOFTIRQ_OFFSET on entering or leaving softirq
+ *   processing.
+ *
+ * - count is changed by SOFTIRQ_DISABLE_OFFSET (= 2 * SOFTIRQ_OFFSET)
+ *   on local_bh_disable or local_bh_enable.
+ *
+ * This lets us distinguish between whether we are currently processing
+ * softirq and whether we just have bh disabled.
+ */
+#ifdef CONFIG_PREEMPT_RT
+
 /**
  * local_bh_blocked() - Check for idle whether BH processing is blocked
  *
@@ -270,11 +275,6 @@ static inline void ksoftirqd_run_end(void)
 static inline void softirq_handle_begin(void) { }
 static inline void softirq_handle_end(void) { }
 
-static inline bool should_wake_ksoftirqd(void)
-{
-	return !this_cpu_read(softirq_ctrl.cnt);
-}
-
 static inline void invoke_softirq(void)
 {
 	if (should_wake_ksoftirqd())
@@ -419,11 +419,6 @@ static inline void ksoftirqd_run_end(void)
 	local_irq_enable();
 }
 
-static inline bool should_wake_ksoftirqd(void)
-{
-	return true;
-}
-
 static inline void invoke_softirq(void)
 {
 	if (!force_irqthreads() || !__this_cpu_read(ksoftirqd)) {
-- 
2.34.1