netdev - [PATCH 3/9] softirq: Factor loop termination condition

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Date: Fri, 5 May 2023 19:33:09 +0800
From: Liu Jian <liujian56@...wei.com>
To: <corbet@....net>, <paulmck@...nel.org>, <frederic@...nel.org>,
	<quic_neeraju@...cinc.com>, <joel@...lfernandes.org>,
	<josh@...htriplett.org>, <boqun.feng@...il.com>, <rostedt@...dmis.org>,
	<mathieu.desnoyers@...icios.com>, <jiangshanlai@...il.com>,
	<qiang1.zhang@...el.com>, <jstultz@...gle.com>, <tglx@...utronix.de>,
	<sboyd@...nel.org>, <davem@...emloft.net>, <edumazet@...gle.com>,
	<kuba@...nel.org>, <pabeni@...hat.com>, <peterz@...radead.org>,
	<frankwoo@...gle.com>, <Rhinewuwu@...gle.com>
CC: <liujian56@...wei.com>, <linux-doc@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <rcu@...r.kernel.org>,
	<netdev@...r.kernel.org>
Subject: [PATCH 3/9] softirq: Factor loop termination condition

From: Peter Zijlstra <peterz@...radead.org>

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Liu Jian <liujian56@...wei.com>
---
 kernel/softirq.c | 44 +++++++++++++++++++++++++-------------------
 1 file changed, 25 insertions(+), 19 deletions(-)

diff --git a/kernel/softirq.c b/kernel/softirq.c
index 59f16a9af5d1..48a81d8ae49a 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -477,22 +477,6 @@ asmlinkage __visible void do_softirq(void)
 
 #endif /* !CONFIG_PREEMPT_RT */
 
-/*
- * We restart softirq processing for at most MAX_SOFTIRQ_RESTART times,
- * but break the loop if need_resched() is set or after 2 ms.
- * The MAX_SOFTIRQ_TIME provides a nice upper bound in most cases, but in
- * certain cases, such as stop_machine(), jiffies may cease to
- * increment and so we need the MAX_SOFTIRQ_RESTART limit as
- * well to make sure we eventually return from this method.
- *
- * These limits have been established via experimentation.
- * The two things to balance is latency against fairness -
- * we want to handle softirqs as soon as possible, but they
- * should not be able to lock up the box.
- */
-#define MAX_SOFTIRQ_TIME	(2 * NSEC_PER_MSEC)
-#define MAX_SOFTIRQ_RESTART 10
-
 #ifdef CONFIG_TRACE_IRQFLAGS
 /*
  * When we run softirqs from irq_exit() and thus on the hardirq stack we need
@@ -526,10 +510,33 @@ static inline bool lockdep_softirq_start(void) { return false; }
 static inline void lockdep_softirq_end(bool in_hardirq) { }
 #endif
 
+/*
+ * We restart softirq processing but break the loop if need_resched() is set or
+ * after 2 ms. The MAX_SOFTIRQ_RESTART guarantees a loop termination if
+ * sched_clock() were ever to stall.
+ *
+ * These limits have been established via experimentation.  The two things to
+ * balance is latency against fairness - we want to handle softirqs as soon as
+ * possible, but they should not be able to lock up the box.
+ */
+#define MAX_SOFTIRQ_TIME	(2 * NSEC_PER_MSEC)
+#define MAX_SOFTIRQ_RESTART	10
+
+static inline bool __softirq_needs_break(u64 start)
+{
+	if (need_resched())
+		return true;
+
+	if (sched_clock() - start >= MAX_SOFTIRQ_TIME)
+		return true;
+
+	return false;
+}
+
 asmlinkage __visible void __softirq_entry __do_softirq(void)
 {
+	unsigned int max_restart = MAX_SOFTIRQ_RESTART;
 	unsigned long old_flags = current->flags;
-	int max_restart = MAX_SOFTIRQ_RESTART;
 	u64 start = sched_clock();
 	struct softirq_action *h;
 	unsigned long pending;
@@ -585,8 +592,7 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
 
 	pending = local_softirq_pending();
 	if (pending) {
-		if (sched_clock() - start < MAX_SOFTIRQ_TIME && !need_resched() &&
-		    --max_restart)
+		if (!__softirq_needs_break(start) && --max_restart)
 			goto restart;
 
 		wakeup_softirqd();
-- 
2.34.1