[PATCH V6 1/1] Softirq:avoid large sched delay from the pending softirqs

* [PATCH V6 1/1] Softirq:avoid large sched delay from the pending softirqs
@ 2020-09-09  9:09 qianjun.kernel
  2020-09-11 15:55 ` peterz
  2020-09-11 16:46 ` Qais Yousef
  0 siblings, 2 replies; 9+ messages in thread
From: qianjun.kernel @ 2020-09-09  9:09 UTC (permalink / raw)
  To: tglx, peterz, will, luto, linux-kernel; +Cc: laoar.shao, urezki, jun qian

From: jun qian <qianjun.kernel@gmail.com>

When get the pending softirqs, it need to process all the pending
softirqs in the while loop. If the processing time of each pending
softirq is need more than 2 msec in this loop, or one of the softirq
will running a long time, according to the original code logic, it
will process all the pending softirqs without wakeuping ksoftirqd,
which will cause a relatively large scheduling delay on the
corresponding CPU, which we do not wish to see. The patch will check
the total time to process pending softirq, if the time exceeds 2 ms
we need to wakeup the ksofirqd to aviod large sched delay.

Signed-off-by: jun qian <qianjun.kernel@gmail.com>
---
 kernel/softirq.c | 83 ++++++++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 69 insertions(+), 14 deletions(-)

diff --git a/kernel/softirq.c b/kernel/softirq.c
index c4201b7f..1f696c8 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -25,6 +25,7 @@
 #include <linux/smpboot.h>
 #include <linux/tick.h>
 #include <linux/irq.h>
+#include <linux/sched/clock.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/irq.h>
@@ -199,18 +200,17 @@ void __local_bh_enable_ip(unsigned long ip, unsigned int cnt)
 
 /*
  * 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.
+ * but break the loop if need_resched() is set or after MAX_SOFTIRQ_TIME_NS
+ * ns. In the loop, if the processing time of the softirq has exceeded
+ * MAX_SOFTIRQ_TIME_NS ns, we also need to break the loop to wakeup the
+ * ksofirqd.
  *
  * 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  msecs_to_jiffies(2)
+#define MAX_SOFTIRQ_TIME_NS 2000000
 #define MAX_SOFTIRQ_RESTART 10
 
 #ifdef CONFIG_TRACE_IRQFLAGS
@@ -246,15 +246,20 @@ static inline void lockdep_softirq_end(bool in_hardirq)
 static inline void lockdep_softirq_end(bool in_hardirq) { }
 #endif
 
+DEFINE_PER_CPU(__u32, pending_new_flag);
+DEFINE_PER_CPU(__u32, pending_next_bit);
+#define SOFTIRQ_PENDING_MASK ((1UL << NR_SOFTIRQS) - 1)
+
 asmlinkage __visible void __softirq_entry __do_softirq(void)
 {
-	unsigned long end = jiffies + MAX_SOFTIRQ_TIME;
+	u64 end = sched_clock() + MAX_SOFTIRQ_TIME_NS;
 	unsigned long old_flags = current->flags;
 	int max_restart = MAX_SOFTIRQ_RESTART;
 	struct softirq_action *h;
 	bool in_hardirq;
-	__u32 pending;
-	int softirq_bit;
+	__u32 pending, pending_left, pending_new;
+	int softirq_bit, next_bit;
+	unsigned long flags;
 
 	/*
 	 * Mask out PF_MEMALLOC as the current task context is borrowed for the
@@ -277,10 +282,33 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
 
 	h = softirq_vec;
 
-	while ((softirq_bit = ffs(pending))) {
-		unsigned int vec_nr;
+	next_bit = per_cpu(pending_next_bit, smp_processor_id());
+	per_cpu(pending_new_flag, smp_processor_id()) = 0;
+
+	pending_left = pending &
+		(SOFTIRQ_PENDING_MASK << next_bit);
+	pending_new = pending &
+		(SOFTIRQ_PENDING_MASK >> (NR_SOFTIRQS - next_bit));
+
+	/*
+	 * In order to be fair, we shold process the pengding bits by the
+	 * last processing order.
+	 */
+	while ((softirq_bit = ffs(pending_left)) ||
+		(softirq_bit = ffs(pending_new))) {
 		int prev_count;
+		unsigned int vec_nr = 0;
 
+		/*
+		 * when the left pengding bits have been handled, we should
+		 * to reset the h to softirq_vec.
+		 */
+		if (!ffs(pending_left)) {
+			if (per_cpu(pending_new_flag, smp_processor_id()) == 0) {
+				h = softirq_vec;
+				per_cpu(pending_new_flag, smp_processor_id()) = 1;
+			}
+		}
 		h += softirq_bit - 1;
 
 		vec_nr = h - softirq_vec;
@@ -298,17 +326,44 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
 			preempt_count_set(prev_count);
 		}
 		h++;
-		pending >>= softirq_bit;
+
+		if (ffs(pending_left))
+			pending_left >>= softirq_bit;
+		else
+			pending_new >>= softirq_bit;
+
+		/*
+		 * the softirq's action has been run too much time,
+		 * so it may need to wakeup the ksoftirqd
+		 */
+		if (need_resched() && sched_clock() > end) {
+			/*
+			 * Ensure that the remaining pending bits will be
+			 * handled.
+			 */
+			local_irq_save(flags);
+			if (ffs(pending_left))
+				or_softirq_pending((pending_left << (vec_nr + 1)) |
+							pending_new);
+			else
+				or_softirq_pending(pending_new << (vec_nr + 1));
+			local_irq_restore(flags);
+			per_cpu(pending_next_bit, smp_processor_id()) = vec_nr + 1;
+			break;
+		}
 	}
 
+	/* reset the pending_next_bit */
+	per_cpu(pending_next_bit, smp_processor_id()) = 0;
+
 	if (__this_cpu_read(ksoftirqd) == current)
 		rcu_softirq_qs();
 	local_irq_disable();
 
 	pending = local_softirq_pending();
 	if (pending) {
-		if (time_before(jiffies, end) && !need_resched() &&
-		    --max_restart)
+		if (!need_resched() && --max_restart &&
+		    sched_clock() <= end)
 			goto restart;
 
 		wakeup_softirqd();
-- 
1.8.3.1


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