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