[RFC PATCH 1/2] softirq: Account time and iteration stats per vector

[Frederic Weisbecker <frederic@kernel.org>]

As we plan to be able to defer some specific softurq vector processing
to workqueues when those vectors need more time than IRQs can offer,
let's first count the time spent and the number of occurences per vector.

For now we still defer to ksoftirqd when the per vector limits are reached

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Dmitry Safonov <dima@arista.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Miller <davem@davemloft.net>
Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Levin Alexander <alexander.levin@verizon.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Radu Rendec <rrendec@arista.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Stanislaw Gruszka <sgruszka@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wanpeng Li <wanpeng.li@hotmail.com>
---
 kernel/softirq.c | 37 +++++++++++++++++++++++++++++--------
 1 file changed, 29 insertions(+), 8 deletions(-)

diff --git a/kernel/softirq.c b/kernel/softirq.c
index 2f5e87f..fa267f7 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -26,6 +26,7 @@
 #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>
@@ -62,6 +63,17 @@ const char * const softirq_to_name[NR_SOFTIRQS] = {
 	"TASKLET", "SCHED", "HRTIMER", "RCU"
 };
 
+struct vector_stat {
+	u64 time;
+	int count;
+};
+
+struct softirq_stat {
+	struct vector_stat stat[NR_SOFTIRQS];
+};
+
+static DEFINE_PER_CPU(struct softirq_stat, softirq_stat_cpu);
+
 /*
  * we cannot loop indefinitely here to avoid userspace starvation,
  * but we also don't want to introduce a worst case 1/HZ latency
@@ -203,7 +215,7 @@ EXPORT_SYMBOL(__local_bh_enable_ip);
  * 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  (2 * NSEC_PER_MSEC)
 #define MAX_SOFTIRQ_RESTART 10
 
 #ifdef CONFIG_TRACE_IRQFLAGS
@@ -241,12 +253,11 @@ static inline void lockdep_softirq_end(bool in_hardirq) { }
 
 asmlinkage __visible void __softirq_entry __do_softirq(void)
 {
-	unsigned long end = jiffies + MAX_SOFTIRQ_TIME;
+	struct softirq_stat *sstat = this_cpu_ptr(&softirq_stat_cpu);
 	unsigned long old_flags = current->flags;
-	int max_restart = MAX_SOFTIRQ_RESTART;
 	struct softirq_action *h;
 	bool in_hardirq;
-	__u32 pending;
+	__u32 pending, overrun = 0;
 	int softirq_bit;
 
 	/*
@@ -262,6 +273,7 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
 	__local_bh_disable_ip(_RET_IP_, SOFTIRQ_OFFSET);
 	in_hardirq = lockdep_softirq_start();
 
+	memzero_explicit(sstat, sizeof(*sstat));
 restart:
 	/* Reset the pending bitmask before enabling irqs */
 	set_softirq_pending(0);
@@ -271,8 +283,10 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
 	h = softirq_vec;
 
 	while ((softirq_bit = ffs(pending))) {
+		struct vector_stat *vstat;
 		unsigned int vec_nr;
 		int prev_count;
+		u64 startime;
 
 		h += softirq_bit - 1;
 
@@ -280,10 +294,18 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
 		prev_count = preempt_count();
 
 		kstat_incr_softirqs_this_cpu(vec_nr);
+		vstat = &sstat->stat[vec_nr];
 
 		trace_softirq_entry(vec_nr);
+		startime = local_clock();
 		h->action(h);
+		vstat->time += local_clock() - startime;
+		vstat->count++;
 		trace_softirq_exit(vec_nr);
+
+		if (vstat->time > MAX_SOFTIRQ_TIME || vstat->count > MAX_SOFTIRQ_RESTART)
+			overrun |= 1 << vec_nr;
+
 		if (unlikely(prev_count != preempt_count())) {
 			pr_err("huh, entered softirq %u %s %p with preempt_count %08x, exited with %08x?\n",
 			       vec_nr, softirq_to_name[vec_nr], h->action,
@@ -299,11 +321,10 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
 
 	pending = local_softirq_pending();
 	if (pending) {
-		if (time_before(jiffies, end) && !need_resched() &&
-		    --max_restart)
+		if (overrun || need_resched())
+			wakeup_softirqd();
+		else
 			goto restart;
-
-		wakeup_softirqd();
 	}
 
 	lockdep_softirq_end(in_hardirq);
-- 
2.7.4