[RFC PATCH 2/2] softirq: Per vector thread deferment

[Frederic Weisbecker]

Some softirq vectors can be more CPU hungry than others. Especially
networking may sometimes deal with packet storm and need more CPU than
IRQ tail can offer without inducing scheduler latencies. In this case
the current code defers to ksoftirqd that behaves nicer. Now this nice
behaviour can be bad for other IRQ vectors that usually need quick
processing.

To solve this we only defer to threading the vectors that outreached the
time limit on IRQ tail processing and leave the others inline on real
Soft-IRQs service. This is achieved using workqueues with
per-CPU/per-vector worklets.

Note ksoftirqd is not removed as it is still needed for threaded IRQs
mode.

Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Cc: Dmitry Safonov <dima@xxxxxxxxxx>
Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: David Miller <davem@xxxxxxxxxxxxx>
Cc: Hannes Frederic Sowa <hannes@xxxxxxxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Levin Alexander <alexander.levin@xxxxxxxxxxx>
Cc: Paolo Abeni <pabeni@xxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Radu Rendec <rrendec@xxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Stanislaw Gruszka <sgruszka@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
---
 kernel/softirq.c | 90 ++++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 87 insertions(+), 3 deletions(-)

diff --git a/kernel/softirq.c b/kernel/softirq.c
index fa267f7..0c817ec6 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -74,6 +74,13 @@ struct softirq_stat {
 
 static DEFINE_PER_CPU(struct softirq_stat, softirq_stat_cpu);
 
+struct vector_work {
+	int vec;
+	struct work_struct work;
+};
+
+static DEFINE_PER_CPU(struct vector_work[NR_SOFTIRQS], vector_work_cpu);
+
 /*
  * we cannot loop indefinitely here to avoid userspace starvation,
  * but we also don't want to introduce a worst case 1/HZ latency
@@ -251,6 +258,70 @@ static inline bool lockdep_softirq_start(void) { return false; }
 static inline void lockdep_softirq_end(bool in_hardirq) { }
 #endif
 
+static void vector_work_func(struct work_struct *work)
+{
+	struct vector_work *vector_work;
+	u32 pending;
+	int vec;
+
+	vector_work = container_of(work, struct vector_work, work);
+	vec = vector_work->vec;
+
+	local_irq_disable();
+	pending = local_softirq_pending();
+	account_irq_enter_time(current);
+	__local_bh_disable_ip(_RET_IP_, SOFTIRQ_OFFSET);
+	lockdep_softirq_enter();
+	set_softirq_pending(pending & ~(1 << vec));
+	local_irq_enable();
+
+	if (pending & (1 << vec)) {
+		struct softirq_action *sa = &softirq_vec[vec];
+
+		kstat_incr_softirqs_this_cpu(vec);
+		trace_softirq_entry(vec);
+		sa->action(sa);
+		trace_softirq_exit(vec);
+	}
+
+	local_irq_disable();
+
+	pending = local_softirq_pending();
+	if (pending & (1 << vec))
+		schedule_work_on(smp_processor_id(), work);
+
+	lockdep_softirq_exit();
+	account_irq_exit_time(current);
+	__local_bh_enable(SOFTIRQ_OFFSET);
+	local_irq_enable();
+}
+
+static int do_softirq_overrun(u32 overrun, u32 pending)
+{
+	struct softirq_action *h = softirq_vec;
+	int softirq_bit;
+
+	if (!overrun)
+		return pending;
+
+	overrun &= pending;
+	pending &= ~overrun;
+
+	while ((softirq_bit = ffs(overrun))) {
+		struct vector_work *work;
+		unsigned int vec_nr;
+
+		h += softirq_bit - 1;
+		vec_nr = h - softirq_vec;
+		work = this_cpu_ptr(&vector_work_cpu[vec_nr]);
+		schedule_work_on(smp_processor_id(), &work->work);
+		h++;
+		overrun >>= softirq_bit;
+	}
+
+	return pending;
+}
+
 asmlinkage __visible void __softirq_entry __do_softirq(void)
 {
 	struct softirq_stat *sstat = this_cpu_ptr(&softirq_stat_cpu);
@@ -321,10 +392,13 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
 
 	pending = local_softirq_pending();
 	if (pending) {
-		if (overrun || need_resched())
+		if (need_resched()) {
 			wakeup_softirqd();
-		else
-			goto restart;
+		} else {
+			pending = do_softirq_overrun(overrun, pending);
+			if (pending)
+				goto restart;
+		}
 	}
 
 	lockdep_softirq_end(in_hardirq);
@@ -661,10 +735,20 @@ void __init softirq_init(void)
 	int cpu;
 
 	for_each_possible_cpu(cpu) {
+		int i;
+
 		per_cpu(tasklet_vec, cpu).tail =
 			&per_cpu(tasklet_vec, cpu).head;
 		per_cpu(tasklet_hi_vec, cpu).tail =
 			&per_cpu(tasklet_hi_vec, cpu).head;
+
+		for (i = 0; i < NR_SOFTIRQS; i++) {
+			struct vector_work *work;
+
+			work = &per_cpu(vector_work_cpu[i], cpu);
+			work->vec = i;
+			INIT_WORK(&work->work, vector_work_func);
+		}
 	}
 
 	open_softirq(TASKLET_SOFTIRQ, tasklet_action);
-- 
2.7.4