Re: [RFC PATCH 0/2] net: threadable napi poll loop

[Eric Dumazet]

On Wed, 2016-05-11 at 08:55 +0200, Peter Zijlstra wrote:
> On Tue, May 10, 2016 at 03:51:37PM -0700, Eric Dumazet wrote:
> > diff --git a/kernel/softirq.c b/kernel/softirq.c
> > index 17caf4b63342..22463217e3cf 100644
> > --- a/kernel/softirq.c
> > +++ b/kernel/softirq.c
> > @@ -56,6 +56,7 @@ EXPORT_SYMBOL(irq_stat);
> >  static struct softirq_action softirq_vec[NR_SOFTIRQS] __cacheline_aligned_in_smp;
> >  
> >  DEFINE_PER_CPU(struct task_struct *, ksoftirqd);
> > +DEFINE_PER_CPU(bool, ksoftirqd_scheduled);
> >  
> >  const char * const softirq_to_name[NR_SOFTIRQS] = {
> >  	"HI", "TIMER", "NET_TX", "NET_RX", "BLOCK", "BLOCK_IOPOLL",
> > @@ -73,8 +74,10 @@ static void wakeup_softirqd(void)
> >  	/* Interrupts are disabled: no need to stop preemption */
> >  	struct task_struct *tsk = __this_cpu_read(ksoftirqd);
> >  
> > -	if (tsk && tsk->state != TASK_RUNNING)
> > +	if (tsk && tsk->state != TASK_RUNNING) {
> > +		__this_cpu_write(ksoftirqd_scheduled, true);
> >  		wake_up_process(tsk);
> 
> Since we're already looking at tsk->state, and the wake_up_process()
> ensures the thing becomes TASK_RUNNING, you could add:
> 
> static inline bool ksoftirqd_running(void)
> {
> 	return __this_cpu_read(ksoftirqd)->state == TASK_RUNNING;
> }

Indeed, and the patch looks quite simple now ;)

diff --git a/kernel/softirq.c b/kernel/softirq.c
index 17caf4b63342d7839528f367b283a386413b0362..23c364485d03618773c385d943c0ef39f5931d09 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -57,6 +57,11 @@ static struct softirq_action softirq_vec[NR_SOFTIRQS] __cacheline_aligned_in_smp
 
 DEFINE_PER_CPU(struct task_struct *, ksoftirqd);
 
+static inline bool ksoftirqd_running(void)
+{
+	return __this_cpu_read(ksoftirqd)->state == TASK_RUNNING;
+}
+
 const char * const softirq_to_name[NR_SOFTIRQS] = {
 	"HI", "TIMER", "NET_TX", "NET_RX", "BLOCK", "BLOCK_IOPOLL",
 	"TASKLET", "SCHED", "HRTIMER", "RCU"
@@ -313,7 +318,7 @@ asmlinkage __visible void do_softirq(void)
 
 	pending = local_softirq_pending();
 
-	if (pending)
+	if (pending && !ksoftirqd_running())
 		do_softirq_own_stack();
 
 	local_irq_restore(flags);
@@ -340,6 +345,9 @@ void irq_enter(void)
 
 static inline void invoke_softirq(void)
 {
+	if (ksoftirqd_running())
+		return;
+
 	if (!force_irqthreads) {
 #ifdef CONFIG_HAVE_IRQ_EXIT_ON_IRQ_STACK
 		/*