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    SubjectRe: [RFC PATCH 0/2] net: threadable napi poll loop
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    On Wed, 2016-05-11 at 14:56 -0700, Eric Dumazet wrote:
    > 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;

    here something like:

    struct task_struct *tsk = __this_cpu_read(ksoftirqd);
    return tsk && (tsk->state == TASK_RUNNING);

    is needed since __this_cpu_read(ksoftirqd) can be NULL on boot.

    > > }
    >
    > 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
    > /*
    >
    >


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