On Fri, 26 Feb 2021 17:35:21 -0800 Wei Wang wrote:
> On Fri, Feb 26, 2021 at 5:22 PM Jakub Kicinski <k...@kernel.org> wrote:
> >
> > On Fri, 26 Feb 2021 17:02:17 -0800 Wei Wang wrote:
> > > static int napi_thread_wait(struct napi_struct *napi)
> > > {
> > > + bool woken = false;
> > > +
> > > set_current_state(TASK_INTERRUPTIBLE);
> > >
> > > while (!kthread_should_stop() && !napi_disable_pending(napi)) {
> > > - if (test_bit(NAPI_STATE_SCHED, &napi->state)) {
> > > + unsigned long state = READ_ONCE(napi->state);
> > > +
> > > + if ((state & NAPIF_STATE_SCHED) &&
> > > + ((state & NAPIF_STATE_SCHED_THREAD) || woken)) {
> > > WARN_ON(!list_empty(&napi->poll_list));
> > > __set_current_state(TASK_RUNNING);
> > > return 0;
> > > + } else {
> > > + WARN_ON(woken);
> > > }
> > >
> > > schedule();
> > > + woken = true;
> > > set_current_state(TASK_INTERRUPTIBLE);
> > > }
> > > __set_current_state(TASK_RUNNING);
> > >
> > > I don't think it is sufficient to only set SCHED_THREADED bit when the
> > > thread is in RUNNING state.
> > > In fact, the thread is most likely NOT in RUNNING mode before we call
> > > wake_up_process() in ____napi_schedule(), because it has finished the
> > > previous round of napi->poll() and SCHED bit was cleared, so
> > > napi_thread_wait() sets the state to INTERRUPTIBLE and schedule() call
> > > should already put it in sleep.
> >
> > That's why the check says "|| woken":
> >
> > ((state & NAPIF_STATE_SCHED_THREAD) || woken))
> >
> > thread knows it owns the NAPI if:
> >
> > (a) the NAPI has the explicit flag set
> > or
> > (b) it was just worken up and !kthread_should_stop(), since only
> > someone who just claimed the normal SCHED on thread's behalf
> > will wake it up
>
> The 'woken' is set after schedule(). If it is the first time
> napi_threaded_wait() is called, and SCHED_THREADED is not set, and
> woken is not set either, this thread will be put to sleep when it
> reaches schedule(), even though there is work waiting to be done on
> that napi. And I think this kthread will not be woken up again
> afterwards, since the SCHED bit is already grabbed.
Indeed, looks like the task will be in WAKING state until it runs?
We can switch the check in ____napi_schedule() from
if (thread->state == TASK_RUNNING)
to
if (!(thread->state & TASK_INTERRUPTIBLE))
?
