On Fri, Mar 12, 2021 at 12:23:57AM +0100, Frederic Weisbecker wrote:
> On Wed, Mar 03, 2021 at 04:00:16PM -0800, paulmck@xxxxxxxxxx wrote:
> > From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> >
> > After interrupts have enabled at boot but before some random point
> > in early_initcall() processing, softirq processing is unreliable.
> > If softirq sees a need to push softirq-handler invocation to ksoftirqd
> > during this time, then those handlers can be delayed until the ksoftirqd
> > kthreads have been spawned, which happens at some random point in the
> > early_initcall() processing. In many cases, this delay is just fine.
> > However, if the boot sequence blocks waiting for a wakeup from a softirq
> > handler, this delay will result in a silent-hang deadlock.
> >
> > This commit therefore prevents these hangs by ensuring that the tick
> > stays active until after the ksoftirqd kthreads have been spawned.
> > This change causes the tick to eventually drain the backlog of delayed
> > softirq handlers, breaking this deadlock.
> >
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > ---
> > kernel/rcu/tree_plugin.h | 11 +++++++++++
> > 1 file changed, 11 insertions(+)
> >
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index 2d60377..36212de 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -1255,6 +1255,11 @@ static void rcu_prepare_kthreads(int cpu)
> > */
> > int rcu_needs_cpu(u64 basemono, u64 *nextevt)
> > {
> > + /* Through early_initcall(), need tick for softirq handlers. */
> > + if (!IS_ENABLED(CONFIG_HZ_PERIODIC) && !this_cpu_ksoftirqd()) {
> > + *nextevt = 1;
> > + return 1;
> > + }
> > *nextevt = KTIME_MAX;
> > return !rcu_segcblist_empty(&this_cpu_ptr(&rcu_data)->cblist) &&
> > !rcu_segcblist_is_offloaded(&this_cpu_ptr(&rcu_data)->cblist);
> > @@ -1350,6 +1355,12 @@ int rcu_needs_cpu(u64 basemono, u64 *nextevt)
> >
> > lockdep_assert_irqs_disabled();
> >
> > + /* Through early_initcall(), need tick for softirq handlers. */
> > + if (!IS_ENABLED(CONFIG_HZ_PERIODIC) && !this_cpu_ksoftirqd()) {
> > + *nextevt = 1;
> > + return 1;
> > + }
> > +
> > /* If no non-offloaded callbacks, RCU doesn't need the CPU. */
> > if (rcu_segcblist_empty(&rdp->cblist) ||
> > rcu_segcblist_is_offloaded(&this_cpu_ptr(&rcu_data)->cblist)) {
>
>
> I suspect rcutiny should be concerned as well?
>
> In fact this patch doesn't look necessary because can_stop_idle_tick() refuse
> to stop the tick when softirqs are pending.
So it does, thank you! I will drop this patch.
Thanx, Paul
