On Tue, Sep 06, 2022 at 05:17:57PM +0200, Frederic Weisbecker wrote:
> On Tue, Sep 06, 2022 at 03:05:46AM +0000, Joel Fernandes wrote:
> > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> > index 4dc86274b3e8..b201606f7c4f 100644
> > --- a/kernel/rcu/tree_nocb.h
> > +++ b/kernel/rcu/tree_nocb.h
> > @@ -256,6 +256,31 @@ static bool wake_nocb_gp(struct rcu_data *rdp, bool force)
> > return __wake_nocb_gp(rdp_gp, rdp, force, flags);
> > }
> >
> > +/*
> > + * LAZY_FLUSH_JIFFIES decides the maximum amount of time that
> > + * can elapse before lazy callbacks are flushed. Lazy callbacks
> > + * could be flushed much earlier for a number of other reasons
> > + * however, LAZY_FLUSH_JIFFIES will ensure no lazy callbacks are
> > + * left unsubmitted to RCU after those many jiffies.
> > + */
> > +#define LAZY_FLUSH_JIFFIES (10 * HZ)
> > +unsigned long jiffies_till_flush = LAZY_FLUSH_JIFFIES;
>
> Still not static.
>
> > @@ -293,12 +322,16 @@ static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
> > * proves to be initially empty, just return false because the no-CB GP
> > * kthread may need to be awakened in this case.
> > *
> > + * Return true if there was something to be flushed and it succeeded, otherwise
> > + * false.
> > + *
>
> This kind of contradict the comment that follows. Not sure you need to add
> that line because the existing comment seem to cover it.
>
> > * Note that this function always returns true if rhp is NULL.
>
> > */
> > static bool rcu_nocb_do_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
> > - unsigned long j)
> > + unsigned long j, unsigned long flush_flags)
> > {
> > struct rcu_cblist rcl;
> > + bool lazy = flush_flags & FLUSH_BP_LAZY;
> >
> > WARN_ON_ONCE(!rcu_rdp_is_offloaded(rdp));
> > rcu_lockdep_assert_cblist_protected(rdp);
> > @@ -326,13 +372,20 @@ static bool rcu_nocb_do_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
> > * Note that this function always returns true if rhp is NULL.
> > */
> > static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
> > - unsigned long j)
> > + unsigned long j, unsigned long flush_flags)
> > {
> > + bool ret;
> > +
> > if (!rcu_rdp_is_offloaded(rdp))
> > return true;
> > rcu_lockdep_assert_cblist_protected(rdp);
> > rcu_nocb_bypass_lock(rdp);
> > - return rcu_nocb_do_flush_bypass(rdp, rhp, j);
> > + ret = rcu_nocb_do_flush_bypass(rdp, rhp, j, flush_flags);
> > +
> > + if (flush_flags & FLUSH_BP_WAKE)
> > + wake_nocb_gp(rdp, true);
>
> Why the true above?
>
> Also should we check if the wake up is really necessary (otherwise it means we
> force a wake up for all rdp's from rcu_barrier())?
>
> was_alldone = rcu_segcblist_pend_cbs(&rdp->cblist);
> ret = rcu_nocb_do_flush_bypass(rdp, rhp, j, flush_flags);
> if (was_alldone && rcu_segcblist_pend_cbs(&rdp->cblist))
> wake_nocb_gp(rdp, false);
You mean something like the following right? Though I'm thinking if its
better to call wake_nocb_gp() from tree.c in entrain() and let that handle
the wake. That way, we can get rid of the extra FLUSH_BP flags as well and
let the flush callers deal with the wakeups..
Anyway, for testing this should be good...
---8<-----------------------
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index bd8f39ee2cd0..e3344c262672 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -382,15 +382,19 @@ static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
unsigned long j, unsigned long flush_flags)
{
bool ret;
+ bool was_alldone;
if (!rcu_rdp_is_offloaded(rdp))
return true;
rcu_lockdep_assert_cblist_protected(rdp);
rcu_nocb_bypass_lock(rdp);
+ if (flush_flags & FLUSH_BP_WAKE)
+ was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
+
ret = rcu_nocb_do_flush_bypass(rdp, rhp, j, flush_flags);
- if (flush_flags & FLUSH_BP_WAKE)
- wake_nocb_gp(rdp, true);
+ if (flush_flags & FLUSH_BP_WAKE && was_alldone)
+ wake_nocb_gp(rdp, false);
return ret;
}
--
2.37.2.789.g6183377224-goog
