Re: [PATCH v5 06/18] rcu: Introduce call_rcu_lazy() API implementation

[Joel Fernandes <joel@joelfernandes.org>]

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