linux-kernel - Re: [PATCH v2 1/8] rcu: Introduce call_rcu

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Date:   Sat, 25 Jun 2022 21:00:54 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        rushikesh.s.kadam@...el.com, urezki@...il.com,
        neeraj.iitr10@...il.com, frederic@...nel.org, rostedt@...dmis.org,
        vineeth@...byteword.org
Subject: Re: [PATCH v2 1/8] rcu: Introduce call_rcu_lazy() API implementation

On Wed, Jun 22, 2022 at 11:18:02PM +0000, Joel Fernandes wrote:
> On Wed, Jun 22, 2022 at 10:50:55PM +0000, Joel Fernandes (Google) wrote:
> [..]
> > diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> > index 2ccf5845957d..fec4fad6654b 100644
> > --- a/kernel/rcu/tree.h
> > +++ b/kernel/rcu/tree.h
> > @@ -267,8 +267,9 @@ struct rcu_data {
> >  /* Values for nocb_defer_wakeup field in struct rcu_data. */
> >  #define RCU_NOCB_WAKE_NOT	0
> >  #define RCU_NOCB_WAKE_BYPASS	1
> > -#define RCU_NOCB_WAKE		2
> > -#define RCU_NOCB_WAKE_FORCE	3
> > +#define RCU_NOCB_WAKE_LAZY	2
> > +#define RCU_NOCB_WAKE		3
> > +#define RCU_NOCB_WAKE_FORCE	4
> >  
> >  #define RCU_JIFFIES_TILL_FORCE_QS (1 + (HZ > 250) + (HZ > 500))
> >  					/* For jiffies_till_first_fqs and */
> > @@ -436,9 +437,10 @@ static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp);
> >  static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq);
> >  static void rcu_init_one_nocb(struct rcu_node *rnp);
> >  static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
> > -				  unsigned long j);
> > +				  unsigned long j, bool lazy);
> >  static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
> > -				bool *was_alldone, unsigned long flags);
> > +				bool *was_alldone, unsigned long flags,
> > +				bool lazy);
> >  static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_empty,
> >  				 unsigned long flags);
> >  static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp, int level);
> > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> > index e369efe94fda..b9244f22e102 100644
> > --- a/kernel/rcu/tree_nocb.h
> > +++ b/kernel/rcu/tree_nocb.h
> > @@ -256,6 +256,8 @@ static bool wake_nocb_gp(struct rcu_data *rdp, bool force)
> >  	return __wake_nocb_gp(rdp_gp, rdp, force, flags);
> >  }
> >  
> > +#define LAZY_FLUSH_JIFFIES (10 * HZ)
> > +
> >  /*
> >   * Arrange to wake the GP kthread for this NOCB group at some future
> >   * time when it is safe to do so.
> > @@ -272,7 +274,10 @@ static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
> >  	 * Bypass wakeup overrides previous deferments. In case
> >  	 * of callback storm, no need to wake up too early.
> >  	 */
> > -	if (waketype == RCU_NOCB_WAKE_BYPASS) {
> > +	if (waketype == RCU_NOCB_WAKE_LAZY) {
> > +		mod_timer(&rdp_gp->nocb_timer, jiffies + LAZY_FLUSH_JIFFIES);
> > +		WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
> > +	} else if (waketype == RCU_NOCB_WAKE_BYPASS) {
> >  		mod_timer(&rdp_gp->nocb_timer, jiffies + 2);
> >  		WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
> >  	} else {
> > @@ -296,7 +301,7 @@ static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
> >   * 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, bool lazy)
> >  {
> >  	struct rcu_cblist rcl;
> >  
> > @@ -310,7 +315,13 @@ static bool rcu_nocb_do_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
> >  	/* Note: ->cblist.len already accounts for ->nocb_bypass contents. */
> >  	if (rhp)
> >  		rcu_segcblist_inc_len(&rdp->cblist); /* Must precede enqueue. */
> > -	rcu_cblist_flush_enqueue(&rcl, &rdp->nocb_bypass, rhp);
> > +
> > +	trace_printk("call_rcu_lazy callbacks = %ld\n", READ_ONCE(rdp->nocb_bypass.lazy_len));
> 
> Before anyone yells at me, that trace_printk() has been politely asked to take
> a walk :-). It got mad at me, but on the next iteration, it wont be there.

;-) ;-) ;-)

							Thanx, Paul