linux-kernel - Re: [PATCH v8 2/6] rcu/segcblist: Add counters to segcblist datastructure

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Message-ID: <20201026054043.GA4192074@google.com>
Date:   Mon, 26 Oct 2020 01:40:43 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Marco Elver <elver@...gle.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        "Uladzislau Rezki (Sony)" <urezki@...il.com>, fweisbec@...il.com,
        neeraj.iitr10@...il.com
Subject: Re: [PATCH v8 2/6] rcu/segcblist: Add counters to segcblist
 datastructure

On Mon, Oct 26, 2020 at 01:32:12AM +0100, Frederic Weisbecker wrote:
> On Wed, Oct 21, 2020 at 03:08:09PM -0400, Joel Fernandes (Google) wrote:
> > @@ -307,6 +317,7 @@ void rcu_segcblist_extract_done_cbs(struct rcu_segcblist *rsclp,
> >  
> >  	if (!rcu_segcblist_ready_cbs(rsclp))
> >  		return; /* Nothing to do. */
> > +	rclp->len = rcu_segcblist_get_seglen(rsclp, RCU_DONE_TAIL);
> 
> I realize, doesn't it break the unsegmented count in srcu_invoke_callbacks() ?
> 
> It still does rcu_segcblist_insert_count(), so it adds zero to rsclp->len
> which thus doesn't get cleared and probably keeps growing.

You are right. This needs changing :-( Its my fault, I did not test SRCU
torture tests which are indeed failing.

I fixed it with the diff attached to the end of the email and will test it
more.

> >  	*rclp->tail = rsclp->head;
> >  	WRITE_ONCE(rsclp->head, *rsclp->tails[RCU_DONE_TAIL]);
> >  	WRITE_ONCE(*rsclp->tails[RCU_DONE_TAIL], NULL);
> > @@ -314,6 +325,7 @@ void rcu_segcblist_extract_done_cbs(struct rcu_segcblist *rsclp,
> >  	for (i = RCU_CBLIST_NSEGS - 1; i >= RCU_DONE_TAIL; i--)
> >  		if (rsclp->tails[i] == rsclp->tails[RCU_DONE_TAIL])
> >  			WRITE_ONCE(rsclp->tails[i], &rsclp->head);
> > +	rcu_segcblist_set_seglen(rsclp, RCU_DONE_TAIL, 0);
> >  }
> >  
> >  /*
> > @@ -330,11 +342,16 @@ void rcu_segcblist_extract_pend_cbs(struct rcu_segcblist *rsclp,
> >  
> >  	if (!rcu_segcblist_pend_cbs(rsclp))
> >  		return; /* Nothing to do. */
> > +	rclp->len = rcu_segcblist_get_seglen(rsclp, RCU_WAIT_TAIL) +
> > +		    rcu_segcblist_get_seglen(rsclp, RCU_NEXT_READY_TAIL) +
> > +		    rcu_segcblist_get_seglen(rsclp, RCU_NEXT_TAIL);
> >  	*rclp->tail = *rsclp->tails[RCU_DONE_TAIL];
> >  	rclp->tail = rsclp->tails[RCU_NEXT_TAIL];
> >  	WRITE_ONCE(*rsclp->tails[RCU_DONE_TAIL], NULL);
> > -	for (i = RCU_DONE_TAIL + 1; i < RCU_CBLIST_NSEGS; i++)
> > +	for (i = RCU_DONE_TAIL + 1; i < RCU_CBLIST_NSEGS; i++) {
> >  		WRITE_ONCE(rsclp->tails[i], rsclp->tails[RCU_DONE_TAIL]);
> > +		rcu_segcblist_set_seglen(rsclp, i, 0);
> 
> You seem to have forgotten the suggestion?
> 
>     rclp->len += rcu_segcblist_get_seglen(rsclp, i)

I decided to keep it this way as I did not see how it could be better.
You mentioned you are Ok with either option so I left it as is.

Thanks!

 - Joel

---8<-----------------------

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 0f23d20d485a..79b7081143a7 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -1160,6 +1160,7 @@ static void srcu_advance_state(struct srcu_struct *ssp)
  */
 static void srcu_invoke_callbacks(struct work_struct *work)
 {
+	long len;
 	bool more;
 	struct rcu_cblist ready_cbs;
 	struct rcu_head *rhp;
@@ -1182,6 +1183,7 @@ static void srcu_invoke_callbacks(struct work_struct *work)
 	/* We are on the job!  Extract and invoke ready callbacks. */
 	sdp->srcu_cblist_invoking = true;
 	rcu_segcblist_extract_done_cbs(&sdp->srcu_cblist, &ready_cbs);
+	len = ready_cbs.len;
 	spin_unlock_irq_rcu_node(sdp);
 	rhp = rcu_cblist_dequeue(&ready_cbs);
 	for (; rhp != NULL; rhp = rcu_cblist_dequeue(&ready_cbs)) {
@@ -1190,13 +1192,14 @@ static void srcu_invoke_callbacks(struct work_struct *work)
 		rhp->func(rhp);
 		local_bh_enable();
 	}
+	WARN_ON_ONCE(ready_cbs.len);
 
 	/*
 	 * Update counts, accelerate new callbacks, and if needed,
 	 * schedule another round of callback invocation.
 	 */
 	spin_lock_irq_rcu_node(sdp);
-	rcu_segcblist_insert_count(&sdp->srcu_cblist, &ready_cbs);
+	rcu_segcblist_add_len(&sdp->srcu_cblist, -len);
 	(void)rcu_segcblist_accelerate(&sdp->srcu_cblist,
 				       rcu_seq_snap(&ssp->srcu_gp_seq));
 	sdp->srcu_cblist_invoking = false;