linux-kernel - Re: [PATCH v9 2/7] rcu/segcblist: Add counters to segcblist datastructure

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Message-ID: <20201107004815.GD3249@paulmck-ThinkPad-P72>
Date:   Fri, 6 Nov 2020 16:48:15 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org,
        Frederic Weisbecker <frederic@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Marco Elver <elver@...gle.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        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 v9 2/7] rcu/segcblist: Add counters to segcblist
 datastructure

On Fri, Nov 06, 2020 at 07:18:47PM -0500, Joel Fernandes wrote:
> On Fri, Nov 06, 2020 at 07:01:57PM -0500, Joel Fernandes wrote:
> > On Wed, Nov 04, 2020 at 09:01:33AM -0800, Paul E. McKenney wrote:
> > 
> > > A casual reader might be forgiven for being confused by the combination
> > > of "Return" in the above comment and the "void" function type below.
> > > So shouldn't this comment be something like "Add the specified number
> > > of callbacks to the specified segment..."?
> > 
> > You are right, sorry and will fix it.
> > 
> > > > @@ -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);
> > > 
> > > This should be a "for" loop.  Yes, the number and names of the segments
> > > hasn't changed for a good long time, but nothing like code as above to
> > > inspire Murphy to more mischief.  :-/
> > > 
> > > Actually, why not put the summation in the existing "for" loop below?
> > > That would save a line of code in addition to providing less inspiration
> > > for Mr. Murphy.
> > 
> > I can do that. Actually Frederic suggested the same thing but I was reluctant
> > as I felt it did not give much LOC benefit. Will revisit it.
> 
> It reduces 1 line of code :) I changed it to the below, will update the patch:

Thank you!  And yes, I am much more concerned about the constraints on
Mr. Murphy than on the lines of code.  ;-)

							Thanx, Paul

> ---8<-----------------------
> 
> diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
> index 9b43d686b1f3..bff9b2253e50 100644
> --- a/kernel/rcu/rcu_segcblist.c
> +++ b/kernel/rcu/rcu_segcblist.c
> @@ -101,7 +101,7 @@ static void rcu_segcblist_set_seglen(struct rcu_segcblist *rsclp, int seg, long
>  	WRITE_ONCE(rsclp->seglen[seg], v);
>  }
>  
> -/* Return number of callbacks in a segment of the segmented callback list. */
> +/* Increase the numeric length of a segment by a specified amount. */
>  static void rcu_segcblist_add_seglen(struct rcu_segcblist *rsclp, int seg, long v)
>  {
>  	WRITE_ONCE(rsclp->seglen[seg], rsclp->seglen[seg] + v);
> @@ -406,13 +406,12 @@ 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->len = 0;
>  	*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++) {
> +		rclp->len += rcu_segcblist_get_seglen(rsclp, i);
>  		WRITE_ONCE(rsclp->tails[i], rsclp->tails[RCU_DONE_TAIL]);
>  		rcu_segcblist_set_seglen(rsclp, i, 0);
>  	}