linux-kernel - Re: [PATCH v7 2/6] rcu/segcblist: Add counters to segcblist datastructure

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Message-ID: <20201017013123.GC4015033@google.com>
Date:   Fri, 16 Oct 2020 21:31:23 -0400
From:   joel@...lfernandes.org
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        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 v7 2/6] rcu/segcblist: Add counters to segcblist
 datastructure

On Thu, Oct 15, 2020 at 02:21:58PM +0200, Frederic Weisbecker wrote:
> On Wed, Oct 14, 2020 at 08:22:57PM -0400, Joel Fernandes (Google) wrote:
> > Add counting of segment lengths of segmented callback list.
> > 
> > This will be useful for a number of things such as knowing how big the
> > ready-to-execute segment have gotten. The immediate benefit is ability
> > to trace how the callbacks in the segmented callback list change.
> > 
> > Also this patch remove hacks related to using donecbs's ->len field as a
> > temporary variable to save the segmented callback list's length. This cannot be
> > done anymore and is not needed.
> > 
> > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > ---
> >  include/linux/rcu_segcblist.h |   2 +
> >  kernel/rcu/rcu_segcblist.c    | 133 +++++++++++++++++++++++-----------
> >  kernel/rcu/rcu_segcblist.h    |   2 -
> >  3 files changed, 92 insertions(+), 45 deletions(-)
> > 
> > diff --git a/include/linux/rcu_segcblist.h b/include/linux/rcu_segcblist.h
> > index b36afe7b22c9..d462ae5e340a 100644
> > --- a/include/linux/rcu_segcblist.h
> > +++ b/include/linux/rcu_segcblist.h
> > @@ -69,8 +69,10 @@ struct rcu_segcblist {
> >  	unsigned long gp_seq[RCU_CBLIST_NSEGS];
> >  #ifdef CONFIG_RCU_NOCB_CPU
> >  	atomic_long_t len;
> > +	atomic_long_t seglen[RCU_CBLIST_NSEGS];
> 
> Also does it really need to be atomic?

Yeah I think not. In fact, I am not even sure if ->len in the existing code
needs to be atomic. I am considering vetting all code paths. Paul told me it
used to be the case that ->len could be lockless written (possibly without
IRQs disabled) I think but that maybe nowadays it is not the case. Let us
double check to be sure.

> > @@ -245,7 +280,7 @@ void rcu_segcblist_enqueue(struct rcu_segcblist *rsclp,
> >  			   struct rcu_head *rhp)
> >  {
> >  	rcu_segcblist_inc_len(rsclp);
> > -	smp_mb(); /* Ensure counts are updated before callback is enqueued. */
> 
> That's a significant change that shouldn't be hidden and unexplained in an unrelated
> patch or it may be easily missed. I'd suggest to move this line together in
> "rcu/tree: Remove redundant smp_mb() in rcu_do_batch" (with the title updated perhaps)
> and maybe put it in the beginning of the series?

Ok I can do that.

> > +	rcu_segcblist_inc_seglen(rsclp, RCU_NEXT_TAIL);
> >  	rhp->next = NULL;
> >  	WRITE_ONCE(*rsclp->tails[RCU_NEXT_TAIL], rhp);
> >  	WRITE_ONCE(rsclp->tails[RCU_NEXT_TAIL], &rhp->next);
> [...]
> > @@ -330,11 +353,16 @@ 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);
> > +	}
> 
> So, that's probably just a matter of personal preference, so feel free to
> ignore but I'd rather do:
> 
>     rclp->len += rcu_segcblist_get_seglen(rsclp, i);
>     rcu_segcblist_set_seglen(rsclp, i, 0);
> 
> instead of the big addition above. That way, if a new index ever gets added/renamed
> to the segcblist, we'll take it into account. Also that spares a few lines.

Yeah your way is better, I will do it this way.

thanks,

 - Joel