Re: [PATCH v6 1/4] rcu: Make call_rcu() lazy to save power

["Paul E. McKenney" <paulmck@kernel.org>]

On Mon, Sep 26, 2022 at 03:04:38PM +0000, Joel Fernandes wrote:
> On Mon, Sep 26, 2022 at 12:00:45AM +0200, Frederic Weisbecker wrote:
> > On Sat, Sep 24, 2022 at 09:00:39PM -0400, Joel Fernandes wrote:
> > > 
> > > 
> > > > On Sep 24, 2022, at 7:28 PM, Joel Fernandes <joel@joelfernandes.org> wrote:
> > > > 
> > > > Hi Frederic, thanks for the response, replies
> > > > below courtesy fruit company’s device:
> > > > 
> > > >>> On Sep 24, 2022, at 6:46 PM, Frederic Weisbecker <frederic@kernel.org> wrote:
> > > >>> 
> > > >>> On Thu, Sep 22, 2022 at 10:01:01PM +0000, Joel Fernandes (Google) wrote:
> > > >>> @@ -3902,7 +3939,11 @@ static void rcu_barrier_entrain(struct rcu_data *rdp)
> > > >>>   rdp->barrier_head.func = rcu_barrier_callback;
> > > >>>   debug_rcu_head_queue(&rdp->barrier_head);
> > > >>>   rcu_nocb_lock(rdp);
> > > >>> -    WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies));
> > > >>> +    /*
> > > >>> +     * Flush the bypass list, but also wake up the GP thread as otherwise
> > > >>> +     * bypass/lazy CBs maynot be noticed, and can cause real long delays!
> > > >>> +     */
> > > >>> +    WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies, FLUSH_BP_WAKE));
> > > >> 
> > > >> This fixes an issue that goes beyond lazy implementation. It should be done
> > > >> in a separate patch, handling rcu_segcblist_entrain() as well, with "Fixes: " tag.
> > > > 
> > > > I wanted to do that, however on discussion with
> > > > Paul I thought of making this optimization only for
> > > > all lazy bypass CBs. That makes it directly related
> > > > this patch since the laziness notion is first
> > > > introduced here. On the other hand I could make
> > > > this change in a later patch since we are not
> > > > super bisectable anyway courtesy of the last
> > > > patch (which is not really an issue if the CONFIG
> > > > is kept off during someone’s bisection.
> > > 
> > > Or are we saying it’s worth doing the wake up for rcu barrier even for
> > > regular bypass CB? That’d save 2 jiffies on rcu barrier. If we agree it’s
> > > needed, then yes splitting the patch makes sense.
> > > 
> > > Please let me know your opinions, thanks,
> > > 
> > >  - Joel
> > 
> > Sure, I mean since we are fixing the buggy rcu_barrier_entrain() anyway, let's
> > just fix bypass as well. Such as in the following (untested):
> 
> Got it. This sounds good to me, and will simplify the code a bit more for sure.
> 
> I guess a question for Paul - are you Ok with rcu_barrier() causing wake ups
> if the bypass list has any non-lazy CBs as well? That should be OK, IMO.

In theory, I am OK with it.  In practice, you are the guys with the
hardware that can measure power consumption, not me!  ;-)

> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index b39e97175a9e..a0df964abb0e 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -3834,6 +3834,8 @@ static void rcu_barrier_entrain(struct rcu_data *rdp)
> >  {
> >  	unsigned long gseq = READ_ONCE(rcu_state.barrier_sequence);
> >  	unsigned long lseq = READ_ONCE(rdp->barrier_seq_snap);
> > +	bool wake_nocb = false;
> > +	bool was_alldone = false;
> >  
> >  	lockdep_assert_held(&rcu_state.barrier_lock);
> >  	if (rcu_seq_state(lseq) || !rcu_seq_state(gseq) || rcu_seq_ctr(lseq) != rcu_seq_ctr(gseq))
> > @@ -3842,6 +3844,8 @@ static void rcu_barrier_entrain(struct rcu_data *rdp)
> >  	rdp->barrier_head.func = rcu_barrier_callback;
> >  	debug_rcu_head_queue(&rdp->barrier_head);
> >  	rcu_nocb_lock(rdp);
> > +	if (rcu_rdp_is_offloaded(rdp) && !rcu_segcblist_pend_cbs(&rdp->cblist))
> > +		was_alldone = true;
> >  	WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies));
> >  	if (rcu_segcblist_entrain(&rdp->cblist, &rdp->barrier_head)) {
> >  		atomic_inc(&rcu_state.barrier_cpu_count);
> > @@ -3849,7 +3853,12 @@ static void rcu_barrier_entrain(struct rcu_data *rdp)
> >  		debug_rcu_head_unqueue(&rdp->barrier_head);
> >  		rcu_barrier_trace(TPS("IRQNQ"), -1, rcu_state.barrier_sequence);
> >  	}
> > +	if (was_alldone && rcu_segcblist_pend_cbs(&rdp->cblist))
> > +		wake_nocb = true;
> >  	rcu_nocb_unlock(rdp);
> > +	if (wake_nocb)
> > +		wake_nocb_gp(rdp, false);
> > +
> 
> Thanks for the code snippet, I like how you are checking if the bypass list
> is empty, without actually checking it ;-)

That certainly is consistent with the RCU philosophy.  :-)

							Thanx, Paul