linux-kernel - Re: [PATCH 10/13] rcu/nocb: Delete bypass_timer upon nocb

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Message-ID: <YE908FC8F8/0p07q@boqun-archlinux>
Date:   Mon, 15 Mar 2021 22:53:36 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Josh Triplett <josh@...htriplett.org>,
        Stable <stable@...r.kernel.org>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH 10/13] rcu/nocb: Delete bypass_timer upon nocb_gp wakeup

On Wed, Mar 10, 2021 at 11:17:02PM +0100, Frederic Weisbecker wrote:
> On Tue, Mar 02, 2021 at 05:24:56PM -0800, Paul E. McKenney wrote:
> > On Tue, Feb 23, 2021 at 01:10:08AM +0100, Frederic Weisbecker wrote:
> > > A NOCB-gp wake up can safely delete the nocb_bypass_timer. nocb_gp_wait()
> > > is going to check again the bypass state and rearm the bypass timer if
> > > necessary.
> > > 
> > > Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> > > Cc: Josh Triplett <josh@...htriplett.org>
> > > Cc: Lai Jiangshan <jiangshanlai@...il.com>
> > > Cc: Joel Fernandes <joel@...lfernandes.org>
> > > Cc: Neeraj Upadhyay <neeraju@...eaurora.org>
> > > Cc: Boqun Feng <boqun.feng@...il.com>
> > 
> > Give that you delete this code a couple of patches later in this series,
> > why not just leave it out entirely?  ;-)
> 
> It's not exactly deleted later, it's rather merged within the
> "del_timer(&rdp_gp->nocb_timer)".
> 
> The purpose of that patch is to make it clear that we explicitly cancel
> the nocb_bypass_timer here before we do it implicitly later with the
> merge of nocb_bypass_timer into nocb_timer.
> 
> We could drop that patch, the resulting code in the end of the patchset
> will be the same of course but the behaviour detail described here might
> slip out of the reviewers attention :-)
> 

How about merging the timers first and adding those small improvements
later? i.e. move patch #12 #13 right after #7 (IIUC, #7 is the last
requirement you need for merging timers), and then patch #8~#11 just
follow, because IIUC, those patches are not about correctness but more
about avoiding necessary timer fire-ups, right?

Just my 2 cents. The overall patchset looks good to me ;-)

Feel free to add

Reviewed-by: Boqun Feng <boqun.feng@...il.com>

Regards,
Boqun

> > 
> > 							Thanx, Paul
> > 
> > > ---
> > >  kernel/rcu/tree_plugin.h | 2 ++
> > >  1 file changed, 2 insertions(+)
> > > 
> > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > > index b62ad79bbda5..9da67b0d3997 100644
> > > --- a/kernel/rcu/tree_plugin.h
> > > +++ b/kernel/rcu/tree_plugin.h
> > > @@ -1711,6 +1711,8 @@ static bool __wake_nocb_gp(struct rcu_data *rdp_gp,
> > >  		del_timer(&rdp_gp->nocb_timer);
> > >  	}
> > >  
> > > +	del_timer(&rdp_gp->nocb_bypass_timer);
> > > +
> > >  	if (force || READ_ONCE(rdp_gp->nocb_gp_sleep)) {
> > >  		WRITE_ONCE(rdp_gp->nocb_gp_sleep, false);
> > >  		needwake = true;
> 
> Thanks.