linux-kernel - Re: [PATCH 04/10] rcu: Implement rcu_segcblist_is

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Date:   Thu, 14 May 2020 08:47:54 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>
Subject: Re: [PATCH 04/10] rcu: Implement rcu_segcblist_is_offloaded() config
 dependent

On Thu, May 14, 2020 at 01:03:31AM +0200, Frederic Weisbecker wrote:
> On Wed, May 13, 2020 at 11:20:29AM -0700, Paul E. McKenney wrote:
> > On Wed, May 13, 2020 at 06:47:08PM +0200, Frederic Weisbecker wrote:
> > > This simplify the usage of this API and avoid checking the kernel
> > > config from the callers.
> > > 
> > > Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> > > Cc: Paul E. McKenney <paulmck@...nel.org>
> > > Cc: Josh Triplett <josh@...htriplett.org>
> > > Cc: Steven Rostedt <rostedt@...dmis.org>
> > > Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> > > Cc: Lai Jiangshan <jiangshanlai@...il.com>
> > > Cc: Joel Fernandes <joel@...lfernandes.org>
> > > ---
> > >  include/linux/rcu_segcblist.h |  2 ++
> > >  kernel/rcu/rcu_segcblist.c    |  2 ++
> > >  kernel/rcu/rcu_segcblist.h    |  6 ++++++
> > >  kernel/rcu/tree.c             | 21 +++++++--------------
> > >  4 files changed, 17 insertions(+), 14 deletions(-)
> > > 
> > > diff --git a/include/linux/rcu_segcblist.h b/include/linux/rcu_segcblist.h
> > > index b36afe7b22c9..0ced0a0ecbcf 100644
> > > --- a/include/linux/rcu_segcblist.h
> > > +++ b/include/linux/rcu_segcblist.h
> > > @@ -73,7 +73,9 @@ struct rcu_segcblist {
> > >  	long len;
> > >  #endif
> > >  	u8 enabled;
> > > +#ifdef CONFIG_RCU_NOCB_CPU
> > >  	u8 offloaded;
> > > +#endif
> > 
> > Given that this is only one byte and that removing it won't actually
> > save any memory on most architectures, why not just leave it and
> > adjust as shown below?
> 
> Right, the point was to make it private to that config and trigger
> a build error otherwise. But if we have an off case that's fine.
> 
> > 
> > >  };
> > >  
> > >  #define RCU_SEGCBLIST_INITIALIZER(n) \
> > > diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
> > > index 9a0f66133b4b..d8ea2bef5574 100644
> > > --- a/kernel/rcu/rcu_segcblist.c
> > > +++ b/kernel/rcu/rcu_segcblist.c
> > > @@ -166,6 +166,7 @@ void rcu_segcblist_disable(struct rcu_segcblist *rsclp)
> > >  	rsclp->enabled = 0;
> > >  }
> > >  
> > > +#ifdef CONFIG_RCU_NOCB_CPU
> > >  /*
> > >   * Mark the specified rcu_segcblist structure as offloaded.  This
> > >   * structure must be empty.
> > > @@ -174,6 +175,7 @@ void rcu_segcblist_offload(struct rcu_segcblist *rsclp)
> > >  {
> > >  	rsclp->offloaded = 1;
> > >  }
> > > +#endif
> > 
> > Leave this unconditional, as it is nowhere near a fastpath.
> 
> The point was to not raise false hopes to those who want to
> offload when it's not supported.
> 
> Let's perhaps have at least a WARN_ON_ONCE(1) if it is called
> when !CONFIG_RCU_NOCB_CPU ?

Sounds like a good choice to me!

> > >  /*
> > >   * Does the specified rcu_segcblist structure contain callbacks that
> > > diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h
> > > index 5c293afc07b8..4c1503a82492 100644
> > > --- a/kernel/rcu/rcu_segcblist.h
> > > +++ b/kernel/rcu/rcu_segcblist.h
> > > @@ -62,7 +62,11 @@ static inline bool rcu_segcblist_is_enabled(struct rcu_segcblist *rsclp)
> > >  /* Is the specified rcu_segcblist offloaded?  */
> > >  static inline bool rcu_segcblist_is_offloaded(struct rcu_segcblist *rsclp)
> > >  {
> > > +#ifdef CONFIG_RCU_NOCB_CPU
> > >  	return rsclp->offloaded;
> > > +#else
> > > +	return false;
> > > +#endif
> > >  }
> > 
> > Then this can just be:
> > 
> > 	return IS_ENABLED(CONFIG_RCU_NOCB_CPU) && rsclp->offloaded;
> 
> Ok.
> 
> > > @@ -1401,8 +1401,7 @@ static bool __note_gp_changes(struct rcu_node *rnp, struct rcu_data *rdp)
> > >  {
> > >  	bool ret = false;
> > >  	bool need_qs;
> > > -	const bool offloaded = IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
> > > -			       rcu_segcblist_is_offloaded(&rdp->cblist);
> > > +	const bool offloaded = rcu_segcblist_is_offloaded(&rdp->cblist);
> > 
> > The adjustment to rcu_segcblist_is_offloaded() allows this (welcome!)
> > simplification to remain.
> 
> Ok thanks!
> 
> > > @@ -3243,8 +3237,7 @@ static int rcu_pending(int user)
> > >  
> > >  	/* Has RCU gone idle with this CPU needing another grace period? */
> > >  	if (!gp_in_progress && rcu_segcblist_is_enabled(&rdp->cblist) &&
> > > -	    (!IS_ENABLED(CONFIG_RCU_NOCB_CPU) ||
> > > -	     !rcu_segcblist_is_offloaded(&rdp->cblist)) &&
> > > +	    !rcu_segcblist_is_offloaded(&rdp->cblist) &&
> > 
> > Ditto.
> > 
> > As in "Why didn't I do it that way to start with???"  ;-)
> 
> You say that to someone who's too lazy to script short commands typed
> 100 times a day ;-)

;-) ;-) ;-)

							Thanx, Paul