Re: [PATCH 04/10] rcu: Implement rcu_segcblist_is_offloaded() config dependent

[Paul E. McKenney]

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@xxxxxxxxxx>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
> Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> ---
>  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?

>  };
>  
>  #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.

>  /*
>   * 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;

>  /*
> @@ -78,7 +82,9 @@ static inline bool rcu_segcblist_restempty(struct rcu_segcblist *rsclp, int seg)
>  void rcu_segcblist_inc_len(struct rcu_segcblist *rsclp);
>  void rcu_segcblist_init(struct rcu_segcblist *rsclp);
>  void rcu_segcblist_disable(struct rcu_segcblist *rsclp);
> +#ifdef CONFIG_RCU_NOCB_CPU
>  void rcu_segcblist_offload(struct rcu_segcblist *rsclp);
> +#endif

This can then remain unconditional.

>  bool rcu_segcblist_ready_cbs(struct rcu_segcblist *rsclp);
>  bool rcu_segcblist_pend_cbs(struct rcu_segcblist *rsclp);
>  struct rcu_head *rcu_segcblist_first_cb(struct rcu_segcblist *rsclp);
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index d9a49cd6065a..804cf7dfff03 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -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.

>  	raw_lockdep_assert_held_rcu_node(rnp);
>  
> @@ -1790,8 +1789,7 @@ static void rcu_gp_cleanup(void)
>  		needgp = true;
>  	}
>  	/* Advance CBs to reduce false positives below. */
> -	offloaded = IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
> -		    rcu_segcblist_is_offloaded(&rdp->cblist);
> +	offloaded = rcu_segcblist_is_offloaded(&rdp->cblist);

And here as well.

>  	if ((offloaded || !rcu_accelerate_cbs(rnp, rdp)) && needgp) {
>  		WRITE_ONCE(rcu_state.gp_flags, RCU_GP_FLAG_INIT);
>  		WRITE_ONCE(rcu_state.gp_req_activity, jiffies);
> @@ -1985,8 +1983,7 @@ rcu_report_qs_rdp(int cpu, struct rcu_data *rdp)
>  	unsigned long flags;
>  	unsigned long mask;
>  	bool needwake = false;
> -	const bool offloaded = IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
> -			       rcu_segcblist_is_offloaded(&rdp->cblist);
> +	const bool offloaded = rcu_segcblist_is_offloaded(&rdp->cblist);

Ditto.

>  	struct rcu_node *rnp;
>  
>  	rnp = rdp->mynode;
> @@ -2153,8 +2150,7 @@ int rcutree_dead_cpu(unsigned int cpu)
>  static void rcu_do_batch(struct rcu_data *rdp)
>  {
>  	unsigned long flags;
> -	const bool offloaded = IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
> -			       rcu_segcblist_is_offloaded(&rdp->cblist);
> +	const bool offloaded = rcu_segcblist_is_offloaded(&rdp->cblist);

Ditto.

>  	struct rcu_head *rhp;
>  	struct rcu_cblist rcl = RCU_CBLIST_INITIALIZER(rcl);
>  	long bl, count;
> @@ -2397,8 +2393,7 @@ static __latent_entropy void rcu_core(void)
>  	unsigned long flags;
>  	struct rcu_data *rdp = raw_cpu_ptr(&rcu_data);
>  	struct rcu_node *rnp = rdp->mynode;
> -	const bool offloaded = IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
> -			       rcu_segcblist_is_offloaded(&rdp->cblist);
> +	const bool offloaded = rcu_segcblist_is_offloaded(&rdp->cblist);

Ditto.

>  
>  	if (cpu_is_offline(smp_processor_id()))
>  		return;
> @@ -2695,8 +2690,7 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func)
>  				   rcu_segcblist_n_cbs(&rdp->cblist));
>  
>  	/* Go handle any RCU core processing required. */
> -	if (IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
> -	    unlikely(rcu_segcblist_is_offloaded(&rdp->cblist))) {
> +	if (unlikely(rcu_segcblist_is_offloaded(&rdp->cblist))) {

Ditto.

>  		__call_rcu_nocb_wake(rdp, was_alldone, flags); /* unlocks */
>  	} else {
>  		__call_rcu_core(rdp, head, flags);
> @@ -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???"  ;-)

							Thanx, Paul

>  	    !rcu_segcblist_restempty(&rdp->cblist, RCU_NEXT_READY_TAIL))
>  		return 1;
>  
> -- 
> 2.25.0
>