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

Paul E. McKenney
2020-05-13T18:20:33Z

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@kernel.org>
> Cc: Paul E. McKenney <paulmck@kernel.org>
> Cc: Josh Triplett <josh@joshtriplett.org>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> Cc: Joel Fernandes <joel@joelfernandes.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?

> };
>
> #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
>