Re: [PATCH v6 1/4] rcu/tree: Make rcu_do_batch count how many callbacks were executed

[Neeraj Upadhyay]

On 9/23/2020 8:52 PM, Joel Fernandes (Google) wrote:
> Currently, rcu_do_batch() depends on the unsegmented callback list's len field
> to know how many CBs are executed. This fields counts down from 0 as CBs are
> dequeued.  It is possible that all CBs could not be run because of reaching
> limits in which case the remaining unexecuted callbacks are requeued in the
> CPU's segcblist.
>
> The number of callbacks that were not requeued are then the negative count (how
> many CBs were run) stored in the rcl->len which has been counting down on every
> dequeue. This negative count is then added to the per-cpu segmented callback
> list's to correct its count.
>
> Such a design works against future efforts to track the length of each segment
> of the segmented callback list. The reason is because
> rcu_segcblist_extract_done_cbs() will be populating the unsegmented callback
> list's length field (rcl->len) during extraction.
>
> Also, the design of counting down from 0 is confusing and error-prone IMHO.
>
> This commit therefore explicitly counts have many callbacks were executed in
> rcu_do_batch() itself, and uses that to update the per-CPU segcb list's ->len
> field, without relying on the negativity of rcl->len.
>
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> ---
>   kernel/rcu/rcu_segcblist.c | 2 +-
>   kernel/rcu/rcu_segcblist.h | 1 +
>   kernel/rcu/tree.c          | 9 ++++-----
>   3 files changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
> index 2d2a6b6b9dfb..bb246d8c6ef1 100644
> --- a/kernel/rcu/rcu_segcblist.c
> +++ b/kernel/rcu/rcu_segcblist.c
> @@ -95,7 +95,7 @@ static void rcu_segcblist_set_len(struct rcu_segcblist *rsclp, long v)
>    * This increase is fully ordered with respect to the callers accesses
>    * both before and after.
>    */
> -static void rcu_segcblist_add_len(struct rcu_segcblist *rsclp, long v)
> +void rcu_segcblist_add_len(struct rcu_segcblist *rsclp, long v)
>   {
>   #ifdef CONFIG_RCU_NOCB_CPU
>   	smp_mb__before_atomic(); /* Up to the caller! */
> diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h
> index 5c293afc07b8..b90725f81d77 100644
> --- a/kernel/rcu/rcu_segcblist.h
> +++ b/kernel/rcu/rcu_segcblist.h
> @@ -76,6 +76,7 @@ static inline bool rcu_segcblist_restempty(struct rcu_segcblist *rsclp, int seg)
>   }
>   
>   void rcu_segcblist_inc_len(struct rcu_segcblist *rsclp);
> +void rcu_segcblist_add_len(struct rcu_segcblist *rsclp, long v);
>   void rcu_segcblist_init(struct rcu_segcblist *rsclp);
>   void rcu_segcblist_disable(struct rcu_segcblist *rsclp);
>   void rcu_segcblist_offload(struct rcu_segcblist *rsclp);
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 7623128d0020..50af465729f4 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2427,7 +2427,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
>   			       rcu_segcblist_is_offloaded(&rdp->cblist);
>   	struct rcu_head *rhp;
>   	struct rcu_cblist rcl = RCU_CBLIST_INITIALIZER(rcl);
> -	long bl, count;
> +	long bl, count = 0;
>   	long pending, tlimit = 0;
>   
>   	/* If no callbacks are ready, just return. */
> @@ -2472,6 +2472,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
>   	for (; rhp; rhp = rcu_cblist_dequeue(&rcl)) {
>   		rcu_callback_t f;
>   
> +		count++;
>   		debug_rcu_head_unqueue(rhp);
>   
>   		rcu_lock_acquire(&rcu_callback_map);
> @@ -2485,9 +2486,8 @@ static void rcu_do_batch(struct rcu_data *rdp)
>   
>   		/*
>   		 * Stop only if limit reached and CPU has something to do.
> -		 * Note: The rcl structure counts down from zero.
>   		 */
> -		if (-rcl.len >= bl && !offloaded &&
> +		if (count >= bl && !offloaded &&
>   		    (need_resched() ||
>   		     (!is_idle_task(current) && !rcu_is_callbacks_kthread())))
>   			break;

Update below usage of -rcl.len also?

if (likely((-rcl.len & 31) || local_clock() < tlimit))


Thanks
Neeraj

> @@ -2510,7 +2510,6 @@ static void rcu_do_batch(struct rcu_data *rdp)
>   
>   	local_irq_save(flags);
>   	rcu_nocb_lock(rdp);
> -	count = -rcl.len;
>   	rdp->n_cbs_invoked += count;
>   	trace_rcu_batch_end(rcu_state.name, count, !!rcl.head, need_resched(),
>   			    is_idle_task(current), rcu_is_callbacks_kthread());
> @@ -2518,7 +2517,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
>   	/* Update counts and requeue any remaining callbacks. */
>   	rcu_segcblist_insert_done_cbs(&rdp->cblist, &rcl);
>   	smp_mb(); /* List handling before counting for rcu_barrier(). */
> -	rcu_segcblist_insert_count(&rdp->cblist, &rcl);
> +	rcu_segcblist_add_len(&rdp->cblist, -count);
>   
>   	/* Reinstate batch limit if we have worked down the excess. */
>   	count = rcu_segcblist_n_cbs(&rdp->cblist);

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