Re: [PATCH v9 1/7] rcu/tree: Make rcu_do_batch count how many callbacks were executed

["Paul E. McKenney" <paulmck@kernel.org>]

On Tue, Nov 03, 2020 at 09:25:57AM -0500, 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 how 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.
> 
> Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
> Reviewed-by: Neeraj Upadhyay <neeraju@codeaurora.org>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

Queued for testing and further review, thank you all!

I will be pulling these in as I look them over, and running tests
in between times, so this will take some time.

As usual, I could not resist editing the commit log, so please
see below to check whether I messed something up.

							Thanx, Paul

------------------------------------------------------------------------

commit d77ef2684cbff728c14b3c84a356139c52ca3a5e
Author: Joel Fernandes (Google) <joel@joelfernandes.org>
Date:   Tue Nov 3 09:25:57 2020 -0500

    rcu/tree: Make rcu_do_batch count how many callbacks were executed
    
    The rcu_do_batch() function extracts the ready-to-invoke callbacks
    from the rcu_segcblist located in the ->cblist field of the current
    CPU's rcu_data structure.  These callbacks are first moved to a local
    (unsegmented) rcu_cblist.  The rcu_do_batch() function then uses this
    rcu_cblist's ->len field to count how many CBs it has invoked, but it
    does so by counting that field down from zero.  Finally, this function
    negates the value in this ->len field (resulting in a positive number)
    and subtracts the result from the ->len field of the current CPU's
    ->cblist field.
    
    Except that it is sometimes necessary for rcu_do_batch() to stop invoking
    callbacks mid-stream, despite there being more ready to invoke, for
    example, if a high-priority task wakes up.  In this case the remaining
    not-yet-invoked callbacks are requeued back onto the CPU's ->cblist,
    but remain in the ready-to-invoke segment of that list.  As above, the
    negative of the local rcu_cblist's ->len field is still subtracted from
    the ->len field of the current CPU's ->cblist field.
    
    The design of counting down from 0 is confusing and error-prone, plus
    use of a positive count will make it easier to provide a uniform and
    consistent API to deal with the per-segment counts that are added
    later in this series.  For example, rcu_segcblist_extract_done_cbs()
    can unconditionally populate the resulting unsegmented list's ->len
    field during extraction.
    
    This commit therefore explicitly counts how many callbacks were executed
    in rcu_do_batch() itself, counting up from zero, and then uses that
    to update the per-CPU segcb list's ->len field, without relying on the
    downcounting of rcl->len from zero.
    
    Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
    Reviewed-by: Neeraj Upadhyay <neeraju@codeaurora.org>
    Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
index 2d2a6b6b9..bb246d8 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 492262b..1d2d614 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 b6ec565..c4035e7 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2436,7 +2436,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
 	const bool offloaded = 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. */
@@ -2481,6 +2481,7 @@ 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);
@@ -2494,15 +2495,14 @@ 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;
 		if (unlikely(tlimit)) {
 			/* only call local_clock() every 32 callbacks */
-			if (likely((-rcl.len & 31) || local_clock() < tlimit))
+			if (likely((count & 31) || local_clock() < tlimit))
 				continue;
 			/* Exceeded the time limit, so leave. */
 			break;
@@ -2519,7 +2519,6 @@ 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());
@@ -2527,7 +2526,7 @@ 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);