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

Paul E. McKenney
2020-11-04T00:05:59Z

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);