workqueue: Mark barrier work with WORK_STRUCT_INACTIVE

Currently, WORK_NO_COLOR has two meanings:
	Not participate in flushing
	Not participate in nr_active

And only non-barrier work items are marked with WORK_STRUCT_INACTIVE
when they are in inactive_works list.  The barrier work items are not
marked INACTIVE even linked in inactive_works list since these tail
items are always moved together with the head work item.

These definitions are simple, clean and practical. (Except a small
blemish that only the first meaning of WORK_NO_COLOR is documented in
include/linux/workqueue.h while both meanings are in workqueue.c)

But dual-purpose WORK_NO_COLOR used for barrier work items has proven to
be problematical[1].  Only the second purpose is obligatory.  So we plan
to make barrier work items participate in flushing but keep them still
not participating in nr_active.

So the plan is to mark barrier work items inactive without using
WORK_NO_COLOR in this patch so that we can assign a flushing color to
them in next patch.

The reasonable way is to add or reuse a bit in work data of the work
item.  But adding a bit will double the size of pool_workqueue.

Currently, WORK_STRUCT_INACTIVE is only used in try_to_grab_pending()
for user-queued work items and try_to_grab_pending() can't work for
barrier work items.  So we extend WORK_STRUCT_INACTIVE to also mark
barrier work items no matter which list they are in because we don't
need to determind which list a barrier work item is in.

So the meaning of WORK_STRUCT_INACTIVE becomes just "the work items don't
participate in nr_active" (no matter whether it is a barrier work item or
a user-queued work item).  And WORK_STRUCT_INACTIVE for user-queued work
items means they are in inactive_works list.

This patch does it by setting WORK_STRUCT_INACTIVE for barrier work items
in insert_wq_barrier() and checking WORK_STRUCT_INACTIVE first in
pwq_dec_nr_in_flight().  And the meaning of WORK_NO_COLOR is reduced to
only "not participating in flushing".

There is no functionality change intended in this patch.  Because
WORK_NO_COLOR+WORK_STRUCT_INACTIVE represents the previous WORK_NO_COLOR
in meaning and try_to_grab_pending() doesn't use for barrier work items
and avoids being confused by this extended WORK_STRUCT_INACTIVE.

A bunch of comment for nr_active & WORK_STRUCT_INACTIVE is also added for
documenting how WORK_STRUCT_INACTIVE works in nr_active management.

[1]: https://lore.kernel.org/lkml/20210812083814.32453-1-lizhe.67@bytedance.com/Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -205,6 +205,23 @@ struct pool_workqueue {
 	int			refcnt;		/* L: reference count */
 	int			nr_in_flight[WORK_NR_COLORS];
 						/* L: nr of in_flight works */
+
+	/*
+	 * nr_active management and WORK_STRUCT_INACTIVE:
+	 *
+	 * When pwq->nr_active >= max_active, new work item is queued to
+	 * pwq->inactive_works instead of pool->worklist and marked with
+	 * WORK_STRUCT_INACTIVE.
+	 *
+	 * All work items marked with WORK_STRUCT_INACTIVE do not participate
+	 * in pwq->nr_active and all work items in pwq->inactive_works are
+	 * marked with WORK_STRUCT_INACTIVE.  But not all WORK_STRUCT_INACTIVE
+	 * work items are in pwq->inactive_works.  Some of them are ready to
+	 * run in pool->worklist or worker->scheduled.  Those work itmes are
+	 * only struct wq_barrier which is used for flush_work() and should
+	 * not participate in pwq->nr_active.  For non-barrier work item, it
+	 * is marked with WORK_STRUCT_INACTIVE iff it is in pwq->inactive_works.
+	 */
 	int			nr_active;	/* L: nr of active works */
 	int			max_active;	/* L: max active works */
 	struct list_head	inactive_works;	/* L: inactive works */
@@ -1171,19 +1188,21 @@ static void pwq_dec_nr_in_flight(struct pool_workqueue *pwq, unsigned long work_
 {
 	int color = get_work_color(work_data);
 
-	/* uncolored work items don't participate in flushing or nr_active */
+	if (!(work_data & WORK_STRUCT_INACTIVE)) {
+		pwq->nr_active--;
+		if (!list_empty(&pwq->inactive_works)) {
+			/* one down, submit an inactive one */
+			if (pwq->nr_active < pwq->max_active)
+				pwq_activate_first_inactive(pwq);
+		}
+	}
+
+	/* uncolored work items don't participate in flushing */
 	if (color == WORK_NO_COLOR)
 		goto out_put;
 
 	pwq->nr_in_flight[color]--;
 
-	pwq->nr_active--;
-	if (!list_empty(&pwq->inactive_works)) {
-		/* one down, submit an inactive one */
-		if (pwq->nr_active < pwq->max_active)
-			pwq_activate_first_inactive(pwq);
-	}
-
 	/* is flush in progress and are we at the flushing tip? */
 	if (likely(pwq->flush_color != color))
 		goto out_put;
@@ -1283,6 +1302,10 @@ static int try_to_grab_pending(struct work_struct *work, bool is_dwork,
 		debug_work_deactivate(work);
 
 		/*
+		 * A cancelable inactive work item must be in the
+		 * pwq->inactive_works since a queued barrier can't be
+		 * canceled (see the comments in insert_wq_barrier()).
+		 *
 		 * An inactive work item cannot be grabbed directly because
 		 * it might have linked NO_COLOR work items which, if left
 		 * on the inactive_works list, will confuse pwq->nr_active
@@ -2675,6 +2698,9 @@ static void insert_wq_barrier(struct pool_workqueue *pwq,
 
 	barr->task = current;
 
+	/* The barrier work item does not participate in pwq->nr_active. */
+	work_flags |= WORK_STRUCT_INACTIVE;
+
 	/*
 	 * If @target is currently being executed, schedule the
 	 * barrier to the worker; otherwise, put it after @target.