[PATCH 4/6] workqueue: Mark barrier work with WORK_STRUCT_INACTIVE

From: Lai Jiangshan
Date: Tue Aug 17 2021 - 07:18:24 EST

From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>

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
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@xxxxxxxxxxxxx/
Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
kernel/workqueue.c | 42 ++++++++++++++++++++++++++++++++++--------
1 file changed, 34 insertions(+), 8 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 55fc2d1688d9..1b2792b397f0 100644
--- a/kernel/workqueue.c
+++ b/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
+ *
+ * All work items marked with WORK_STRUCT_INACTIVE do not participate
+ * in pwq->nr_active and all work items in pwq->inactive_works are
+ * 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_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,

+ * 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
@@ -2676,6 +2699,9 @@ 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.