Re: [PATCH v4] sched/wake_q: Reduce reference counting for special users
From: Davidlohr Bueso
Date: Mon Jan 21 2019 - 11:02:33 EST
Hi - considering that the wake_q patches were picked up for tip/urgent, can
this one make it in as well?
Thanks,
Davidlohr
On Tue, 18 Dec 2018, Waiman Long wrote:
On 12/18/2018 02:53 PM, Davidlohr Bueso wrote:
Some users, specifically futexes and rwsems, required fixes
that allowed the callers to be safe when wakeups occur before
they are expected by wake_up_q(). Such scenarios also play
games and rely on reference counting, and until now were
pivoting on wake_q doing it. With the wake_q_add() call being
moved down, this can no longer be the case. As such we end up
with a a double task refcounting overhead; and these callers
care enough about this (being rather core-ish).
This patch introduces a wake_q_add_safe() call that serves
for callers that have already done refcounting and therefore the
task is 'safe' from wake_q point of view (int that it requires
reference throughout the entire queue/>wakeup cycle). In the one
case it has internal reference counting, in the other case it
consumes the reference counting.
Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>
---
- Changes from v3: fixed wake_q_add_safe. While previous version
had been tested with a bootup, the failed cmpxchg path obviously
hadn't been exercised. Sorry about the noise.
include/linux/sched/wake_q.h | 4 +--
kernel/futex.c | 3 +--
kernel/locking/rwsem-xadd.c | 4 +--
kernel/sched/core.c | 60
++++++++++++++++++++++++++++++++------------
4 files changed, 48 insertions(+), 23 deletions(-)
diff --git a/include/linux/sched/wake_q.h b/include/linux/sched/wake_q.h
index 545f37138057..ad826d2a4557 100644
--- a/include/linux/sched/wake_q.h
+++ b/include/linux/sched/wake_q.h
@@ -51,8 +51,8 @@ static inline void wake_q_init(struct wake_q_head
*head)
head->lastp = &head->first;
}
-extern void wake_q_add(struct wake_q_head *head,
- struct task_struct *task);
+extern void wake_q_add(struct wake_q_head *head, struct task_struct
*task);
+extern void wake_q_add_safe(struct wake_q_head *head, struct
task_struct *task);
extern void wake_up_q(struct wake_q_head *head);
#endif /* _LINUX_SCHED_WAKE_Q_H */
diff --git a/kernel/futex.c b/kernel/futex.c
index d14971f6ed3d..6218d98f649b 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1402,8 +1402,7 @@ static void mark_wake_futex(struct wake_q_head
*wake_q, struct futex_q *q)
* Queue the task for later wakeup for after we've released
* the hb->lock. wake_q_add() grabs reference to p.
*/
- wake_q_add(wake_q, p);
- put_task_struct(p);
+ wake_q_add_safe(wake_q, p);
}
/*
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 50d9af615dc4..fbe96341beee 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -211,9 +211,7 @@ static void __rwsem_mark_wake(struct rw_semaphore
*sem,
* Ensure issuing the wakeup (either by us or someone else)
* after setting the reader waiter to nil.
*/
- wake_q_add(wake_q, tsk);
- /* wake_q_add() already take the task ref */
- put_task_struct(tsk);
+ wake_q_add_safe(wake_q, tsk);
}
adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d740d7a3608d..be977df66a21 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -396,19 +396,7 @@ static bool set_nr_if_polling(struct task_struct *p)
#endif
#endif
-/**
- * wake_q_add() - queue a wakeup for 'later' waking.
- * @head: the wake_q_head to add @task to
- * @task: the task to queue for 'later' wakeup
- *
- * Queue a task for later wakeup, most likely by the wake_up_q() call
in the
- * same context, _HOWEVER_ this is not guaranteed, the wakeup can come
- * instantly.
- *
- * This function must be used as-if it were wake_up_process(); IOW
the task
- * must be ready to be woken at this location.
- */
-void wake_q_add(struct wake_q_head *head, struct task_struct *task)
+static bool __wake_q_add(struct wake_q_head *head, struct task_struct
*task)
{
struct wake_q_node *node = &task->wake_q;
@@ -422,15 +410,55 @@ void wake_q_add(struct wake_q_head *head, struct
task_struct *task)
*/
smp_mb__before_atomic();
if (unlikely(cmpxchg_relaxed(&node->next, NULL, WAKE_Q_TAIL)))
- return;
-
- get_task_struct(task);
+ return false;
/*
* The head is context local, there can be no concurrency.
*/
*head->lastp = node;
head->lastp = &node->next;
+ return true;
+}
+
+/**
+ * wake_q_add() - queue a wakeup for 'later' waking.
+ * @head: the wake_q_head to add @task to
+ * @task: the task to queue for 'later' wakeup
+ *
+ * Queue a task for later wakeup, most likely by the wake_up_q() call
in the
+ * same context, _HOWEVER_ this is not guaranteed, the wakeup can come
+ * instantly.
+ *
+ * This function must be used as-if it were wake_up_process(); IOW
the task
+ * must be ready to be woken at this location.
+ */
+void wake_q_add(struct wake_q_head *head, struct task_struct *task)
+{
+ if (__wake_q_add(head, task))
+ get_task_struct(task);
+}
+
+/**
+ * wake_q_add_safe() - safely queue a wakeup for 'later' waking.
+ * @head: the wake_q_head to add @task to
+ * @task: the task to queue for 'later' wakeup
+ *
+ * Queue a task for later wakeup, most likely by the wake_up_q() call
in the
+ * same context, _HOWEVER_ this is not guaranteed, the wakeup can come
+ * instantly.
+ *
+ * This function must be used as-if it were wake_up_process(); IOW
the task
+ * must be ready to be woken at this location.
+ *
+ * This function is essentially a task-safe equivalent to
wake_q_add(). Callers
+ * that already hold reference to @task can call the 'safe' version
and trust
+ * wake_q to do the right thing depending whether or not the @task is
already
+ * queued for wakeup.
+ */
+void wake_q_add_safe(struct wake_q_head *head, struct task_struct *task)
+{
+ if (!__wake_q_add(head, task))
+ put_task_struct(task);
}
void wake_up_q(struct wake_q_head *head)
Acked-by: Waiman Long <longman@xxxxxxxxxx>