[PATCH v5 6/9] block: Change the runtime power management approach (2/2)

Bart Van Assche
2018-08-08T01:08:12Z

Instead of allowing requests that are not power management requests
to enter the queue in runtime suspended status (RPM_SUSPENDED), make
the blk_get_request() caller block. This change fixes a starvation
issue: it is now guaranteed that power management requests will be
executed no matter how many blk_get_request() callers are waiting.
Instead of maintaining the q->nr_pending counter, rely on
q->q_usage_counter. Call pm_runtime_mark_last_busy() every time a
request finishes instead of only if the queue depth drops to zero.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Jianchao Wang <jianchao.w.wang@oracle.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Cc: Alan Stern <stern@rowland.harvard.edu>
---
block/blk-core.c | 37 ++++++++-----------------------------
block/blk-mq-debugfs.c | 1 -
block/blk-pm.c | 40 +++++++++++++++++++++++++++++++++++-----
block/blk-pm.h | 6 ++----
include/linux/blkdev.h | 1 -
5 files changed, 45 insertions(+), 40 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 179a13be0fca..a2ef253edfbd 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -2737,30 +2737,6 @@ void blk_account_io_done(struct request *req, u64 now)
}
}

-#ifdef CONFIG_PM
-/*
- * Don't process normal requests when queue is suspended
- * or in the process of suspending/resuming
- */
-static bool blk_pm_allow_request(struct request *rq)
-{
- switch (rq->q->rpm_status) {
- case RPM_RESUMING:
- case RPM_SUSPENDING:
- return rq->rq_flags & RQF_PM;
- case RPM_SUSPENDED:
- return false;
- default:
- return true;
- }
-}
-#else
-static bool blk_pm_allow_request(struct request *rq)
-{
- return true;
-}
-#endif
-
void blk_account_io_start(struct request *rq, bool new_io)
{
struct hd_struct *part;
@@ -2806,11 +2782,14 @@ static struct request *elv_next_request(struct request_queue *q)

while (1) {
list_for_each_entry(rq, &q->queue_head, queuelist) {
- if (blk_pm_allow_request(rq))
- return rq;
-
- if (rq->rq_flags & RQF_SOFTBARRIER)
- break;
+#ifdef CONFIG_PM
+ /*
+ * If a request gets queued in state RPM_SUSPENDED
+ * then that's a kernel bug.
+ */
+ WARN_ON_ONCE(q->rpm_status == RPM_SUSPENDED);
+#endif
+ return rq;
}

/*
diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
index a5ea86835fcb..7d74d53dc098 100644
--- a/block/blk-mq-debugfs.c
+++ b/block/blk-mq-debugfs.c
@@ -332,7 +332,6 @@ static const char *const rqf_name[] = {
RQF_NAME(ELVPRIV),
RQF_NAME(IO_STAT),
RQF_NAME(ALLOCED),
- RQF_NAME(PM),
RQF_NAME(HASHED),
RQF_NAME(STATS),
RQF_NAME(SPECIAL_PAYLOAD),
diff --git a/block/blk-pm.c b/block/blk-pm.c
index bf8532da952d..d6b65cef9764 100644
--- a/block/blk-pm.c
+++ b/block/blk-pm.c
@@ -86,14 +86,39 @@ int blk_pre_runtime_suspend(struct request_queue *q)
if (!q->dev)
return ret;

+ WARN_ON_ONCE(q->rpm_status != RPM_ACTIVE);
+
+ blk_set_pm_only(q);
+ /*
+ * This function only gets called if the most recent
+ * pm_request_resume() call occurred at least autosuspend_delay_ms
+ * ago. Since blk_queue_enter() is called by the request allocation
+ * code before pm_request_resume(), if q_usage_counter indicates that
+ * no requests are in flight it is safe to suspend the device.
+ */
+ ret = -EBUSY;
+ if (!percpu_ref_is_in_use(&q->q_usage_counter)) {
+ /*
+ * Switch to preempt-only mode before calling
+ * synchronize_rcu() such that later blk_queue_enter() calls
+ * see the preempt-only state. See also
+ * http://lwn.net/Articles/573497/.
+ */
+ synchronize_rcu();
+ if (!percpu_ref_is_in_use(&q->q_usage_counter))
+ ret = 0;
+ }
+
spin_lock_irq(q->queue_lock);
- if (q->nr_pending) {
- ret = -EBUSY;
+ if (ret < 0)
pm_runtime_mark_last_busy(q->dev);
- } else {
+ else
q->rpm_status = RPM_SUSPENDING;
- }
spin_unlock_irq(q->queue_lock);
+
+ if (ret)
+ blk_clear_pm_only(q);
+
return ret;
}
EXPORT_SYMBOL(blk_pre_runtime_suspend);
@@ -124,6 +149,9 @@ void blk_post_runtime_suspend(struct request_queue *q, int err)
pm_runtime_mark_last_busy(q->dev);
}
spin_unlock_irq(q->queue_lock);
+
+ if (err)
+ blk_clear_pm_only(q);
}
EXPORT_SYMBOL(blk_post_runtime_suspend);

@@ -171,13 +199,15 @@ void blk_post_runtime_resume(struct request_queue *q, int err)
spin_lock_irq(q->queue_lock);
if (!err) {
q->rpm_status = RPM_ACTIVE;
- __blk_run_queue(q);
pm_runtime_mark_last_busy(q->dev);
pm_request_autosuspend(q->dev);
} else {
q->rpm_status = RPM_SUSPENDED;
}
spin_unlock_irq(q->queue_lock);
+
+ if (!err)
+ blk_clear_pm_only(q);
}
EXPORT_SYMBOL(blk_post_runtime_resume);

diff --git a/block/blk-pm.h b/block/blk-pm.h
index 1ffc8ef203ec..fcb507a29e99 100644
--- a/block/blk-pm.h
+++ b/block/blk-pm.h
@@ -8,21 +8,19 @@
#ifdef CONFIG_PM
static inline void blk_pm_requeue_request(struct request *rq)
{
- if (rq->q->dev && !(rq->rq_flags & RQF_PM))
- rq->q->nr_pending--;
}

static inline void blk_pm_add_request(struct request_queue *q,
struct request *rq)
{
- if (q->dev && !(rq->rq_flags & RQF_PM) && q->nr_pending++ == 0 &&
+ if (q->dev && !(rq->rq_flags & RQF_PM) &&
(q->rpm_status == RPM_SUSPENDED || q->rpm_status == RPM_SUSPENDING))
pm_request_resume(q->dev);
}

static inline void blk_pm_put_request(struct request *rq)
{
- if (rq->q->dev && !(rq->rq_flags & RQF_PM) && !--rq->q->nr_pending)
+ if (rq->q->dev && !(rq->rq_flags & RQF_PM))
pm_runtime_mark_last_busy(rq->q->dev);
}
#else
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index cc5ef316eb39..b10787fdc69e 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -549,7 +549,6 @@ struct request_queue {
#ifdef CONFIG_PM
struct device *dev;
int rpm_status;
- unsigned int nr_pending;
#endif

/*
--
2.18.0