[PATCH v6 11/12] block: Change the runtime power management approach (2/2)

Bart Van Assche
2018-08-09T19:41:48Z

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-pm.c | 72 ++++++++++++++++++++++++++++++++++++++++++++----
2 files changed, 75 insertions(+), 34 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index f30545fb2de2..b0bb6b5320fe 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -2772,30 +2772,6 @@ 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;
@@ -2841,11 +2817,14 @@ 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-pm.c b/block/blk-pm.c
index bf8532da952d..977beffdccd2 100644
--- a/block/blk-pm.c
+++ b/block/blk-pm.c
@@ -1,8 +1,11 @@
// SPDX-License-Identifier: GPL-2.0

+#include <linux/blk-mq.h>
#include <linux/blk-pm.h>
#include <linux/blkdev.h>
#include <linux/pm_runtime.h>
+#include "blk-mq.h"
+#include "blk-mq-tag.h"

/**
* blk_pm_runtime_init - Block layer runtime PM initialization routine
@@ -58,6 +61,36 @@ void blk_pm_runtime_exit(struct request_queue *q)
}
EXPORT_SYMBOL(blk_pm_runtime_exit);

+struct in_flight_data {
+ struct request_queue *q;
+ int in_flight;
+};
+
+static void blk_count_in_flight(struct blk_mq_hw_ctx *hctx, struct request *rq,
+ void *priv, bool reserved)
+{
+ struct in_flight_data *in_flight = priv;
+
+ if (rq->q == in_flight->q)
+ in_flight->in_flight++;
+}
+
+/*
+ * Count the number of requests that are in flight for request queue @q. Use
+ * @q->nr_pending for legacy queues. Iterate over the tag set for blk-mq
+ * queues. Use blk_mq_queue_tag_busy_iter() instead of
+ * blk_mq_tagset_busy_iter() because the latter only considers requests that
+ * have already been started.
+ */
+static int blk_requests_in_flight(struct request_queue *q)
+{
+ struct in_flight_data in_flight = { .q = q };
+
+ if (q->mq_ops)
+ blk_mq_queue_tag_busy_iter(q, blk_count_in_flight, &in_flight);
+ return q->nr_pending + in_flight.in_flight;
+}
+
/**
* blk_pre_runtime_suspend - Pre runtime suspend check
* @q: the queue of the device
@@ -86,14 +119,38 @@ 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 no requests have a tag assigned
+ * it is safe to suspend the device.
+ */
+ ret = -EBUSY;
+ if (blk_requests_in_flight(q) == 0) {
+ /*
+ * Call synchronize_rcu() such that later blk_queue_enter()
+ * calls see the preempt-only state. See also
+ * http://lwn.net/Articles/573497/.
+ */
+ synchronize_rcu();
+ if (blk_requests_in_flight(q) == 0)
+ 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 +181,9 @@ 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 +231,15 @@ 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);

--
2.18.0