On Wed, Sep 19, 2018 at 03:45:29PM -0700, Bart Van Assche wrote:
> 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.
> For blk-mq, 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 <bvanassche@xxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Ming Lei <ming.lei@xxxxxxxxxx>
> Cc: Jianchao Wang <jianchao.w.wang@xxxxxxxxxx>
> Cc: Hannes Reinecke <hare@xxxxxxxx>
> Cc: Johannes Thumshirn <jthumshirn@xxxxxxx>
> Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> ---
> block/blk-core.c | 37 +++++-----------------
> block/blk-pm.c | 81 +++++++++++++++++++++++++++++++++++++++++++++---
> 2 files changed, 84 insertions(+), 34 deletions(-)
>
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 18b874d5c9c9..ae092ca121d5 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -2747,30 +2747,6 @@ 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;
> @@ -2816,11 +2792,14 @@ 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 9b636960d285..3ad5a7334baa 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
> @@ -40,6 +43,34 @@ void blk_pm_runtime_init(struct request_queue *q, struct device *dev)
> }
> EXPORT_SYMBOL(blk_pm_runtime_init);
>
> +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.
> + */
> +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_rq_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
> @@ -68,14 +99,49 @@ int blk_pre_runtime_suspend(struct request_queue *q)
> if (!q->dev)
> return ret;
>
> + WARN_ON_ONCE(q->rpm_status != RPM_ACTIVE);
> +
> + /*
> + * Increase the pm_only counter before checking whether any
> + * non-PM blk_queue_enter() calls are in progress to avoid that any
> + * new non-PM blk_queue_enter() calls succeed before the pm_only
> + * counter is decreased again.
> + */
> + blk_set_pm_only(q);
> + /*
> + * This function only gets called if the most recent request activity
> + * 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) {
> + blk_freeze_queue_start(q);
> + /*
> + * Freezing a queue starts a transition of the queue
> + * usage counter to atomic mode. Wait until atomic
> + * mode has been reached. This involves calling
> + * call_rcu(). That call guarantees that later
> + * blk_queue_enter() calls see the pm-only state. See
> + * also http://lwn.net/Articles/573497/.
> + */
> + percpu_ref_switch_to_atomic_sync(&q->q_usage_counter);
> + if (percpu_ref_is_zero(&q->q_usage_counter))
> + ret = 0;
> + blk_mq_unfreeze_queue(q);
Tejun doesn't agree on this kind of usage yet, so the ref has to be
dropped before calling blk_mq_unfreeze_queue().
Also, this way still can't address the race in the following link:
https://marc.info/?l=linux-block&m=153732992701093&w=2
Thanks,
Ming
