On Wed, Jul 18, 2018 at 7:49 AM, Bart Van Assche <bart.vanassche@xxxxxxx> 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.
> 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.
> Use RQF_PREEMPT to mark power management requests instead of RQF_PM.
> This is safe because the power management core serializes system-wide
> suspend/resume and runtime power management state changes.
>
> Signed-off-by: Bart Van Assche <bart.vanassche@xxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Ming Lei <ming.lei@xxxxxxxxxx>
> Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> Cc: Johannes Thumshirn <jthumshirn@xxxxxxx>
> ---
> block/blk-core.c | 56 +++++++++++++++++----------------------
> block/blk-mq-debugfs.c | 1 -
> block/blk-mq.c | 12 ++++-----
> block/elevator.c | 11 +-------
> drivers/scsi/sd.c | 4 +--
> drivers/scsi/ufs/ufshcd.c | 10 +++----
> include/linux/blk-mq.h | 1 +
> include/linux/blkdev.h | 7 ++---
> 8 files changed, 41 insertions(+), 61 deletions(-)
>
> diff --git a/block/blk-core.c b/block/blk-core.c
> index f84a9b7b6f5a..65d7f27c8c22 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -1719,7 +1719,7 @@ EXPORT_SYMBOL_GPL(part_round_stats);
> #ifdef CONFIG_PM
> static 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_PREEMPT))
> pm_runtime_mark_last_busy(rq->q->dev);
> }
> #else
> @@ -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;
> - }
> -
> - 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 +2782,12 @@ 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;
> + /*
> + * If a request gets queued in state RPM_SUSPENDED
> + * then that's a kernel bug.
> + */
> + WARN_ON_ONCE(q->rpm_status == RPM_SUSPENDED);
> + return rq;
> }
>
> /*
> @@ -3801,8 +3778,11 @@ int blk_pre_runtime_suspend(struct request_queue *q)
> if (!q->dev)
> return ret;
>
> + blk_set_preempt_only(q);
> + blk_freeze_queue_start(q);
> +
> spin_lock_irq(q->queue_lock);
> - if (q->nr_pending) {
> + if (!percpu_ref_is_zero(&q->q_usage_counter)) {
This way can't work reliably because the percpu ref isn't in atomic mode
yet after blk_freeze_queue_start() returns, then percpu_ref_is_zero() won't
see accurate value of the counter, finally the device may be put down before
in-flight requests are completed by hardware.
Thanks,
Ming Lei
