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

Ming Lei
2018-08-08T08:50:18Z

On Tue, Aug 07, 2018 at 03:51:30PM -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

Looks not see the related change which blocks blk_get_request() in
this patchset.

BTW, blk_pm_add_request() won't block since it uses the async version
of runtime resume.

> 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;
> + }
> +

In blk_queue_enter(), V5 starts to allow all RQF_PREEMPT requests
to enter queue even though pm_only is set. That means any scsi_execute()
may issue a new command to host just after the above percpu_ref_is_in_use()
returns 0, meantime the suspend is in-progress.

This case is illegal given RQF_PM is the only kind of request which can be
issued to hardware during suspend.

Thanks,
Ming