Re: [PATCH 3/4] blk-mq: implement hybrid poll mode for sync O_DIRECT

From: Ming Lei
Date: Thu Nov 03 2016 - 08:27:50 EST

On Wed, Nov 2, 2016 at 5:05 AM, Jens Axboe <axboe@xxxxxx> wrote:
> This patch enables a hybrid polling mode. Instead of polling after IO
> submission, we can induce an artificial delay, and then poll after that.
> For example, if the IO is presumed to complete in 8 usecs from now, we
> can sleep for 4 usecs, wake up, and then do our polling. This still puts

I guess in reality it isn't easy to figure a perfect poll time:

- for one driver, different CPU and different drive/disk may cause different
completion time

- for requests with different size, the completion time can be different too

Is there one way to figure out the poll time automatically?

> a sleep/wakeup cycle in the IO path, but instead of the wakeup happening
> after the IO has completed, it'll happen before. With this hybrid
> scheme, we can achieve big latency reductions while still using the same
> (or less) amount of CPU.
>
> Signed-off-by: Jens Axboe <axboe@xxxxxx>
> ---
> block/blk-mq.c | 38 ++++++++++++++++++++++++++++++++++++++
> block/blk-sysfs.c | 29 +++++++++++++++++++++++++++++
> block/blk.h | 1 +
> include/linux/blkdev.h | 1 +
> 4 files changed, 69 insertions(+)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 4ef35588c299..caa55bec9411 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -302,6 +302,7 @@ static void __blk_mq_free_request(struct blk_mq_hw_ctx *hctx,
> rq->rq_flags = 0;
>
> clear_bit(REQ_ATOM_STARTED, &rq->atomic_flags);
> + clear_bit(REQ_ATOM_POLL_SLEPT, &rq->atomic_flags);
> blk_mq_put_tag(hctx, ctx, tag);
> blk_queue_exit(q);
> }
> @@ -2352,11 +2353,48 @@ void blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, int nr_hw_queues)
> }
> EXPORT_SYMBOL_GPL(blk_mq_update_nr_hw_queues);
>
> +static void blk_mq_poll_hybrid_sleep(struct request_queue *q,
> + struct request *rq)
> +{
> + struct hrtimer_sleeper hs;
> + ktime_t kt;
> +
> + if (!q->poll_nsec || test_bit(REQ_ATOM_POLL_SLEPT, &rq->atomic_flags))
> + return;
> +
> + set_bit(REQ_ATOM_POLL_SLEPT, &rq->atomic_flags);
> +
> + /*
> + * This will be replaced with the stats tracking code, using
> + * 'avg_completion_time / 2' as the pre-sleep target.
> + */
> + kt = ktime_set(0, q->poll_nsec);
> +
> + hrtimer_init_on_stack(&hs.timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
> + hrtimer_set_expires(&hs.timer, kt);
> +
> + hrtimer_init_sleeper(&hs, current);
> + do {
> + if (test_bit(REQ_ATOM_COMPLETE, &rq->atomic_flags))
> + break;
> + set_current_state(TASK_INTERRUPTIBLE);
> + hrtimer_start_expires(&hs.timer, HRTIMER_MODE_REL);
> + if (hs.task)
> + io_schedule();
> + hrtimer_cancel(&hs.timer);
> + } while (hs.task && !signal_pending(current));
> +
> + __set_current_state(TASK_RUNNING);
> + destroy_hrtimer_on_stack(&hs.timer);
> +}
> +
> bool blk_mq_poll(struct blk_mq_hw_ctx *hctx, struct request *rq)
> {
> struct request_queue *q = hctx->queue;
> long state;
>
> + blk_mq_poll_hybrid_sleep(q, rq);
> +
> hctx->poll_considered++;
>
> state = current->state;
> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> index 5bb4648f434a..467b81c6713c 100644
> --- a/block/blk-sysfs.c
> +++ b/block/blk-sysfs.c
> @@ -336,6 +336,28 @@ queue_rq_affinity_store(struct request_queue *q, const char *page, size_t count)
> return ret;
> }
>
> +static ssize_t queue_poll_delay_show(struct request_queue *q, char *page)
> +{
> + return queue_var_show(q->poll_nsec / 1000, page);
> +}
> +
> +static ssize_t queue_poll_delay_store(struct request_queue *q, const char *page,
> + size_t count)
> +{
> + unsigned long poll_usec;
> + ssize_t ret;
> +
> + if (!q->mq_ops || !q->mq_ops->poll)
> + return -EINVAL;
> +
> + ret = queue_var_store(&poll_usec, page, count);
> + if (ret < 0)
> + return ret;
> +
> + q->poll_nsec = poll_usec * 1000;
> + return ret;
> +}
> +
> static ssize_t queue_poll_show(struct request_queue *q, char *page)
> {
> return queue_var_show(test_bit(QUEUE_FLAG_POLL, &q->queue_flags), page);
> @@ -562,6 +584,12 @@ static struct queue_sysfs_entry queue_poll_entry = {
> .store = queue_poll_store,
> };
>
> +static struct queue_sysfs_entry queue_poll_delay_entry = {
> + .attr = {.name = "io_poll_delay", .mode = S_IRUGO | S_IWUSR },
> + .show = queue_poll_delay_show,
> + .store = queue_poll_delay_store,
> +};
> +
> static struct queue_sysfs_entry queue_wc_entry = {
> .attr = {.name = "write_cache", .mode = S_IRUGO | S_IWUSR },
> .show = queue_wc_show,
> @@ -608,6 +636,7 @@ static struct attribute *default_attrs[] = {
> &queue_wc_entry.attr,
> &queue_dax_entry.attr,
> &queue_stats_entry.attr,
> + &queue_poll_delay_entry.attr,
> NULL,
> };
>
> diff --git a/block/blk.h b/block/blk.h
> index aa132dea598c..041185e5f129 100644
> --- a/block/blk.h
> +++ b/block/blk.h
> @@ -111,6 +111,7 @@ void blk_account_io_done(struct request *req);
> enum rq_atomic_flags {
> REQ_ATOM_COMPLETE = 0,
> REQ_ATOM_STARTED,
> + REQ_ATOM_POLL_SLEPT,
> };
>
> /*
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index dcd8d6e8801f..6acd220dc3f3 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -502,6 +502,7 @@ struct request_queue {
> unsigned int request_fn_active;
>
> unsigned int rq_timeout;
> + unsigned int poll_nsec;
> struct timer_list timeout;
> struct work_struct timeout_work;
> struct list_head timeout_list;
> --
> 2.7.4
>
> --
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--
Ming Lei