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Date: Tue, 28 May 2019 11:37:26 -0700
From: Omar Sandoval <osandov@...ndov.com>
To: "Pavel Begunkov (Silence)" <asml.silence@...il.com>
Cc: Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
linux-kernel@...r.kernel.org, osandov@...com, ming.lei@...hat.com,
Hou Tao <houtao1@...wei.com>
Subject: Re: [PATCH 1/1] blk-mq: Fix disabled hybrid polling
On Sat, May 25, 2019 at 04:42:11PM +0300, Pavel Begunkov (Silence) wrote:
> From: Pavel Begunkov <asml.silence@...il.com>
>
> Commit 4bc6339a583cec650b05 ("block: move blk_stat_add() to
> __blk_mq_end_request()") moved blk_stat_add(), so now it's called after
> blk_update_request(), which zeroes rq->__data_len. Without length,
> blk_stat_add() can't calculate stat bucket and returns error,
> effectively disabling hybrid polling.
I don't see how this patch fixes this problem, am I missing something?
The timing issue seems orthogonal.
> __blk_mq_end_request() is a right place to call blk_stat_add(), as it's
> guaranteed to be called for each request. Yet, calculating time there
> won't provide sufficient accuracy/precision for finer tuned hybrid
> polling, because a path from __blk_mq_complete_request() to
> __blk_mq_end_request() adds unpredictable overhead.
>
> Add io_end_time_ns field in struct request, save time as soon as
> possible (at __blk_mq_complete_request()) and reuse later.
>
> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
> ---
> block/blk-mq.c | 13 ++++++++++---
> block/blk-stat.c | 4 ++--
> block/blk-stat.h | 2 +-
> include/linux/blkdev.h | 11 +++++++++++
> 4 files changed, 24 insertions(+), 6 deletions(-)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 32b8ad3d341b..8f6b6bfe0ccb 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -330,6 +330,7 @@ static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data,
> else
> rq->start_time_ns = 0;
> rq->io_start_time_ns = 0;
> + rq->io_end_time_ns = 0;
> rq->nr_phys_segments = 0;
> #if defined(CONFIG_BLK_DEV_INTEGRITY)
> rq->nr_integrity_segments = 0;
> @@ -532,14 +533,17 @@ EXPORT_SYMBOL_GPL(blk_mq_free_request);
>
> inline void __blk_mq_end_request(struct request *rq, blk_status_t error)
> {
> - u64 now = 0;
> + u64 now = rq->io_end_time_ns;
Kyber expects the timestamp passed in to kyber_complete_request() to
include the software overhead. iostat should probably include the
software overhead, too. So, we probably won't be able to avoid calling
ktime_get() twice, once for I/O time and one for the end-to-end time.
> - if (blk_mq_need_time_stamp(rq))
> + /* called directly bypassing __blk_mq_complete_request */
> + if (blk_mq_need_time_stamp(rq) && !now) {
> now = ktime_get_ns();
> + rq->io_end_time_ns = now;
> + }
>
> if (rq->rq_flags & RQF_STATS) {
> blk_mq_poll_stats_start(rq->q);
> - blk_stat_add(rq, now);
> + blk_stat_add(rq);
> }
>
> if (rq->internal_tag != -1)
> @@ -579,6 +583,9 @@ static void __blk_mq_complete_request(struct request *rq)
> bool shared = false;
> int cpu;
>
> + if (blk_mq_need_time_stamp(rq))
> + rq->io_end_time_ns = ktime_get_ns();
> +
> WRITE_ONCE(rq->state, MQ_RQ_COMPLETE);
> /*
> * Most of single queue controllers, there is only one irq vector
> diff --git a/block/blk-stat.c b/block/blk-stat.c
> index 940f15d600f8..9b9b30927ea8 100644
> --- a/block/blk-stat.c
> +++ b/block/blk-stat.c
> @@ -48,7 +48,7 @@ void blk_rq_stat_add(struct blk_rq_stat *stat, u64 value)
> stat->nr_samples++;
> }
>
> -void blk_stat_add(struct request *rq, u64 now)
> +void blk_stat_add(struct request *rq)
> {
> struct request_queue *q = rq->q;
> struct blk_stat_callback *cb;
> @@ -56,7 +56,7 @@ void blk_stat_add(struct request *rq, u64 now)
> int bucket;
> u64 value;
>
> - value = (now >= rq->io_start_time_ns) ? now - rq->io_start_time_ns : 0;
> + value = blk_rq_io_time(rq);
>
> blk_throtl_stat_add(rq, value);
>
> diff --git a/block/blk-stat.h b/block/blk-stat.h
> index 17b47a86eefb..2653818cee36 100644
> --- a/block/blk-stat.h
> +++ b/block/blk-stat.h
> @@ -65,7 +65,7 @@ struct blk_stat_callback {
> struct blk_queue_stats *blk_alloc_queue_stats(void);
> void blk_free_queue_stats(struct blk_queue_stats *);
>
> -void blk_stat_add(struct request *rq, u64 now);
> +void blk_stat_add(struct request *rq);
>
> /* record time/size info in request but not add a callback */
> void blk_stat_enable_accounting(struct request_queue *q);
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 592669bcc536..2a8d4b68d707 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -198,6 +198,9 @@ struct request {
> u64 start_time_ns;
> /* Time that I/O was submitted to the device. */
> u64 io_start_time_ns;
> + /* Time that I/O was reported completed by the device. */
> + u64 io_end_time_ns;
> +
>
> #ifdef CONFIG_BLK_WBT
> unsigned short wbt_flags;
> @@ -385,6 +388,14 @@ static inline int blkdev_reset_zones_ioctl(struct block_device *bdev,
>
> #endif /* CONFIG_BLK_DEV_ZONED */
>
> +static inline u64 blk_rq_io_time(struct request *rq)
> +{
> + u64 start = rq->io_start_time_ns;
> + u64 end = rq->io_end_time_ns;
> +
> + return (end - start) ? end - start : 0;
I think you meant:
return end >= start ? end - start : 0;
> +}
> +
> struct request_queue {
> /*
> * Together with queue_head for cacheline sharing
> --
> 2.21.0
>
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