That's the same bug I posted a patch about 3+ weeks ago. No answer yet, however. https://www.spinics.net/lists/linux-block/msg40049.html
I think putting time sampling at __blk_mq_end_request() won't give sufficient precision for more sophisticated hybrid polling, because path __blk_mq_complete_request() -> __blk_mq_end_request() adds a lot of overhead (redirect rq to another cpu, dma unmap, etc). That's the case for mentioned patchset https://www.spinics.net/lists/linux-block/msg40044.html It works OK for 50-80us requests (e.g. SSD read), but is not good enough for 20us and less (e.g. SSD write). I think it would be better to save the time in advance, and use it later. By the way, it seems, that __blk_mq_end_request() is really better place to record stats, as not all drivers use __blk_mq_complete_request(), even though they don't implement blk_poll(). On 21/05/2019 10:59, Hou Tao wrote: > Currently rq->data_len will be decreased by partial completion or > zeroed by completion, so when blk_stat_add() is invoked, data_len > will be zero and there will never be samples in poll_cb because > blk_mq_poll_stats_bkt() will return -1 if data_len is zero. > > We could move blk_stat_add() back to __blk_mq_complete_request(), > but that would make the effort of trying to call ktime_get_ns() > once in vain. Instead we can reuse throtl_size field, and use > it for both block stats and block throttle, and adjust the > logic in blk_mq_poll_stats_bkt() accordingly. > > Fixes: 4bc6339a583c ("block: move blk_stat_add() to __blk_mq_end_request()") > Signed-off-by: Hou Tao <[email protected]> > --- > block/blk-mq.c | 11 +++++------ > block/blk-throttle.c | 3 ++- > include/linux/blkdev.h | 15 ++++++++++++--- > 3 files changed, 19 insertions(+), 10 deletions(-) > > diff --git a/block/blk-mq.c b/block/blk-mq.c > index 08a6248d8536..4d1462172f0f 100644 > --- a/block/blk-mq.c > +++ b/block/blk-mq.c > @@ -44,12 +44,12 @@ static void blk_mq_poll_stats_fn(struct blk_stat_callback > *cb); > > static int blk_mq_poll_stats_bkt(const struct request *rq) > { > - int ddir, bytes, bucket; > + int ddir, sectors, bucket; > > ddir = rq_data_dir(rq); > - bytes = blk_rq_bytes(rq); > + sectors = blk_rq_stats_sectors(rq); > > - bucket = ddir + 2*(ilog2(bytes) - 9); > + bucket = ddir + 2 * ilog2(sectors); > > if (bucket < 0) > return -1; > @@ -329,6 +329,7 @@ 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->stats_sectors = 0; > rq->nr_phys_segments = 0; > #if defined(CONFIG_BLK_DEV_INTEGRITY) > rq->nr_integrity_segments = 0; > @@ -678,9 +679,7 @@ void blk_mq_start_request(struct request *rq) > > if (test_bit(QUEUE_FLAG_STATS, &q->queue_flags)) { > rq->io_start_time_ns = ktime_get_ns(); > -#ifdef CONFIG_BLK_DEV_THROTTLING_LOW > - rq->throtl_size = blk_rq_sectors(rq); > -#endif > + rq->stats_sectors = blk_rq_sectors(rq); > rq->rq_flags |= RQF_STATS; > rq_qos_issue(q, rq); > } > diff --git a/block/blk-throttle.c b/block/blk-throttle.c > index 1b97a73d2fb1..88459a4ac704 100644 > --- a/block/blk-throttle.c > +++ b/block/blk-throttle.c > @@ -2249,7 +2249,8 @@ void blk_throtl_stat_add(struct request *rq, u64 > time_ns) > struct request_queue *q = rq->q; > struct throtl_data *td = q->td; > > - throtl_track_latency(td, rq->throtl_size, req_op(rq), time_ns >> 10); > + throtl_track_latency(td, blk_rq_stats_sectors(rq), req_op(rq), > + time_ns >> 10); > } > > void blk_throtl_bio_endio(struct bio *bio) > diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h > index 1aafeb923e7b..68a0841d3554 100644 > --- a/include/linux/blkdev.h > +++ b/include/linux/blkdev.h > @@ -202,9 +202,12 @@ struct request { > #ifdef CONFIG_BLK_WBT > unsigned short wbt_flags; > #endif > -#ifdef CONFIG_BLK_DEV_THROTTLING_LOW > - unsigned short throtl_size; > -#endif > + /* > + * rq sectors used for blk stats. It has the same value > + * with blk_rq_sectors(rq), except that it never be zeroed > + * by completion. > + */ > + unsigned short stats_sectors; > > /* > * Number of scatter-gather DMA addr+len pairs after > @@ -892,6 +895,7 @@ static inline struct request_queue *bdev_get_queue(struct > block_device *bdev) > * blk_rq_err_bytes() : bytes left till the next error > boundary > * blk_rq_sectors() : sectors left in the entire request > * blk_rq_cur_sectors() : sectors left in the current segment > + * blk_rq_stats_sectors() : sectors of the entire request used for stats > */ > static inline sector_t blk_rq_pos(const struct request *rq) > { > @@ -920,6 +924,11 @@ static inline unsigned int blk_rq_cur_sectors(const > struct request *rq) > return blk_rq_cur_bytes(rq) >> SECTOR_SHIFT; > } > > +static inline unsigned int blk_rq_stats_sectors(const struct request *rq) > +{ > + return rq->stats_sectors; > +} > + > #ifdef CONFIG_BLK_DEV_ZONED > static inline unsigned int blk_rq_zone_no(struct request *rq) > { > -- Yours sincerely, Pavel Begunkov
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