On Mon, Oct 26, 2020 at 02:15:34PM +0800, Weiping Zhang wrote:
> Do not add io_ticks if there is no infligh io when start a new IO,
> otherwise an extra 1 jiffy will be add to this IO.
>
> I run the following command on a host, with different kernel version.
>
> fio -name=test -ioengine=sync -bs=4K -rw=write
> -filename=/home/test.fio.log -size=100M -time_based=1 -direct=1
> -runtime=300 -rate=2m,2m
>
> If we run fio in a sync direct io mode, IO will be proccessed one by one,
> you can see that there are 512 IOs completed in one second.
>
> kernel: 4.19.0
>
> Device: rrqm/s wrqm/s r/s w/s rMB/s wMB/s avgrq-sz avgqu-sz await r_await w_await svctm %util
> vda 0.00 0.00 0.00 512.00 0.00 2.00 8.00 0.21 0.40 0.00 0.40 0.40 20.60
>
> The averate io.latency is 0.4ms, so the disk time cost in one second
> should be 0.4 * 512 = 204.8 ms, that means, %util should be 20%.
>
> Becase update_io_ticks will add a extra 1 jiffy(1ms) for every IO, the
> io.latency will be 1 + 0.4 = 1.4ms, 1.4 * 512 = 716.8ms,
> so the %util show it about 72%.
>
> Device r/s w/s rMB/s wMB/s rrqm/s wrqm/s %rrqm %wrqm r_await w_await aqu-sz rareq-sz wareq-sz svctm %util
> vda 0.00 512.00 0.00 2.00 0.00 0.00 0.00 0.00 0.00 0.40 0.20 0.00 4.00 1.41 72.10
>
> After this patch:
> Device r/s w/s rMB/s wMB/s rrqm/s wrqm/s %rrqm %wrqm r_await w_await aqu-sz rareq-sz wareq-sz svctm %util
> vda 0.00 512.00 0.00 2.00 0.00 0.00 0.00 0.00 0.00 0.40 0.20 0.00 4.00 0.39 20.00
>
> Fixes: 5b18b5a73760 ("block: delete part_round_stats and switch to less precise counting")
> Fixes: 2b8bd423614c ("block/diskstats: more accurate approximation of io_ticks for slow disks")
> Reported-by: Yabin Li <liyabinliyabin@xxxxxxxxxxxxxx>
> Signed-off-by: Weiping Zhang <zhangweiping@xxxxxxxxxxxxxx>
> ---
> block/blk-core.c | 19 ++++++++++++++-----
> block/blk-mq.c | 26 ++++++++++++++++++++++++++
> block/blk-mq.h | 1 +
> block/blk.h | 1 +
> block/genhd.c | 13 +++++++++++++
> 5 files changed, 55 insertions(+), 5 deletions(-)
>
> diff --git a/block/blk-core.c b/block/blk-core.c
> index ac00d2fa4eb4..9dad92355125 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -1256,14 +1256,14 @@ unsigned int blk_rq_err_bytes(const struct request *rq)
> }
> EXPORT_SYMBOL_GPL(blk_rq_err_bytes);
>
> -static void update_io_ticks(struct hd_struct *part, unsigned long now, bool end)
> +static void update_io_ticks(struct hd_struct *part, unsigned long now, bool inflight)
> {
> unsigned long stamp;
> again:
> stamp = READ_ONCE(part->stamp);
> if (unlikely(stamp != now)) {
> - if (likely(cmpxchg(&part->stamp, stamp, now) == stamp))
> - __part_stat_add(part, io_ticks, end ? now - stamp : 1);
> + if (likely(cmpxchg(&part->stamp, stamp, now) == stamp) && inflight)
> + __part_stat_add(part, io_ticks, now - stamp);
> }
> if (part->partno) {
> part = &part_to_disk(part)->part0;
> @@ -1310,13 +1310,20 @@ void blk_account_io_done(struct request *req, u64 now)
>
> void blk_account_io_start(struct request *rq)
> {
> + struct hd_struct *part;
> + struct request_queue *q;
> + bool inflight;
> +
> if (!blk_do_io_stat(rq))
> return;
>
> rq->part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq));
>
> part_stat_lock();
> - update_io_ticks(rq->part, jiffies, false);
> + part = rq->part;
> + q = part_to_disk(part)->queue;
> + inflight = blk_mq_part_is_in_flight(q, part);
> + update_io_ticks(part, jiffies, inflight);
This way is much worse than before commit 5b18b5a73760("block: delete part_round_stats
and switch to less precise counting") which only reads inflight once in every jiffy.
But you switch to read inflight for _every_ IO, and this way is too bad, IMO.
Thanks,
Ming
