On Mon, Oct 26, 2020 at 6:15 PM Ming Lei <ming.lei@xxxxxxxxxx> wrote:
>
> 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.
>
Ya, the inflight count should be counted when part's stamp was changed,
How about like this:
+static int blk_part_get_inflight(struct hd_struct *part)
+{
+ struct request_queue *q;
+ int inflight;
+
+ q = part_to_disk(part)->queue;
+ if (queue_is_mq(q))
+ inflight = blk_mq_part_is_in_flight(q, part);
+ else
+ inflight = part_is_in_flight(part);
+
+ return inflight;
+}
+
+static void update_io_ticks(struct hd_struct *part, unsigned long
now, bool end)
{
unsigned long stamp;
+ int inflight = -1;
again:
stamp = READ_ONCE(part->stamp);
if (unlikely(stamp != now)) {
- if (likely(cmpxchg(&part->stamp, stamp, now) == stamp)
&& inflight)
- __part_stat_add(part, io_ticks, now - stamp);
+ if (likely(cmpxchg(&part->stamp, stamp, now) == stamp)) {
+ if(end) {
+ __part_stat_add(part, io_ticks, now - stamp);
+ } else {
+ if (inflight == -1)
+ inflight = blk_part_get_inflight(part);
+ if (inflight > 0)
+ __part_stat_add(part,
io_ticks, now - stamp);
+ }
+ }
}
> Thanks,
> Ming
>
