Re: [PATCH v4 0/4] blk-iocost: charge flushes and zone appends

[Tao Cui <cui.tao@linux.dev>]

Hi Jens,

Gentle ping on this series, Tejun has acked it. Could you pick it up?

Thanks,
Tao

在 2026/9/22 12:38, Tao Cui 写道:
> From: Tao Cui <cuitao@kylinos.cn>
> 
> 
> While testing iocost's weight-based throttling under concurrent IO, we
> observed that a cgroup limited to 1% weight could issue an unbounded
> number of flushes without being throttled: an fsync loop produced ~510k
> flushes in 12s with cost.usage staying at zero the entire time.  The
> device was monopolized while iocost reported no activity.  Zone appends
> were in the same position: the builtin linear cost model defines
> coefficients only for READ and WRITE, so REQ_OP_ZONE_APPEND is priced
> at zero and excluded from the latency statistics as well.
> 
> This series prices both, without changing the behavior of existing
> setups:
> 
>  1/4: add a flushiops entry to io.cost.model, translated like the
>       other iops coefficients (VTIME_PER_SEC / flushiops).  A bio with
>       REQ_PREFLUSH is charged one flush on top of its data cost, and a
>       bio with REQ_FUA one more flush on devices without native FUA
>       support (bdev_fua()), mirroring the pre-flush and post-flush the
>       block layer issues for them.  flushiops is zero in the builtin
>       profiles with no fallback to the write coefficients, so nothing
>       changes until it is configured.  Also documents flushiops in
>       cgroup-v2.rst.
>  2/4: treat zone appends as writes in the builtin cost model -
>       REQ_OP_ZONE_APPEND falls through to the REQ_OP_WRITE
>       coefficients.
>  3/4: count zone append completions in the latency window so the vrate
>       feedback loop can see ZA-induced latency.
>  4/4: fix a stale comment in ioc_rqos_throttle().
> 
> Measured on virtio-blk with a bio test module issuing bios from a
> cgroup (300 bios per phase):
> 
>   - plain writes cost 23.41 usec, unchanged
>   - with the default model, WRITE|PREFLUSH and standalone flushes cost
>     the same as before: the flush component is free until flushiops is
>     configured
>   - with flushiops=100, WRITE|PREFLUSH and WRITE|FUA are charged
>     +10000 usec per bio and WRITE|PREFLUSH|FUA +20000, exactly
>     VTIME_PER_SEC / 100 per flush (virtio-blk has no native FUA,
>     confirmed via queue/fua)
>   - standalone flushes are charged 10000.06 usec each, matching
>     VTIME_PER_SEC / 100
>   - with flushiops=10, a flush storm from a low-weight cgroup is
>     throttled to one flush per 100ms
> 
> Changes in v4:
> - 1/4 only, addressed Tejun's review comments: remove the always-true
>   !is_merge wrapper and the separate flush_cost accumulator, charge
>   flushes directly into cost before the empty bio check, trim the
>   comment to the FUA case, and update the commit message and
>   cgroup-v2.rst wording. No behavioral change: the whitebox numbers
>   are identical.
> - 2/4, 3/4 and 4/4 are unchanged except for the added Acked-by tags.
> 
> Changes in v2:
> - charge the flush component of data-bearing REQ_PREFLUSH bios too,
>   not only standalone flushes (Christoph)
> - skip the iocg->cursor update for dataless flush bios
> 
> Changes in v3:
> - the write-coefficient flush pricing is replaced by the flushiops
>   model parameter: pricing a flush off the write coefficients is as
>   arbitrary as pricing it at zero and changes what existing setups
>   get charged, so flushes are only priced when flushiops is
>   configured (Tejun)
> - the zone append patch is reduced to the minimal change: ZA falls
>   through to the REQ_OP_WRITE coefficients and the cursor is left
>   alone (Tejun)
> - the Fixes tags on 2/4 and 3/4 point at 0512a75b98f8, which added
>   the op without pricing it; 3/4 is otherwise unchanged from v2
> 
> Tao Cui (4):
>   blk-iocost: add flush cost support with the flushiops model parameter
>   blk-iocost: charge zone appends as writes
>   blk-iocost: account zone append completions in latency stats
>   blk-iocost: fix stale comment in ioc_rqos_throttle()
> 
>  Documentation/admin-guide/cgroup-v2.rst |  7 +++++
>  block/blk-iocost.c                      | 39 ++++++++++++++++++++-----
>  2 files changed, 38 insertions(+), 8 deletions(-)
>