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blk-iocost: charge flushes and zone appends - #1266
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The builtin linear cost model defines coefficients only for READ and WRITE, so the flush component of IOs is priced at zero: standalone flushes (the dataless REQ_OP_WRITE | REQ_PREFLUSH bios from blkdev_issue_flush()) and the pre-flush the block layer issues ahead of data-bearing REQ_PREFLUSH bios are both free. 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, monopolizing the device while iocost reported no activity. On ext4, a write+fsync workload showed the same gap: the write component was charged but the flush component was not. A flush is not like a write and the write coefficients say nothing about what it costs, so instead of pricing it off them, add a flushiops entry to io.cost.model, following the existing iops parameters: it sets the rate at which flushes are charged, LCOEF_FLUSH = VTIME_PER_SEC / flushiops. The linear model cannot express how the cost of a flush depends on the preceding writes, so this is a per-request policy charge: a user-tunable knob rather than a hardware property. 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. Zero (the default and the builtin profiles) means no charge, so nothing changes until the parameter is configured. Also skip the iocg->cursor update for dataless bios: they only reach it once priced, and their bi_sector is not a data position, so setting the cursor from it would misclassify the following IOs. Signed-off-by: Tao Cui <cuitao@kylinos.cn> Acked-by: Tejun Heo <tj@kernel.org>
Zone append is a primary write operation for zoned devices; zoned xfs and btrfs use it for data writes. It is priced at zero, so the zone append portion of zoned workloads runs outside the controller: a 1%-weight cgroup issued 16000 appends at zero cost on a zoned null_blk. Treat REQ_OP_ZONE_APPEND the same as REQ_OP_WRITE so it goes through the existing seq/rand classification and charging. Fixes: 0512a75 ("block: Introduce REQ_OP_ZONE_APPEND") Signed-off-by: Tao Cui <cuitao@kylinos.cn> Acked-by: Tejun Heo <tj@kernel.org>
ioc_rqos_done() only accounts READ and WRITE completions, so zone append completions are excluded from the latency window: the vrate feedback loop cannot see ZA-induced latency, leaving it unable to respond to device saturation caused by zone appends. Similarly, calc_size_vtime_cost_builtin() does not classify zone append as a write operation. Charging (the cost model) and feedback (the latency window) are separate mechanisms, so this is not covered by the previous patch that prices zone appends. Treat zone append as WRITE for both latency accounting and cost classification. Fixes: 0512a75 ("block: Introduce REQ_OP_ZONE_APPEND") Signed-off-by: Tao Cui <cuitao@kylinos.cn> Reviewed-by: Christoph Hellwig <hch@lst.de> Acked-by: Tejun Heo <tj@kernel.org>
The comment says that priority-inversion IOs are "punted to @ioc->aux_iocg", but no aux_iocg field ever existed in struct ioc. The comment was introduced already stale by commit da437b9 ("blk-iocost: grab ioc->lock for debt handling"). Update it to describe the current use_debt / iocg->abs_vdebt mechanism. Signed-off-by: Tao Cui <cuitao@kylinos.cn> Reviewed-by: Christoph Hellwig <hch@lst.de> Acked-by: Tejun Heo <tj@kernel.org>
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Pull request for series with
subject: blk-iocost: charge flushes and zone appends
version: 1
url: https://patchwork.kernel.org/series/1160080/