Re: [PATCH block-5.17] fix rq-qos breakage from skipping rq_qos_done_bio()

[Ming Lei]

On Sun, Mar 13, 2022 at 09:15:02PM -1000, Tejun Heo wrote:
> a647a524a467 ("block: don't call rq_qos_ops->done_bio if the bio isn't
> tracked") made bio_endio() skip rq_qos_done_bio() if BIO_TRACKED is not set.
> While this fixed a potential oops, it also broke blk-iocost by skipping the
> done_bio callback for merged bios.
> 
> Before, whether a bio goes through rq_qos_throttle() or rq_qos_merge(),
> rq_qos_done_bio() would be called on the bio on completion with BIO_TRACKED
> distinguishing the former from the latter. rq_qos_done_bio() is not called
> for bios which wenth through rq_qos_merge(). This royally confuses
> blk-iocost as the merged bios never finish and are considered perpetually
> in-flight.
> 
> One reliably reproducible failure mode is an intermediate cgroup geting
> stuck active preventing its children from being activated due to the
> leaf-only rule, leading to loss of control. The following is from
> resctl-bench protection scenario which emulates isolating a web server like
> workload from a memory bomb run on an iocost configuration which should
> yield a reasonable level of protection.
> 
>   # cat /sys/block/nvme2n1/device/model
>   Samsung SSD 970 PRO 512GB               
>   # cat /sys/fs/cgroup/io.cost.model
>   259:0 ctrl=user model=linear rbps=834913556 rseqiops=93622 rrandiops=102913 wbps=618985353 wseqiops=72325 wrandiops=71025
>   # cat /sys/fs/cgroup/io.cost.qos
>   259:0 enable=1 ctrl=user rpct=95.00 rlat=18776 wpct=95.00 wlat=8897 min=60.00 max=100.00
>   # resctl-bench -m 29.6G -r out.json run protection::scenario=mem-hog,loops=1
>   ...
>   Memory Hog Summary
>   ==================
> 
>   IO Latency: R p50=242u:336u/2.5m p90=794u:1.4m/7.5m p99=2.7m:8.0m/62.5m max=8.0m:36.4m/350m
>               W p50=221u:323u/1.5m p90=709u:1.2m/5.5m p99=1.5m:2.5m/9.5m max=6.9m:35.9m/350m
> 
>   Isolation and Request Latency Impact Distributions:
> 
>                 min   p01   p05   p10   p25   p50   p75   p90   p95   p99   max  mean stdev
>   isol%       15.90 15.90 15.90 40.05 57.24 59.07 60.01 74.63 74.63 90.35 90.35 58.12 15.82 
>   lat-imp%        0     0     0     0     0  4.55 14.68 15.54 233.5 548.1 548.1 53.88 143.6 
> 
>   Result: isol=58.12:15.82% lat_imp=53.88%:143.6 work_csv=100.0% missing=3.96%
> 
> The isolation result of 58.12% is close to what this device would show
> without any IO control.
> 
> Fix it by introducing a new flag BIO_QOS_MERGED to mark merged bios and
> calling rq_qos_done_bio() on them too. For consistency and clarity, rename
> BIO_TRACKED to BIO_QOS_THROTTLED. The flag checks are moved into
> rq_qos_done_bio() so that it's next to the code paths that set the flags.
> 
> With the patch applied, the above same benchmark shows:
> 
>   # resctl-bench -m 29.6G -r out.json run protection::scenario=mem-hog,loops=1
>   ...
>   Memory Hog Summary
>   ==================
> 
>   IO Latency: R p50=123u:84.4u/985u p90=322u:256u/2.5m p99=1.6m:1.4m/9.5m max=11.1m:36.0m/350m
>               W p50=429u:274u/995u p90=1.7m:1.3m/4.5m p99=3.4m:2.7m/11.5m max=7.9m:5.9m/26.5m
> 
>   Isolation and Request Latency Impact Distributions:
> 
>                 min   p01   p05   p10   p25   p50   p75   p90   p95   p99   max  mean stdev
>   isol%       84.91 84.91 89.51 90.73 92.31 94.49 96.36 98.04 98.71 100.0 100.0 94.42  2.81 
>   lat-imp%        0     0     0     0     0  2.81  5.73 11.11 13.92 17.53 22.61  4.10  4.68 
> 
>   Result: isol=94.42:2.81% lat_imp=4.10%:4.68 work_csv=58.34% missing=0%
> 
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> Fixes: a647a524a467 ("block: don't call rq_qos_ops->done_bio if the bio isn't tracked")
> Cc: stable@xxxxxxxxxxxxxxx # v5.15+
> Cc: Ming Lei <ming.lei@xxxxxxxxxx>
> Cc: Yu Kuai <yukuai3@xxxxxxxxxx>

Looks fine since rq always holds one .q_usage_counter in case of merge:

Reviewed-by: Ming Lei <ming.lei@xxxxxxxxxx>



Thanks,
Ming