[PATCH v3] blk-iocost: fix busy_level reset when no IOs complete

Jialin Wang
2026-03-31T10:05:22Z

When a disk is saturated, it is common for no IOs to complete within a
timer period. Currently, in this case, rq_wait_pct and missed_ppm are
calculated as 0, the iocost incorrectly interprets this as meeting QoS
targets and resets busy_level to 0.

This reset prevents busy_level from reaching the threshold (4) needed
to reduce vrate. On certain cloud storage, such as Azure Premium SSD,
we observed that iocost may fail to reduce vrate for tens of seconds
during saturation, failing to mitigate noisy neighbor issues.

Fix this by tracking the number of IO completions (nr_done) in a period.
If nr_done is 0 and there are lagging IOs, the saturation status is
unknown, so we keep busy_level unchanged.

The issue is consistently reproducible on Azure Standard_D8as_v5 (Dasv5)
VMs with 512GB Premium SSD (P20) using the script below. It was not
observed on GCP n2d VMs (with 100G pd-ssd and 1.5T local-ssd), and no
regressions were found with this patch. In this script, cgA performs
large IOs with iodepth=128, while cgB performs small IOs with iodepth=1
rate_iops=100 rw=randrw. With iocost enabled, we expect it to throttle
cgA, the submission latency (slat) of cgA should be significantly higher,
cgB can reach 200 IOPS and the completion latency (clat) should below.

BLK_DEVID="8:0"
MODEL="rbps=173471131 rseqiops=3566 rrandiops=3566 wbps=173333269 wseqiops=3566 wrandiops=3566"
QOS="rpct=90 rlat=3500 wpct=90 wlat=3500 min=80 max=10000"

echo "$BLK_DEVID ctrl=user model=linear $MODEL" > /sys/fs/cgroup/io.cost.model
echo "$BLK_DEVID enable=1 ctrl=user $QOS" > /sys/fs/cgroup/io.cost.qos

CG_A="/sys/fs/cgroup/cgA"
CG_B="/sys/fs/cgroup/cgB"

FILE_A="/path/to/sda/A.fio.testfile"
FILE_B="/path/to/sda/B.fio.testfile"
RESULT_DIR="./iocost_results_$(date +%Y%m%d_%H%M%S)"

mkdir -p "$CG_A" "$CG_B" "$RESULT_DIR"

get_result() {
local file=$1
local label=$2

local results=$(jq -r '
.jobs[0].mixed |
( .iops | tonumber | round ) as $iops |
( .bw_bytes / 1024 / 1024 ) as $bps |
( .slat_ns.mean / 1000000 ) as $slat |
( .clat_ns.mean / 1000000 ) as $avg |
( .clat_ns.max / 1000000 ) as $max |
( .clat_ns.percentile["90.000000"] / 1000000 ) as $p90 |
( .clat_ns.percentile["99.000000"] / 1000000 ) as $p99 |
( .clat_ns.percentile["99.900000"] / 1000000 ) as $p999 |
( .clat_ns.percentile["99.990000"] / 1000000 ) as $p9999 |
"\($iops)|\($bps)|\($slat)|\($avg)|\($max)|\($p90)|\($p99)|\($p999)|\($p9999)"
' "$file")

IFS='|' read -r iops bps slat avg max p90 p99 p999 p9999 <<<"$results"
printf "%-8s %-6s %-7.2f %-8.2f %-8.2f %-8.2f %-8.2f %-8.2f %-8.2f %-8.2f\n" \
"$label" "$iops" "$bps" "$slat" "$avg" "$max" "$p90" "$p99" "$p999" "$p9999"
}

run_fio() {
local cg_path=$1
local filename=$2
local name=$3
local bs=$4
local qd=$5
local out=$6
shift 6
local extra=$@

(
pid=$(sh -c 'echo $PPID')
echo $pid >"${cg_path}/cgroup.procs"
fio --name="$name" --filename="$filename" --direct=1 --rw=randrw --rwmixread=50 \
--ioengine=libaio --bs="$bs" --iodepth="$qd" --size=4G --runtime=10 \
--time_based --group_reporting --unified_rw_reporting=mixed \
--output-format=json --output="$out" $extra >/dev/null 2>&1
) &
}

echo "Starting Test ..."

for bs_b in "4k" "32k" "256k"; do
echo "Running iteration: BS=$bs_b"
out_a="${RESULT_DIR}/cgA_1m.json"
out_b="${RESULT_DIR}/cgB_${bs_b}.json"

# cgA: Heavy background (BS 1MB, QD 128)
run_fio "$CG_A" "$FILE_A" "cgA" "1m" 128 "$out_a"
# cgB: Latency sensitive (Variable BS, QD 1, Read/Write IOPS limit 100)
run_fio "$CG_B" "$FILE_B" "cgB" "$bs_b" 1 "$out_b" "--rate_iops=100"

wait
SUMMARY_DATA+="$(get_result "$out_a" "cgA-1m")"$'\n'
SUMMARY_DATA+="$(get_result "$out_b" "cgB-$bs_b")"$'\n\n'
done

echo -e "\nFinal Results Summary:\n"

printf "%-8s %-6s %-7s %-8s %-8s %-8s %-8s %-8s %-8s %-8s\n" \
"" "" "" "slat" "clat" "clat" "clat" "clat" "clat" "clat"
printf "%-8s %-6s %-7s %-8s %-8s %-8s %-8s %-8s %-8s %-8s\n\n" \
"CGROUP" "IOPS" "MB/s" "avg(ms)" "avg(ms)" "max(ms)" "P90(ms)" "P99" "P99.9" "P99.99"
echo "$SUMMARY_DATA"

echo "Results saved in $RESULT_DIR"

Before:
slat clat clat clat clat clat clat
CGROUP IOPS MB/s avg(ms) avg(ms) max(ms) P90(ms) P99 P99.9 P99.99

cgA-1m 166 166.37 3.44 748.95 1298.29 977.27 1233.13 1300.23 1300.23
cgB-4k 5 0.02 0.02 181.74 761.32 742.39 759.17 759.17 759.17

cgA-1m 167 166.51 1.98 748.68 1549.41 809.50 1451.23 1551.89 1551.89
cgB-32k 6 0.18 0.02 169.98 761.76 742.39 759.17 759.17 759.17

cgA-1m 166 165.55 2.89 750.89 1540.37 851.44 1451.23 1535.12 1535.12
cgB-256k 5 1.30 0.02 191.35 759.51 750.78 759.17 759.17 759.17

After:
slat clat clat clat clat clat clat
CGROUP IOPS MB/s avg(ms) avg(ms) max(ms) P90(ms) P99 P99.9 P99.99

cgA-1m 162 162.48 6.14 749.69 850.02 826.28 834.67 843.06 851.44
cgB-4k 199 0.78 0.01 1.95 42.12 2.57 7.50 34.87 42.21

cgA-1m 146 146.20 6.83 833.04 908.68 893.39 901.78 910.16 910.16
cgB-32k 200 6.25 0.01 2.32 31.40 3.06 7.50 16.58 31.33

cgA-1m 110 110.46 9.04 1082.67 1197.91 1182.79 1199.57 1199.57 1199.57
cgB-256k 200 49.98 0.02 3.69 22.20 4.88 9.11 20.05 22.15

Signed-off-by: Jialin Wang <wjl.linux@gmail.com>
---
Changes in v3:
- Handle only the !nr_done && nr_lagging case and leave the other cases
as they are.
Changes in v2:
- Handle more edge cases to prevent potential regressions.

v2: https://lore.kernel.org/all/20260329154112.526679-1-wjl.linux@gmail.com/
v1: https://lore.kernel.org/all/20260318163351.394528-1-wjl.linux@gmail.com/

block/blk-iocost.c | 23 +++++++++++++++++------
1 file changed, 17 insertions(+), 6 deletions(-)

diff --git a/block/blk-iocost.c b/block/blk-iocost.c
index d145db61e5c3..0cca88a366dc 100644
--- a/block/blk-iocost.c
+++ b/block/blk-iocost.c
@@ -1596,7 +1596,8 @@ static enum hrtimer_restart iocg_waitq_timer_fn(struct hrtimer *timer)
return HRTIMER_NORESTART;
}

-static void ioc_lat_stat(struct ioc *ioc, u32 *missed_ppm_ar, u32 *rq_wait_pct_p)
+static void ioc_lat_stat(struct ioc *ioc, u32 *missed_ppm_ar, u32 *rq_wait_pct_p,
+ u32 *nr_done)
{
u32 nr_met[2] = { };
u32 nr_missed[2] = { };
@@ -1633,6 +1634,8 @@ static void ioc_lat_stat(struct ioc *ioc, u32 *missed_ppm_ar, u32 *rq_wait_pct_p

*rq_wait_pct_p = div64_u64(rq_wait_ns * 100,
ioc->period_us * NSEC_PER_USEC);
+
+ *nr_done = nr_met[READ] + nr_met[WRITE] + nr_missed[READ] + nr_missed[WRITE];
}

/* was iocg idle this period? */
@@ -2250,12 +2253,12 @@ static void ioc_timer_fn(struct timer_list *timer)
u64 usage_us_sum = 0;
u32 ppm_rthr;
u32 ppm_wthr;
- u32 missed_ppm[2], rq_wait_pct;
+ u32 missed_ppm[2], rq_wait_pct, nr_done;
u64 period_vtime;
int prev_busy_level;

/* how were the latencies during the period? */
- ioc_lat_stat(ioc, missed_ppm, &rq_wait_pct);
+ ioc_lat_stat(ioc, missed_ppm, &rq_wait_pct, &nr_done);

/* take care of active iocgs */
spin_lock_irq(&ioc->lock);
@@ -2397,9 +2400,17 @@ static void ioc_timer_fn(struct timer_list *timer)
* and should increase vtime rate.
*/
prev_busy_level = ioc->busy_level;
- if (rq_wait_pct > RQ_WAIT_BUSY_PCT ||
- missed_ppm[READ] > ppm_rthr ||
- missed_ppm[WRITE] > ppm_wthr) {
+ if (!nr_done && nr_lagging) {
+ /*
+ * When there are lagging IOs but no completions, we don't
+ * know if the IO latency will meet the QoS targets. The
+ * disk might be saturated or not. We should not reset
+ * busy_level to 0 (which would prevent vrate from scaling
+ * up or down), but rather to keep it unchanged.
+ */
+ } else if (rq_wait_pct > RQ_WAIT_BUSY_PCT ||
+ missed_ppm[READ] > ppm_rthr ||
+ missed_ppm[WRITE] > ppm_wthr) {
/* clearly missing QoS targets, slow down vrate */
ioc->busy_level = max(ioc->busy_level, 0);
ioc->busy_level++;
--
2.53.0