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    Subject[PATCH 5.17 0141/1126] block: fix rq-qos breakage from skipping rq_qos_done_bio()
    Date
    From: Tejun Heo <tj@kernel.org>

    commit aa1b46dcdc7baaf5fec0be25782ef24b26aa209e upstream.

    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@kernel.org>
    Fixes: a647a524a467 ("block: don't call rq_qos_ops->done_bio if the bio isn't tracked")
    Cc: stable@vger.kernel.org # v5.15+
    Cc: Ming Lei <ming.lei@redhat.com>
    Cc: Yu Kuai <yukuai3@huawei.com>
    Reviewed-by: Ming Lei <ming.lei@redhat.com>
    Link: https://lore.kernel.org/r/Yi7rdrzQEHjJLGKB@slm.duckdns.org
    Signed-off-by: Jens Axboe <axboe@kernel.dk>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    block/bio.c | 3 +--
    block/blk-iolatency.c | 2 +-
    block/blk-rq-qos.h | 20 +++++++++++---------
    include/linux/blk_types.h | 3 ++-
    4 files changed, 15 insertions(+), 13 deletions(-)

    --- a/block/bio.c
    +++ b/block/bio.c
    @@ -1486,8 +1486,7 @@ again:
    if (!bio_integrity_endio(bio))
    return;

    - if (bio->bi_bdev && bio_flagged(bio, BIO_TRACKED))
    - rq_qos_done_bio(bdev_get_queue(bio->bi_bdev), bio);
    + rq_qos_done_bio(bio);

    if (bio->bi_bdev && bio_flagged(bio, BIO_TRACE_COMPLETION)) {
    trace_block_bio_complete(bdev_get_queue(bio->bi_bdev), bio);
    --- a/block/blk-iolatency.c
    +++ b/block/blk-iolatency.c
    @@ -598,7 +598,7 @@ static void blkcg_iolatency_done_bio(str
    int inflight = 0;

    blkg = bio->bi_blkg;
    - if (!blkg || !bio_flagged(bio, BIO_TRACKED))
    + if (!blkg || !bio_flagged(bio, BIO_QOS_THROTTLED))
    return;

    iolat = blkg_to_lat(bio->bi_blkg);
    --- a/block/blk-rq-qos.h
    +++ b/block/blk-rq-qos.h
    @@ -177,20 +177,20 @@ static inline void rq_qos_requeue(struct
    __rq_qos_requeue(q->rq_qos, rq);
    }

    -static inline void rq_qos_done_bio(struct request_queue *q, struct bio *bio)
    +static inline void rq_qos_done_bio(struct bio *bio)
    {
    - if (q->rq_qos)
    - __rq_qos_done_bio(q->rq_qos, bio);
    + if (bio->bi_bdev && (bio_flagged(bio, BIO_QOS_THROTTLED) ||
    + bio_flagged(bio, BIO_QOS_MERGED))) {
    + struct request_queue *q = bdev_get_queue(bio->bi_bdev);
    + if (q->rq_qos)
    + __rq_qos_done_bio(q->rq_qos, bio);
    + }
    }

    static inline void rq_qos_throttle(struct request_queue *q, struct bio *bio)
    {
    - /*
    - * BIO_TRACKED lets controllers know that a bio went through the
    - * normal rq_qos path.
    - */
    if (q->rq_qos) {
    - bio_set_flag(bio, BIO_TRACKED);
    + bio_set_flag(bio, BIO_QOS_THROTTLED);
    __rq_qos_throttle(q->rq_qos, bio);
    }
    }
    @@ -205,8 +205,10 @@ static inline void rq_qos_track(struct r
    static inline void rq_qos_merge(struct request_queue *q, struct request *rq,
    struct bio *bio)
    {
    - if (q->rq_qos)
    + if (q->rq_qos) {
    + bio_set_flag(bio, BIO_QOS_MERGED);
    __rq_qos_merge(q->rq_qos, rq, bio);
    + }
    }

    static inline void rq_qos_queue_depth_changed(struct request_queue *q)
    --- a/include/linux/blk_types.h
    +++ b/include/linux/blk_types.h
    @@ -317,7 +317,8 @@ enum {
    BIO_TRACE_COMPLETION, /* bio_endio() should trace the final completion
    * of this bio. */
    BIO_CGROUP_ACCT, /* has been accounted to a cgroup */
    - BIO_TRACKED, /* set if bio goes through the rq_qos path */
    + BIO_QOS_THROTTLED, /* bio went through rq_qos throttle path */
    + BIO_QOS_MERGED, /* but went through rq_qos merge path */
    BIO_REMAPPED,
    BIO_ZONE_WRITE_LOCKED, /* Owns a zoned device zone write lock */
    BIO_PERCPU_CACHE, /* can participate in per-cpu alloc cache */

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