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    Subject[PATCH v3] blk: fix a wrong accounting of hd_struct->in_flight
    Hi Jens,

    Jens Axboe wrote:
    > On 2010-10-14 14:48, Yasuaki Ishimatsu wrote:
    >> Index: linux-2.6.36-rc7/block/blk-core.c
    >> ===================================================================
    >> --- linux-2.6.36-rc7.orig/block/blk-core.c 2010-10-07 05:39:52.000000000 +0900
    >> +++ linux-2.6.36-rc7/block/blk-core.c 2010-10-14 17:25:43.000000000 +0900
    >> @@ -66,9 +66,15 @@ static void drive_stat_acct(struct reque
    >> cpu = part_stat_lock();
    >> part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq));
    >>
    >> - if (!new_io)
    >> + if (!new_io) {
    >> + if (unlikely(rq->part != part)) {
    >> + part_dec_in_flight(rq->part, rw);
    >> + part_inc_in_flight(part, rw);
    >> + rq->part = part;
    >> + }
    >> part_stat_inc(cpu, part, merges[rw]);
    >> - else {
    >> + } else {
    >> + rq->part = part;
    >> part_round_stats(cpu, part);
    >> part_inc_in_flight(part, rw);
    >> }
    >
    > I was thinking that we'd do away with the lookup always if ->part was
    > already set. It will probably require a quiscing of IO on partition
    > table reload, though.

    O.K.
    I removed extra part lookups. Following patch also fixed a wrong accounting of
    hd_struct->in_flight. But I could not invent how to stop IOs when
    reloading partition table. Do you have some idea?

    Thsanks,
    Yasuaki Ishimatsu
    ===

    From: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>

    /proc/diskstats would display a strange output as follows.

    $ cat /proc/diskstats |grep sda
    8 0 sda 90524 7579 102154 20464 0 0 0 0 0 14096 20089
    8 1 sda1 19085 1352 21841 4209 0 0 0 0 4294967064 15689 4293424691
    ~~~~~~~~~~
    8 2 sda2 71252 3624 74891 15950 0 0 0 0 232 23995 1562390
    8 3 sda3 54 487 2188 92 0 0 0 0 0 88 92
    8 4 sda4 4 0 8 0 0 0 0 0 0 0 0
    8 5 sda5 81 2027 2130 138 0 0 0 0 0 87 137

    Its reason is the wrong way of accounting hd_struct->in_flight. When a bio is
    merged into a request belongs to different partition by ELEVATOR_FRONT_MERGE.

    The detailed root cause is as follows.

    Assuming that there are two partition, sda1 and sda2.

    1. A request for sda2 is in request_queue. Hence sda1's hd_struct->in_flight
    is 0 and sda2's one is 1.

    | hd_struct->in_flight
    ---------------------------
    sda1 | 0
    sda2 | 1
    ---------------------------

    2. A bio belongs to sda1 is issued and is merged into the request mentioned on
    step1 by ELEVATOR_BACK_MERGE. The first sector of the request is changed
    from sda2 region to sda1 region. However the two partition's
    hd_struct->in_flight are not changed.

    | hd_struct->in_flight
    ---------------------------
    sda1 | 0
    sda2 | 1
    ---------------------------

    3. The request is finished and blk_account_io_done() is called. In this case,
    sda2's hd_struct->in_flight, not a sda1's one, is decremented.

    | hd_struct->in_flight
    ---------------------------
    sda1 | -1
    sda2 | 1
    ---------------------------

    The patch fixes the problem.

    Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
    ---
    block/blk-core.c | 13 ++++++++-----
    block/blk-merge.c | 2 +-
    include/linux/blkdev.h | 1 +
    3 files changed, 10 insertions(+), 6 deletions(-)

    Index: linux-2.6.36-rc7/block/blk-core.c
    ===================================================================
    --- linux-2.6.36-rc7.orig/block/blk-core.c 2010-10-15 09:21:37.000000000 +0900
    +++ linux-2.6.36-rc7/block/blk-core.c 2010-10-15 09:44:23.000000000 +0900
    @@ -64,13 +64,15 @@ static void drive_stat_acct(struct reque
    return;

    cpu = part_stat_lock();
    - part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq));

    - if (!new_io)
    + if (!new_io) {
    + part = rq->part;
    part_stat_inc(cpu, part, merges[rw]);
    - else {
    + } else {
    + part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq));
    part_round_stats(cpu, part);
    part_inc_in_flight(part, rw);
    + rq->part = part;
    }

    part_stat_unlock();
    @@ -128,6 +130,7 @@ void blk_rq_init(struct request_queue *q
    rq->ref_count = 1;
    rq->start_time = jiffies;
    set_start_time_ns(rq);
    + rq->part = NULL;
    }
    EXPORT_SYMBOL(blk_rq_init);

    @@ -1759,7 +1762,7 @@ static void blk_account_io_completion(st
    int cpu;

    cpu = part_stat_lock();
    - part = disk_map_sector_rcu(req->rq_disk, blk_rq_pos(req));
    + part = req->part;
    part_stat_add(cpu, part, sectors[rw], bytes >> 9);
    part_stat_unlock();
    }
    @@ -1779,7 +1782,7 @@ static void blk_account_io_done(struct r
    int cpu;

    cpu = part_stat_lock();
    - part = disk_map_sector_rcu(req->rq_disk, blk_rq_pos(req));
    + part = req->part;

    part_stat_inc(cpu, part, ios[rw]);
    part_stat_add(cpu, part, ticks[rw], duration);
    Index: linux-2.6.36-rc7/block/blk-merge.c
    ===================================================================
    --- linux-2.6.36-rc7.orig/block/blk-merge.c 2010-10-07 05:39:52.000000000 +0900
    +++ linux-2.6.36-rc7/block/blk-merge.c 2010-10-15 09:38:45.000000000 +0900
    @@ -343,7 +343,7 @@ static void blk_account_io_merge(struct
    int cpu;

    cpu = part_stat_lock();
    - part = disk_map_sector_rcu(req->rq_disk, blk_rq_pos(req));
    + part = rq->part;

    part_round_stats(cpu, part);
    part_dec_in_flight(part, rq_data_dir(req));
    Index: linux-2.6.36-rc7/include/linux/blkdev.h
    ===================================================================
    --- linux-2.6.36-rc7.orig/include/linux/blkdev.h 2010-10-15 09:21:37.000000000 +0900
    +++ linux-2.6.36-rc7/include/linux/blkdev.h 2010-10-15 09:26:22.000000000 +0900
    @@ -115,6 +115,7 @@ struct request {
    void *elevator_private3;

    struct gendisk *rq_disk;
    + struct hd_struct *part;
    unsigned long start_time;
    #ifdef CONFIG_BLK_CGROUP
    unsigned long long start_time_ns;



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