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    Subject[PATCH 4.19 266/271] blk-throttle: fix zero wait time for iops throttled group
    Date
    From: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>

    commit 3a10f999ffd464d01c5a05592a15470a3c4bbc36 upstream.

    After commit 991f61fe7e1d ("Blk-throttle: reduce tail io latency when
    iops limit is enforced") wait time could be zero even if group is
    throttled and cannot issue requests right now. As a result
    throtl_select_dispatch() turns into busy-loop under irq-safe queue
    spinlock.

    Fix is simple: always round up target time to the next throttle slice.

    Fixes: 991f61fe7e1d ("Blk-throttle: reduce tail io latency when iops limit is enforced")
    Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
    Cc: stable@vger.kernel.org # v4.19+
    Signed-off-by: Jens Axboe <axboe@kernel.dk>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    block/blk-throttle.c | 9 +++------
    1 file changed, 3 insertions(+), 6 deletions(-)

    --- a/block/blk-throttle.c
    +++ b/block/blk-throttle.c
    @@ -892,13 +892,10 @@ static bool tg_with_in_iops_limit(struct
    unsigned long jiffy_elapsed, jiffy_wait, jiffy_elapsed_rnd;
    u64 tmp;

    - jiffy_elapsed = jiffy_elapsed_rnd = jiffies - tg->slice_start[rw];
    + jiffy_elapsed = jiffies - tg->slice_start[rw];

    - /* Slice has just started. Consider one slice interval */
    - if (!jiffy_elapsed)
    - jiffy_elapsed_rnd = tg->td->throtl_slice;
    -
    - jiffy_elapsed_rnd = roundup(jiffy_elapsed_rnd, tg->td->throtl_slice);
    + /* Round up to the next throttle slice, wait time must be nonzero */
    + jiffy_elapsed_rnd = roundup(jiffy_elapsed + 1, tg->td->throtl_slice);

    /*
    * jiffy_elapsed_rnd should not be a big value as minimum iops can be

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