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blk-throttle: fix zero wait time for iops throttled group
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commit 3a10f99 upstream.

After commit 991f61f ("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: 991f61f ("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>
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Konstantin Khlebnikov authored and Greg Kroah-Hartman committed Jul 26, 2019
1 parent 3201a60 commit a613a79
Showing 1 changed file with 3 additions and 6 deletions.
9 changes: 3 additions & 6 deletions block/blk-throttle.c
Original file line number Diff line number Diff line change
Expand Up @@ -881,13 +881,10 @@ static bool tg_with_in_iops_limit(struct throtl_grp *tg, struct bio *bio,
unsigned long jiffy_elapsed, jiffy_wait, jiffy_elapsed_rnd;
u64 tmp;

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

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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