[PATCH v2 04/10] blk-throttle: correct calculation of wait time in tg_may_dispatch

Kemeng Shi
2022-11-29T03:02:24Z

In C language, When executing "if (expression1 && expression2)" and
expression1 return false, the expression2 may not be executed.
For "tg_within_bps_limit(tg, bio, bps_limit, &bps_wait) &&
tg_within_iops_limit(tg, bio, iops_limit, &iops_wait))", if bps is
limited, tg_within_bps_limit will return false and
tg_within_iops_limit will not be called. So even bps and iops are
both limited, iops_wait will not be calculated and is always zero.
So wait time of iops is always ignored.

Fix this by always calling tg_within_bps_limit and tg_within_iops_limit
to get wait time for both bps and iops.

Observed that:
1. Wait time in tg_within_iops_limit/tg_within_bps_limit need always
be stored as wait argument is always passed.
2. wait time is stored to zero if iops/bps is limited otherwise non-zero
is stored.
Simpfy tg_within_iops_limit/tg_within_bps_limit by removing wait argument
and return wait time directly. Caller tg_may_dispatch checks if wait time
is zero to find if iops/bps is limited.

Signed-off-by: Kemeng Shi <shikemeng@huawei.com>
---
block/blk-throttle.c | 38 +++++++++++++-------------------------
1 file changed, 13 insertions(+), 25 deletions(-)

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index ee7dc1a0adfd..06e4193b064e 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -822,17 +822,15 @@ static void tg_update_carryover(struct throtl_grp *tg)
tg->carryover_ios[READ], tg->carryover_ios[WRITE]);
}

-static bool tg_within_iops_limit(struct throtl_grp *tg, struct bio *bio,
- u32 iops_limit, unsigned long *wait)
+static unsigned long tg_within_iops_limit(struct throtl_grp *tg, struct bio *bio,
+ u32 iops_limit)
{
bool rw = bio_data_dir(bio);
unsigned int io_allowed;
unsigned long jiffy_elapsed, jiffy_wait, jiffy_elapsed_rnd;

if (iops_limit == UINT_MAX) {
- if (wait)
- *wait = 0;
- return true;
+ return 0;
}

jiffy_elapsed = jiffies - tg->slice_start[rw];
@@ -842,21 +840,16 @@ static bool tg_within_iops_limit(struct throtl_grp *tg, struct bio *bio,
io_allowed = calculate_io_allowed(iops_limit, jiffy_elapsed_rnd) +
tg->carryover_ios[rw];
if (tg->io_disp[rw] + 1 <= io_allowed) {
- if (wait)
- *wait = 0;
- return true;
+ return 0;
}

/* Calc approx time to dispatch */
jiffy_wait = jiffy_elapsed_rnd - jiffy_elapsed;
-
- if (wait)
- *wait = jiffy_wait;
- return false;
+ return jiffy_wait;
}

-static bool tg_within_bps_limit(struct throtl_grp *tg, struct bio *bio,
- u64 bps_limit, unsigned long *wait)
+static unsigned long tg_within_bps_limit(struct throtl_grp *tg, struct bio *bio,
+ u64 bps_limit)
{
bool rw = bio_data_dir(bio);
u64 bytes_allowed, extra_bytes;
@@ -865,9 +858,7 @@ static bool tg_within_bps_limit(struct throtl_grp *tg, struct bio *bio,

/* no need to throttle if this bio's bytes have been accounted */
if (bps_limit == U64_MAX || bio_flagged(bio, BIO_BPS_THROTTLED)) {
- if (wait)
- *wait = 0;
- return true;
+ return 0;
}

jiffy_elapsed = jiffy_elapsed_rnd = jiffies - tg->slice_start[rw];
@@ -880,9 +871,7 @@ static bool tg_within_bps_limit(struct throtl_grp *tg, struct bio *bio,
bytes_allowed = calculate_bytes_allowed(bps_limit, jiffy_elapsed_rnd) +
tg->carryover_bytes[rw];
if (tg->bytes_disp[rw] + bio_size <= bytes_allowed) {
- if (wait)
- *wait = 0;
- return true;
+ return 0;
}

/* Calc approx time to dispatch */
@@ -897,9 +886,7 @@ static bool tg_within_bps_limit(struct throtl_grp *tg, struct bio *bio,
* up we did. Add that time also.
*/
jiffy_wait = jiffy_wait + (jiffy_elapsed_rnd - jiffy_elapsed);
- if (wait)
- *wait = jiffy_wait;
- return false;
+ return jiffy_wait;
}

/*
@@ -947,8 +934,9 @@ static bool tg_may_dispatch(struct throtl_grp *tg, struct bio *bio,
jiffies + tg->td->throtl_slice);
}

- if (tg_within_bps_limit(tg, bio, bps_limit, &bps_wait) &&
- tg_within_iops_limit(tg, bio, iops_limit, &iops_wait)) {
+ bps_wait = tg_within_bps_limit(tg, bio, bps_limit);
+ iops_wait = tg_within_iops_limit(tg, bio, iops_limit);
+ if (bps_wait + iops_wait == 0) {
if (wait)
*wait = 0;
return true;
--
2.30.0