[PATCH 19/27] blk-iocost: revamp donation amount determination
From: Tejun Heo
Date: Tue Sep 01 2020 - 14:55:23 EST
iocost has various safety nets to combat inuse adjustment calculation
inaccuracies. With Andy's method implemented in transfer_surpluses(), inuse
adjustment calculations are now accurate and we can make donation amount
determinations accurate too.
* Stop keeping track of past usage history and using the maximum. Act on the
immediate usage information.
* Remove donation constraints defined by SURPLUS_* constants. Donate
whatever isn't used.
* Determine the donation amount so that the iocg will end up with
MARGIN_TARGET_PCT budget at the end of the coming period assuming the same
usage as the previous period. TARGET is set at 50% of period, which is the
previous maximum. This provides smooth convergence for most repetitive IO
patterns.
* Apply donation logic early at 20% budget. There's no risk in doing so as
the calculation is based on the delta between the current budget and the
target budget at the end of the coming period.
* Remove preemptive iocg activation for zero cost IOs. As donation can reach
near zero now, the mere activation doesn't provide any protection anymore.
In the unlikely case that this becomes a problem, the right solution is
assigning appropriate costs for such IOs.
This significantly improves the donation determination logic while also
simplifying it. Now all donations are immediate, exact and smooth.
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Cc: Andy Newell <newella@xxxxxx>
---
block/blk-iocost.c | 133 +++++++++++++++++----------------------------
1 file changed, 51 insertions(+), 82 deletions(-)
diff --git a/block/blk-iocost.c b/block/blk-iocost.c
index ecc23b827e5d..694f1487208a 100644
--- a/block/blk-iocost.c
+++ b/block/blk-iocost.c
@@ -217,12 +217,14 @@ enum {
MAX_PERIOD = USEC_PER_SEC,
/*
- * A cgroup's vtime can run 50% behind the device vtime, which
+ * iocg->vtime is targeted at 50% behind the device vtime, which
* serves as its IO credit buffer. Surplus weight adjustment is
* immediately canceled if the vtime margin runs below 10%.
*/
MARGIN_MIN_PCT = 10,
- MARGIN_MAX_PCT = 50,
+ MARGIN_LOW_PCT = 20,
+ MARGIN_TARGET_PCT = 50,
+ MARGIN_MAX_PCT = 100,
/* Have some play in timer operations */
TIMER_SLACK_PCT = 1,
@@ -234,17 +236,6 @@ enum {
*/
VTIME_VALID_DUR = 300 * USEC_PER_SEC,
- /*
- * Remember the past three non-zero usages and use the max for
- * surplus calculation. Three slots guarantee that we remember one
- * full period usage from the last active stretch even after
- * partial deactivation and re-activation periods. Don't start
- * giving away weight before collecting two data points to prevent
- * hweight adjustments based on one partial activation period.
- */
- NR_USAGE_SLOTS = 3,
- MIN_VALID_USAGES = 2,
-
/* 1/64k is granular enough and can easily be handled w/ u32 */
WEIGHT_ONE = 1 << 16,
@@ -280,14 +271,6 @@ enum {
/* don't let cmds which take a very long time pin lagging for too long */
MAX_LAGGING_PERIODS = 10,
- /*
- * If usage% * 1.25 + 2% is lower than hweight% by more than 3%,
- * donate the surplus.
- */
- SURPLUS_SCALE_PCT = 125, /* * 125% */
- SURPLUS_SCALE_ABS = WEIGHT_ONE / 50, /* + 2% */
- SURPLUS_MIN_ADJ_DELTA = WEIGHT_ONE / 33, /* 3% */
-
/* switch iff the conditions are met for longer than this */
AUTOP_CYCLE_NSEC = 10LLU * NSEC_PER_SEC,
@@ -376,6 +359,8 @@ struct ioc_params {
struct ioc_margins {
s64 min;
+ s64 low;
+ s64 target;
s64 max;
};
@@ -514,11 +499,7 @@ struct ioc_gq {
struct iocg_stat desc_stat;
struct iocg_stat last_stat;
u64 last_stat_abs_vusage;
-
- /* usage is recorded as fractions of WEIGHT_ONE */
- u32 usage_delta_us;
- int usage_idx;
- u32 usages[NR_USAGE_SLOTS];
+ u64 usage_delta_us;
/* this iocg's depth in the hierarchy and ancestors including self */
int level;
@@ -737,6 +718,8 @@ static void ioc_refresh_margins(struct ioc *ioc)
u64 vrate = atomic64_read(&ioc->vtime_rate);
margins->min = (period_us * MARGIN_MIN_PCT / 100) * vrate;
+ margins->low = (period_us * MARGIN_LOW_PCT / 100) * vrate;
+ margins->target = (period_us * MARGIN_TARGET_PCT / 100) * vrate;
margins->max = (period_us * MARGIN_MAX_PCT / 100) * vrate;
}
@@ -1228,7 +1211,7 @@ static bool iocg_kick_delay(struct ioc_gq *iocg, struct ioc_now *now)
return false;
}
if (!atomic_read(&blkg->use_delay) &&
- time_before_eq64(vtime, now->vnow + ioc->margins.max))
+ time_before_eq64(vtime, now->vnow + ioc->margins.target))
return false;
/* use delay */
@@ -1527,7 +1510,7 @@ static u32 hweight_after_donation(struct ioc_gq *iocg, u32 hwm, u32 usage,
{
struct ioc *ioc = iocg->ioc;
u64 vtime = atomic64_read(&iocg->vtime);
- s64 excess;
+ s64 excess, delta, target, new_hwi;
/* see whether minimum margin requirement is met */
if (waitqueue_active(&iocg->waitq) ||
@@ -1542,15 +1525,28 @@ static u32 hweight_after_donation(struct ioc_gq *iocg, u32 hwm, u32 usage,
vtime += excess;
}
- /* add margin */
- usage = DIV_ROUND_UP(usage * SURPLUS_SCALE_PCT, 100);
- usage += SURPLUS_SCALE_ABS;
-
- /* don't bother if the surplus is too small */
- if (usage + SURPLUS_MIN_ADJ_DELTA > hwm)
- return hwm;
+ /*
+ * Let's say the distance between iocg's and device's vtimes as a
+ * fraction of period duration is delta. Assuming that the iocg will
+ * consume the usage determined above, we want to determine new_hwi so
+ * that delta equals MARGIN_TARGET at the end of the next period.
+ *
+ * We need to execute usage worth of IOs while spending the sum of the
+ * new budget (1 - MARGIN_TARGET) and the leftover from the last period
+ * (delta):
+ *
+ * usage = (1 - MARGIN_TARGET + delta) * new_hwi
+ *
+ * Therefore, the new_hwi is:
+ *
+ * new_hwi = usage / (1 - MARGIN_TARGET + delta)
+ */
+ delta = div64_s64(WEIGHT_ONE * (now->vnow - vtime),
+ now->vnow - ioc->period_at_vtime);
+ target = WEIGHT_ONE * MARGIN_TARGET_PCT / 100;
+ new_hwi = div64_s64(WEIGHT_ONE * usage, WEIGHT_ONE - target + delta);
- return usage;
+ return clamp_t(s64, new_hwi, 1, hwm);
}
/*
@@ -1812,7 +1808,7 @@ static void ioc_timer_fn(struct timer_list *timer)
u32 ppm_wthr = MILLION - ioc->params.qos[QOS_WPPM];
u32 missed_ppm[2], rq_wait_pct;
u64 period_vtime;
- int prev_busy_level, i;
+ int prev_busy_level;
/* how were the latencies during the period? */
ioc_lat_stat(ioc, missed_ppm, &rq_wait_pct);
@@ -1857,11 +1853,10 @@ static void ioc_timer_fn(struct timer_list *timer)
}
commit_weights(ioc);
- /* calc usages and see whether some weights need to be moved around */
+ /* calc usage and see whether some weights need to be moved around */
list_for_each_entry(iocg, &ioc->active_iocgs, active_list) {
- u64 vdone, vtime, usage_us;
- u32 hw_active, hw_inuse, usage;
- int uidx, nr_valid;
+ u64 vdone, vtime, usage_us, usage_dur;
+ u32 usage, hw_active, hw_inuse;
/*
* Collect unused and wind vtime closer to vnow to prevent
@@ -1886,15 +1881,11 @@ static void ioc_timer_fn(struct timer_list *timer)
nr_lagging++;
/*
- * Determine absolute usage factoring in pending and in-flight
- * IOs to avoid stalls and high-latency completions appearing as
- * idle.
+ * Determine absolute usage factoring in in-flight IOs to avoid
+ * high-latency completions appearing as idle.
*/
usage_us = iocg->usage_delta_us;
- if (waitqueue_active(&iocg->waitq) && time_before64(vtime, now.vnow))
- usage_us += DIV64_U64_ROUND_UP(
- cost_to_abs_cost(now.vnow - vtime, hw_inuse),
- now.vrate);
+
if (vdone != vtime) {
u64 inflight_us = DIV64_U64_ROUND_UP(
cost_to_abs_cost(vtime - vdone, hw_inuse),
@@ -1902,43 +1893,22 @@ static void ioc_timer_fn(struct timer_list *timer)
usage_us = max(usage_us, inflight_us);
}
- /* convert to hweight based usage ratio and record */
- uidx = (iocg->usage_idx + 1) % NR_USAGE_SLOTS;
-
- if (time_after64(vtime, now.vnow - ioc->margins.min)) {
- iocg->usage_idx = uidx;
- iocg->usages[uidx] = WEIGHT_ONE;
- } else if (usage_us) {
- u64 started_at, dur;
-
- if (time_after64(iocg->activated_at, ioc->period_at))
- started_at = iocg->activated_at;
- else
- started_at = ioc->period_at;
-
- dur = max_t(u64, now.now - started_at, 1);
+ /* convert to hweight based usage ratio */
+ if (time_after64(iocg->activated_at, ioc->period_at))
+ usage_dur = max_t(u64, now.now - iocg->activated_at, 1);
+ else
+ usage_dur = max_t(u64, now.now - ioc->period_at, 1);
- iocg->usage_idx = uidx;
- iocg->usages[uidx] = clamp_t(u32,
- DIV64_U64_ROUND_UP(usage_us * WEIGHT_ONE, dur),
+ usage = clamp_t(u32,
+ DIV64_U64_ROUND_UP(usage_us * WEIGHT_ONE,
+ usage_dur),
1, WEIGHT_ONE);
- }
-
- /* base the decision on max historical usage */
- for (i = 0, usage = 0, nr_valid = 0; i < NR_USAGE_SLOTS; i++) {
- if (iocg->usages[i]) {
- usage = max(usage, iocg->usages[i]);
- nr_valid++;
- }
- }
- if (nr_valid < MIN_VALID_USAGES)
- usage = WEIGHT_ONE;
/* see whether there's surplus vtime */
WARN_ON_ONCE(!list_empty(&iocg->surplus_list));
if (hw_inuse < hw_active ||
(!waitqueue_active(&iocg->waitq) &&
- time_before64(vtime, now.vnow - ioc->margins.max))) {
+ time_before64(vtime, now.vnow - ioc->margins.low))) {
u32 hwa, hwm, new_hwi;
/*
@@ -2175,15 +2145,14 @@ static void ioc_rqos_throttle(struct rq_qos *rqos, struct bio *bio)
if (!ioc->enabled || !iocg->level)
return;
- /* always activate so that even 0 cost IOs get protected to some level */
- if (!iocg_activate(iocg, &now))
- return;
-
/* calculate the absolute vtime cost */
abs_cost = calc_vtime_cost(bio, iocg, false);
if (!abs_cost)
return;
+ if (!iocg_activate(iocg, &now))
+ return;
+
iocg->cursor = bio_end_sector(bio);
vtime = atomic64_read(&iocg->vtime);
--
2.26.2