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blk-iocost: revamp in-period donation snapbacks 

When the margin drops below the minimum on a donating iocg, donation is
immediately canceled in full. There are a couple shortcomings with the
current behavior.

* It's abrupt. A small temporary budget deficit can lead to a wide swing in
  weight allocation and a large surplus.

* It's open coded in the issue path but not implemented for the merge path.
  A series of merges at a low inuse can make the iocg incur debts and stall
  incorrectly.

This patch reimplements in-period donation snapbacks so that

* inuse adjustment and cost calculations are factored into
  adjust_inuse_and_calc_cost() which is called from both the issue and merge
  paths.

* Snapbacks are more gradual. It occurs in quarter steps.

* A snapback triggers if the margin goes below the low threshold and is
  lower than the budget at the time of the last adjustment.

* For the above, __propagate_weights() stores the margin in
  iocg->saved_margin. Move iocg->last_inuse storing together into
  __propagate_weights() for consistency.

* Full snapback is guaranteed when there are waiters.

* With precise donation and gradual snapbacks, inuse adjustments are now a
  lot more effective and the value of scaling inuse on weight changes isn't
  clear. Removed inuse scaling from weight_update().

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Tejun Heo 2020-09-01 14:52:50 -04:00 committed by Jens Axboe
parent f1de2439ec
commit b0853ab4a2
1 changed files with 96 additions and 37 deletions
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133
block/blk-iocost.c
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@ -226,6 +226,8 @@ enum {
MARGIN_TARGET_PCT = 50,
MARGIN_MAX_PCT = 100,
INUSE_ADJ_STEP_PCT = 25,
/* Have some play in timer operations */
TIMER_SLACK_PCT = 1,
@ -443,12 +445,17 @@ struct ioc_gq {
*
* `last_inuse` remembers `inuse` while an iocg is idle to persist
* surplus adjustments.
*
* `inuse` may be adjusted dynamically during period. `saved_*` are used
* to determine and track adjustments.
*/
u32 cfg_weight;
u32 weight;
u32 active;
u32 inuse;
u32 last_inuse;
s64 saved_margin;
sector_t cursor; /* to detect randio */
@ -934,9 +941,11 @@ static void ioc_start_period(struct ioc *ioc, struct ioc_now *now)
/*
* Update @iocg's `active` and `inuse` to @active and @inuse, update level
* weight sums and propagate upwards accordingly.
* weight sums and propagate upwards accordingly. If @save, the current margin
* is saved to be used as reference for later inuse in-period adjustments.
*/
static void __propagate_weights(struct ioc_gq *iocg, u32 active, u32 inuse)
static void __propagate_weights(struct ioc_gq *iocg, u32 active, u32 inuse,
bool save, struct ioc_now *now)
{
struct ioc *ioc = iocg->ioc;
int lvl;
@ -945,6 +954,10 @@ static void __propagate_weights(struct ioc_gq *iocg, u32 active, u32 inuse)
inuse = clamp_t(u32, inuse, 1, active);
iocg->last_inuse = iocg->inuse;
if (save)
iocg->saved_margin = now->vnow - atomic64_read(&iocg->vtime);
if (active == iocg->active && inuse == iocg->inuse)
return;
@ -996,9 +1009,10 @@ static void commit_weights(struct ioc *ioc)
}
}
static void propagate_weights(struct ioc_gq *iocg, u32 active, u32 inuse)
static void propagate_weights(struct ioc_gq *iocg, u32 active, u32 inuse,
bool save, struct ioc_now *now)
{
__propagate_weights(iocg, active, inuse);
__propagate_weights(iocg, active, inuse, save, now);
commit_weights(iocg->ioc);
}
@ -1082,7 +1096,7 @@ static u32 current_hweight_max(struct ioc_gq *iocg)
return max_t(u32, hwm, 1);
}
static void weight_updated(struct ioc_gq *iocg)
static void weight_updated(struct ioc_gq *iocg, struct ioc_now *now)
{
struct ioc *ioc = iocg->ioc;
struct blkcg_gq *blkg = iocg_to_blkg(iocg);
@ -1093,9 +1107,7 @@ static void weight_updated(struct ioc_gq *iocg)
weight = iocg->cfg_weight ?: iocc->dfl_weight;
if (weight != iocg->weight && iocg->active)
propagate_weights(iocg, weight,
DIV64_U64_ROUND_UP((u64)iocg->inuse * weight,
iocg->weight));
propagate_weights(iocg, weight, iocg->inuse, true, now);
iocg->weight = weight;
}
@ -1165,8 +1177,9 @@ static bool iocg_activate(struct ioc_gq *iocg, struct ioc_now *now)
*/
iocg->hweight_gen = atomic_read(&ioc->hweight_gen) - 1;
list_add(&iocg->active_list, &ioc->active_iocgs);
propagate_weights(iocg, iocg->weight,
iocg->last_inuse ?: iocg->weight);
iocg->last_inuse ?: iocg->weight, true, now);
TRACE_IOCG_PATH(iocg_activate, iocg, now,
last_period, cur_period, vtime);
@ -1789,7 +1802,7 @@ static void transfer_surpluses(struct list_head *surpluses, struct ioc_now *now)
inuse = DIV64_U64_ROUND_UP(
parent->child_adjusted_sum * iocg->hweight_after_donation,
parent->hweight_inuse);
__propagate_weights(iocg, iocg->active, inuse);
__propagate_weights(iocg, iocg->active, inuse, true, now);
}
/* walk list should be dissolved after use */
@ -1844,8 +1857,7 @@ static void ioc_timer_fn(struct timer_list *timer)
iocg_kick_waitq(iocg, true, &now);
} else if (iocg_is_idle(iocg)) {
/* no waiter and idle, deactivate */
iocg->last_inuse = iocg->inuse;
__propagate_weights(iocg, 0, 0);
__propagate_weights(iocg, 0, 0, false, &now);
list_del_init(&iocg->active_list);
}
@ -1925,7 +1937,7 @@ static void ioc_timer_fn(struct timer_list *timer)
list_add(&iocg->surplus_list, &surpluses);
} else {
__propagate_weights(iocg, iocg->active,
iocg->active);
iocg->active, true, &now);
nr_shortages++;
}
} else {
@ -2055,6 +2067,50 @@ static void ioc_timer_fn(struct timer_list *timer)
spin_unlock_irq(&ioc->lock);
}
static u64 adjust_inuse_and_calc_cost(struct ioc_gq *iocg, u64 vtime,
u64 abs_cost, struct ioc_now *now)
{
struct ioc *ioc = iocg->ioc;
struct ioc_margins *margins = &ioc->margins;
u32 adj_step = DIV_ROUND_UP(iocg->active * INUSE_ADJ_STEP_PCT, 100);
u32 hwi;
s64 margin;
u64 cost, new_inuse;
current_hweight(iocg, NULL, &hwi);
cost = abs_cost_to_cost(abs_cost, hwi);
margin = now->vnow - vtime - cost;
/*
* We only increase inuse during period and do so iff the margin has
* deteriorated since the previous adjustment.
*/
if (margin >= iocg->saved_margin || margin >= margins->low ||
iocg->inuse == iocg->active)
return cost;
spin_lock_irq(&ioc->lock);
/* we own inuse only when @iocg is in the normal active state */
if (list_empty(&iocg->active_list)) {
spin_unlock_irq(&ioc->lock);
return cost;
}
/* bump up inuse till @abs_cost fits in the existing budget */
new_inuse = iocg->inuse;
do {
new_inuse = new_inuse + adj_step;
propagate_weights(iocg, iocg->active, new_inuse, true, now);
current_hweight(iocg, NULL, &hwi);
cost = abs_cost_to_cost(abs_cost, hwi);
} while (time_after64(vtime + cost, now->vnow) &&
iocg->inuse != iocg->active);
spin_unlock_irq(&ioc->lock);
return cost;
}
static void calc_vtime_cost_builtin(struct bio *bio, struct ioc_gq *iocg,
bool is_merge, u64 *costp)
{
@ -2136,7 +2192,6 @@ static void ioc_rqos_throttle(struct rq_qos *rqos, struct bio *bio)
struct ioc_gq *iocg = blkg_to_iocg(blkg);
struct ioc_now now;
struct iocg_wait wait;
u32 hw_active, hw_inuse;
u64 abs_cost, cost, vtime;
bool use_debt, ioc_locked;
unsigned long flags;
@ -2154,21 +2209,8 @@ static void ioc_rqos_throttle(struct rq_qos *rqos, struct bio *bio)
return;
iocg->cursor = bio_end_sector(bio);
vtime = atomic64_read(&iocg->vtime);
current_hweight(iocg, &hw_active, &hw_inuse);
if (hw_inuse < hw_active &&
time_after_eq64(vtime + ioc->margins.min, now.vnow)) {
TRACE_IOCG_PATH(inuse_reset, iocg, &now,
iocg->inuse, iocg->weight, hw_inuse, hw_active);
spin_lock_irq(&ioc->lock);
propagate_weights(iocg, iocg->weight, iocg->weight);
spin_unlock_irq(&ioc->lock);
current_hweight(iocg, &hw_active, &hw_inuse);
}
cost = abs_cost_to_cost(abs_cost, hw_inuse);
cost = adjust_inuse_and_calc_cost(iocg, vtime, abs_cost, &now);
/*
* If no one's waiting and within budget, issue right away. The
@ -2190,7 +2232,7 @@ static void ioc_rqos_throttle(struct rq_qos *rqos, struct bio *bio)
*/
use_debt = bio_issue_as_root_blkg(bio) || fatal_signal_pending(current);
ioc_locked = use_debt || READ_ONCE(iocg->abs_vdebt);
retry_lock:
iocg_lock(iocg, ioc_locked, &flags);
/*
@ -2232,6 +2274,17 @@ static void ioc_rqos_throttle(struct rq_qos *rqos, struct bio *bio)
return;
}
/* guarantee that iocgs w/ waiters have maximum inuse */
if (iocg->inuse != iocg->active) {
if (!ioc_locked) {
iocg_unlock(iocg, false, &flags);
ioc_locked = true;
goto retry_lock;
}
propagate_weights(iocg, iocg->active, iocg->active, true,
&now);
}
/*
* Append self to the waitq and schedule the wakeup timer if we're
* the first waiter. The timer duration is calculated based on the
@ -2274,8 +2327,7 @@ static void ioc_rqos_merge(struct rq_qos *rqos, struct request *rq,
struct ioc *ioc = iocg->ioc;
sector_t bio_end = bio_end_sector(bio);
struct ioc_now now;
u32 hw_inuse;
u64 abs_cost, cost;
u64 vtime, abs_cost, cost;
unsigned long flags;
/* bypass if disabled or for root cgroup */
@ -2287,8 +2339,9 @@ static void ioc_rqos_merge(struct rq_qos *rqos, struct request *rq,
return;
ioc_now(ioc, &now);
current_hweight(iocg, NULL, &hw_inuse);
cost = abs_cost_to_cost(abs_cost, hw_inuse);
vtime = atomic64_read(&iocg->vtime);
cost = adjust_inuse_and_calc_cost(iocg, vtime, abs_cost, &now);
/* update cursor if backmerging into the request at the cursor */
if (blk_rq_pos(rq) < bio_end &&
@ -2530,7 +2583,7 @@ static void ioc_pd_init(struct blkg_policy_data *pd)
}
spin_lock_irqsave(&ioc->lock, flags);
weight_updated(iocg);
weight_updated(iocg, &now);
spin_unlock_irqrestore(&ioc->lock, flags);
}
@ -2544,7 +2597,10 @@ static void ioc_pd_free(struct blkg_policy_data *pd)
spin_lock_irqsave(&ioc->lock, flags);
if (!list_empty(&iocg->active_list)) {
propagate_weights(iocg, 0, 0);
struct ioc_now now;
ioc_now(ioc, &now);
propagate_weights(iocg, 0, 0, false, &now);
list_del_init(&iocg->active_list);
}
@ -2612,6 +2668,7 @@ static ssize_t ioc_weight_write(struct kernfs_open_file *of, char *buf,
struct blkcg *blkcg = css_to_blkcg(of_css(of));
struct ioc_cgrp *iocc = blkcg_to_iocc(blkcg);
struct blkg_conf_ctx ctx;
struct ioc_now now;
struct ioc_gq *iocg;
u32 v;
int ret;
@ -2632,7 +2689,8 @@ static ssize_t ioc_weight_write(struct kernfs_open_file *of, char *buf,
if (iocg) {
spin_lock_irq(&iocg->ioc->lock);
weight_updated(iocg);
ioc_now(iocg->ioc, &now);
weight_updated(iocg, &now);
spin_unlock_irq(&iocg->ioc->lock);
}
}
@ -2658,7 +2716,8 @@ static ssize_t ioc_weight_write(struct kernfs_open_file *of, char *buf,
spin_lock(&iocg->ioc->lock);
iocg->cfg_weight = v * WEIGHT_ONE;
weight_updated(iocg);
ioc_now(iocg->ioc, &now);
weight_updated(iocg, &now);
spin_unlock(&iocg->ioc->lock);
blkg_conf_finish(&ctx);