blk-iocost: implement delay adjustment hysteresis

Curently, iocost syncs the delay duration to the outstanding debt amount,
which seemed enough to protect the system from anon memory hogs. However,
that was mostly because the delay calcuation was using hweight_inuse which
quickly converges towards zero under debt for delay duration calculation,
often pusnishing debtors overly harshly for longer than deserved.

The previous patch fixed the delay calcuation and now the protection against
anonymous memory hogs isn't enough because the effect of delay is indirect
and non-linear and a huge amount of future debt can accumulate abruptly
while unthrottled.

This patch implements delay hysteresis so that delay is decayed
exponentially over time instead of getting cleared immediately as debt is
paid off. While the overall behavior is similar to the blk-cgroup
implementation used by blk-iolatency, a lot of the details are different and
due to the empirical nature of the mechanism, it's challenging to adapt the
mechanism for one controller without negatively impacting the other.

As the delay is gradually decayed now, there's no point in running it from
its own hrtimer. Periodic updates are now performed from ioc_timer_fn() and
the dedicated hrtimer is removed.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-cgroup.c

@@ -1613,16 +1613,24 @@ static void blkcg_scale_delay(struct blkcg_gq *blkg, u64 now)
 static void blkcg_maybe_throttle_blkg(struct blkcg_gq *blkg, bool use_memdelay)
 {
 	unsigned long pflags;
+	bool clamp;
 	u64 now = ktime_to_ns(ktime_get());
 	u64 exp;
 	u64 delay_nsec = 0;
 	int tok;
 
 	while (blkg->parent) {
-		if (atomic_read(&blkg->use_delay)) {
+		int use_delay = atomic_read(&blkg->use_delay);
+
+		if (use_delay) {
+			u64 this_delay;
+
 			blkcg_scale_delay(blkg, now);
-			delay_nsec = max_t(u64, delay_nsec,
-					   atomic64_read(&blkg->delay_nsec));
+			this_delay = atomic64_read(&blkg->delay_nsec);
+			if (this_delay > delay_nsec) {
+				delay_nsec = this_delay;
+				clamp = use_delay > 0;
+			}
 		}
 		blkg = blkg->parent;
 	}
@@ -1634,10 +1642,13 @@ static void blkcg_maybe_throttle_blkg(struct blkcg_gq *blkg, bool use_memdelay)
 	 * Let's not sleep for all eternity if we've amassed a huge delay.
 	 * Swapping or metadata IO can accumulate 10's of seconds worth of
 	 * delay, and we want userspace to be able to do _something_ so cap the
-	 * delays at 1 second.  If there's 10's of seconds worth of delay then
-	 * the tasks will be delayed for 1 second for every syscall.
+	 * delays at 0.25s. If there's 10's of seconds worth of delay then the
+	 * tasks will be delayed for 0.25 second for every syscall. If
+	 * blkcg_set_delay() was used as indicated by negative use_delay, the
+	 * caller is responsible for regulating the range.
 	 */
-	delay_nsec = min_t(u64, delay_nsec, 250 * NSEC_PER_MSEC);
+	if (clamp)
+		delay_nsec = min_t(u64, delay_nsec, 250 * NSEC_PER_MSEC);
 
 	if (use_memdelay)
 		psi_memstall_enter(&pflags);

block/blk-iocost.c

@@ -270,6 +270,31 @@ enum {
 	/* unbusy hysterisis */
 	UNBUSY_THR_PCT		= 75,
 
+	/*
+	 * The effect of delay is indirect and non-linear and a huge amount of
+	 * future debt can accumulate abruptly while unthrottled. Linearly scale
+	 * up delay as debt is going up and then let it decay exponentially.
+	 * This gives us quick ramp ups while delay is accumulating and long
+	 * tails which can help reducing the frequency of debt explosions on
+	 * unthrottle. The parameters are experimentally determined.
+	 *
+	 * The delay mechanism provides adequate protection and behavior in many
+	 * cases. However, this is far from ideal and falls shorts on both
+	 * fronts. The debtors are often throttled too harshly costing a
+	 * significant level of fairness and possibly total work while the
+	 * protection against their impacts on the system can be choppy and
+	 * unreliable.
+	 *
+	 * The shortcoming primarily stems from the fact that, unlike for page
+	 * cache, the kernel doesn't have well-defined back-pressure propagation
+	 * mechanism and policies for anonymous memory. Fully addressing this
+	 * issue will likely require substantial improvements in the area.
+	 */
+	MIN_DELAY_THR_PCT	= 500,
+	MAX_DELAY_THR_PCT	= 25000,
+	MIN_DELAY		= 250,
+	MAX_DELAY		= 250 * USEC_PER_MSEC,
+
 	/* don't let cmds which take a very long time pin lagging for too long */
 	MAX_LAGGING_PERIODS	= 10,
 
@@ -473,6 +498,10 @@ struct ioc_gq {
 	atomic64_t			done_vtime;
 	u64				abs_vdebt;
 
+	/* current delay in effect and when it started */
+	u64				delay;
+	u64				delay_at;
+
 	/*
 	 * The period this iocg was last active in.  Used for deactivation
 	 * and invalidating `vtime`.
@@ -495,7 +524,6 @@ struct ioc_gq {
 
 	struct wait_queue_head		waitq;
 	struct hrtimer			waitq_timer;
-	struct hrtimer			delay_timer;
 
 	/* timestamp at the latest activation */
 	u64				activated_at;
@@ -1204,58 +1232,50 @@ static bool iocg_kick_delay(struct ioc_gq *iocg, struct ioc_now *now)
 {
 	struct ioc *ioc = iocg->ioc;
 	struct blkcg_gq *blkg = iocg_to_blkg(iocg);
-	u64 vtime = atomic64_read(&iocg->vtime);
-	u64 delta_ns, expires, oexpires;
+	u64 tdelta, delay, new_delay;
+	s64 vover, vover_pct;
 	u32 hwa;
 
 	lockdep_assert_held(&iocg->waitq.lock);
 
-	/* debt-adjust vtime */
-	current_hweight(iocg, &hwa, NULL);
-	vtime += abs_cost_to_cost(iocg->abs_vdebt, hwa);
+	/* calculate the current delay in effect - 1/2 every second */
+	tdelta = now->now - iocg->delay_at;
+	if (iocg->delay)
+		delay = iocg->delay >> div64_u64(tdelta, USEC_PER_SEC);
+	else
+		delay = 0;
 
-	/*
-	 * Clear or maintain depending on the overage. Non-zero vdebt is what
-	 * guarantees that @iocg is online and future iocg_kick_delay() will
-	 * clear use_delay. Don't leave it on when there's no vdebt.
-	 */
-	if (!iocg->abs_vdebt || time_before_eq64(vtime, now->vnow)) {
+	/* calculate the new delay from the debt amount */
+	current_hweight(iocg, &hwa, NULL);
+	vover = atomic64_read(&iocg->vtime) +
+		abs_cost_to_cost(iocg->abs_vdebt, hwa) - now->vnow;
+	vover_pct = div64_s64(100 * vover, ioc->period_us * now->vrate);
+
+	if (vover_pct <= MIN_DELAY_THR_PCT)
+		new_delay = 0;
+	else if (vover_pct >= MAX_DELAY_THR_PCT)
+		new_delay = MAX_DELAY;
+	else
+		new_delay = MIN_DELAY +
+			div_u64((MAX_DELAY - MIN_DELAY) *
+				(vover_pct - MIN_DELAY_THR_PCT),
+				MAX_DELAY_THR_PCT - MIN_DELAY_THR_PCT);
+
+	/* pick the higher one and apply */
+	if (new_delay > delay) {
+		iocg->delay = new_delay;
+		iocg->delay_at = now->now;
+		delay = new_delay;
+	}
+
+	if (delay >= MIN_DELAY) {
+		blkcg_set_delay(blkg, delay * NSEC_PER_USEC);
+		return true;
+	} else {
+		iocg->delay = 0;
 		blkcg_clear_delay(blkg);
 		return false;
 	}
-	if (!atomic_read(&blkg->use_delay) &&
-	    time_before_eq64(vtime, now->vnow + ioc->margins.target))
-		return false;
-
-	/* use delay */
-	delta_ns = DIV64_U64_ROUND_UP(vtime - now->vnow,
-				      now->vrate) * NSEC_PER_USEC;
-	blkcg_set_delay(blkg, delta_ns);
-	expires = now->now_ns + delta_ns;
-
-	/* if already active and close enough, don't bother */
-	oexpires = ktime_to_ns(hrtimer_get_softexpires(&iocg->delay_timer));
-	if (hrtimer_is_queued(&iocg->delay_timer) &&
-	    abs(oexpires - expires) <= ioc->timer_slack_ns)
-		return true;
-
-	hrtimer_start_range_ns(&iocg->delay_timer, ns_to_ktime(expires),
-			       ioc->timer_slack_ns, HRTIMER_MODE_ABS);
-	return true;
-}
-
-static enum hrtimer_restart iocg_delay_timer_fn(struct hrtimer *timer)
-{
-	struct ioc_gq *iocg = container_of(timer, struct ioc_gq, delay_timer);
-	struct ioc_now now;
-	unsigned long flags;
-
-	spin_lock_irqsave(&iocg->waitq.lock, flags);
-	ioc_now(iocg->ioc, &now);
-	iocg_kick_delay(iocg, &now);
-	spin_unlock_irqrestore(&iocg->waitq.lock, flags);
-
-	return HRTIMER_NORESTART;
 }
 
 static void iocg_incur_debt(struct ioc_gq *iocg, u64 abs_cost,
@@ -1356,10 +1376,11 @@ static void iocg_kick_waitq(struct ioc_gq *iocg, bool pay_debt,
 		atomic64_add(vpay, &iocg->done_vtime);
 		iocg_pay_debt(iocg, abs_vpay, now);
 		vbudget -= vpay;
-
-		iocg_kick_delay(iocg, now);
 	}
 
+	if (iocg->abs_vdebt || iocg->delay)
+		iocg_kick_delay(iocg, now);
+
 	/*
 	 * Debt can still be outstanding if we haven't paid all yet or the
 	 * caller raced and called without @pay_debt. Shouldn't wake up waiters
@@ -1906,12 +1927,13 @@ static void ioc_timer_fn(struct timer_list *timer)
 	 */
 	list_for_each_entry_safe(iocg, tiocg, &ioc->active_iocgs, active_list) {
 		if (!waitqueue_active(&iocg->waitq) && !iocg->abs_vdebt &&
-		    !iocg_is_idle(iocg))
+		    !iocg->delay && !iocg_is_idle(iocg))
 			continue;
 
 		spin_lock(&iocg->waitq.lock);
 
-		if (waitqueue_active(&iocg->waitq) || iocg->abs_vdebt) {
+		if (waitqueue_active(&iocg->waitq) || iocg->abs_vdebt ||
+		    iocg->delay) {
 			/* might be oversleeping vtime / hweight changes, kick */
 			iocg_kick_waitq(iocg, true, &now);
 		} else if (iocg_is_idle(iocg)) {
@@ -2641,8 +2663,6 @@ static void ioc_pd_init(struct blkg_policy_data *pd)
 	init_waitqueue_head(&iocg->waitq);
 	hrtimer_init(&iocg->waitq_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
 	iocg->waitq_timer.function = iocg_waitq_timer_fn;
-	hrtimer_init(&iocg->delay_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
-	iocg->delay_timer.function = iocg_delay_timer_fn;
 
 	iocg->level = blkg->blkcg->css.cgroup->level;
 
@@ -2679,7 +2699,6 @@ static void ioc_pd_free(struct blkg_policy_data *pd)
 		spin_unlock_irqrestore(&ioc->lock, flags);
 
 		hrtimer_cancel(&iocg->waitq_timer);
-		hrtimer_cancel(&iocg->delay_timer);
 	}
 	free_percpu(iocg->pcpu_stat);
 	kfree(iocg);