blk-iocost: halve debts if device stays idle

A low weight iocg can amass a large amount of debt, for example, when
anonymous memory gets reclaimed aggressively. If the system has a lot of
memory paired with a slow IO device, the debt can span multiple seconds or
more. If there are no other subsequent IO issuers, the in-debt iocg may end
up blocked paying its debt while the IO device is idle.

This patch implements a mechanism to protect against such pathological
cases. If the device has been sufficiently idle for a substantial amount of
time, the debts are halved. The criteria are on the conservative side as we
want to resolve the rare extreme cases without impacting regular operation
by forgiving debts too readily.
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-iocost.c

@@ -295,6 +295,13 @@ enum {
 	MIN_DELAY		= 250,
 	MAX_DELAY		= 250 * USEC_PER_MSEC,
 
+	/*
+	 * Halve debts if total usage keeps staying under 25% w/o any shortages
+	 * for over 100ms.
+	 */
+	DEBT_BUSY_USAGE_PCT	= 25,
+	DEBT_REDUCTION_IDLE_DUR	= 100 * USEC_PER_MSEC,
+
 	/* don't let cmds which take a very long time pin lagging for too long */
 	MAX_LAGGING_PERIODS	= 10,
 
@@ -436,6 +443,9 @@ struct ioc {
 	bool				weights_updated;
 	atomic_t			hweight_gen;	/* for lazy hweights */
 
+	/* the last time debt cancel condition wasn't met */
+	u64				debt_busy_at;
+
 	u64				autop_too_fast_at;
 	u64				autop_too_slow_at;
 	int				autop_idx;
@@ -1216,6 +1226,7 @@ static bool iocg_activate(struct ioc_gq *iocg, struct ioc_now *now)
 
 	if (ioc->running == IOC_IDLE) {
 		ioc->running = IOC_RUNNING;
+		ioc->debt_busy_at = now->now;
 		ioc_start_period(ioc, now);
 	}
 
@@ -1896,7 +1907,8 @@ static void ioc_timer_fn(struct timer_list *timer)
 	struct ioc_gq *iocg, *tiocg;
 	struct ioc_now now;
 	LIST_HEAD(surpluses);
-	int nr_shortages = 0, nr_lagging = 0;
+	int nr_debtors = 0, nr_shortages = 0, nr_lagging = 0;
+	u64 usage_us_sum = 0;
 	u32 ppm_rthr = MILLION - ioc->params.qos[QOS_RPPM];
 	u32 ppm_wthr = MILLION - ioc->params.qos[QOS_WPPM];
 	u32 missed_ppm[2], rq_wait_pct;
@@ -1936,6 +1948,8 @@ static void ioc_timer_fn(struct timer_list *timer)
 		    iocg->delay) {
 			/* might be oversleeping vtime / hweight changes, kick */
 			iocg_kick_waitq(iocg, true, &now);
+			if (iocg->abs_vdebt)
+				nr_debtors++;
 		} else if (iocg_is_idle(iocg)) {
 			/* no waiter and idle, deactivate */
 			__propagate_weights(iocg, 0, 0, false, &now);
@@ -1978,6 +1992,7 @@ static void ioc_timer_fn(struct timer_list *timer)
 		 * high-latency completions appearing as idle.
 		 */
 		usage_us = iocg->usage_delta_us;
+		usage_us_sum += usage_us;
 
 		if (vdone != vtime) {
 			u64 inflight_us = DIV64_U64_ROUND_UP(
@@ -2036,6 +2051,38 @@ static void ioc_timer_fn(struct timer_list *timer)
 	list_for_each_entry_safe(iocg, tiocg, &surpluses, surplus_list)
 		list_del_init(&iocg->surplus_list);
 
+	/*
+	 * A low weight iocg can amass a large amount of debt, for example, when
+	 * anonymous memory gets reclaimed aggressively. If the system has a lot
+	 * of memory paired with a slow IO device, the debt can span multiple
+	 * seconds or more. If there are no other subsequent IO issuers, the
+	 * in-debt iocg may end up blocked paying its debt while the IO device
+	 * is idle.
+	 *
+	 * The following protects against such pathological cases. If the device
+	 * has been sufficiently idle for a substantial amount of time, the
+	 * debts are halved. The criteria are on the conservative side as we
+	 * want to resolve the rare extreme cases without impacting regular
+	 * operation by forgiving debts too readily.
+	 */
+	if (nr_shortages ||
+	    div64_u64(100 * usage_us_sum, now.now - ioc->period_at) >=
+	    DEBT_BUSY_USAGE_PCT)
+		ioc->debt_busy_at = now.now;
+
+	if (nr_debtors &&
+	    now.now - ioc->debt_busy_at >= DEBT_REDUCTION_IDLE_DUR) {
+		list_for_each_entry(iocg, &ioc->active_iocgs, active_list) {
+			if (iocg->abs_vdebt) {
+				spin_lock(&iocg->waitq.lock);
+				iocg->abs_vdebt /= 2;
+				iocg_kick_waitq(iocg, true, &now);
+				spin_unlock(&iocg->waitq.lock);
+			}
+		}
+		ioc->debt_busy_at = now.now;
+	}
+
 	/*
 	 * If q is getting clogged or we're missing too much, we're issuing
 	 * too much IO and should lower vtime rate.  If we're not missing