blk-iocost: calculate iocg->usages[] from iocg->local_stat.usage_us

Currently, iocg->usages[] which are used to guide inuse adjustments are
calculated from vtime deltas. This, however, assumes that the hierarchical
inuse weight at the time of calculation held for the entire period, which
often isn't true and can lead to significant errors.

Now that we have absolute usage information collected, we can derive
iocg->usages[] from iocg->local_stat.usage_us so that inuse adjustment
decisions are made based on actual absolute usage. The calculated usage is
clamped between 1 and WEIGHT_ONE and WEIGHT_ONE is also used to signal
saturation regardless of the current hierarchical inuse weight.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-iocost.c b/block/blk-iocost.c
index f30f9b37fcf0d10dc4ab2a3d36fb4ab1ccb67104..2496674bbbf497e694f6520f8578a250bcf3205d 100644 (file)
--- a/block/blk-iocost.c
+++ b/block/blk-iocost.c
@@ -476,14 +476,10 @@ struct ioc_gq {
         * `vtime_done` is the same but progressed on completion rather
         * than issue.  The delta behind `vtime` represents the cost of
         * currently in-flight IOs.
-        *
-        * `last_vtime` is used to remember `vtime` at the end of the last
-        * period to calculate utilization.
         */
        atomic64_t                      vtime;
        atomic64_t                      done_vtime;
        u64                             abs_vdebt;
-       u64                             last_vtime;
 
        /*
         * The period this iocg was last active in.  Used for deactivation
@@ -506,6 +502,9 @@ struct ioc_gq {
        struct hrtimer                  waitq_timer;
        struct hrtimer                  delay_timer;
 
+       /* timestamp at the latest activation */
+       u64                             activated_at;
+
        /* statistics */
        struct iocg_pcpu_stat __percpu  *pcpu_stat;
        struct iocg_stat                local_stat;
@@ -514,6 +513,7 @@ struct ioc_gq {
        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];
 
@@ -1159,7 +1159,7 @@ static bool iocg_activate(struct ioc_gq *iocg, struct ioc_now *now)
        TRACE_IOCG_PATH(iocg_activate, iocg, now,
                        last_period, cur_period, vtime);
 
-       iocg->last_vtime = vtime;
+       iocg->activated_at = now->now;
 
        if (ioc->running == IOC_IDLE) {
                ioc->running = IOC_RUNNING;
@@ -1451,7 +1451,8 @@ static void iocg_flush_stat_one(struct ioc_gq *iocg, struct ioc_now *now)
        vusage_delta = abs_vusage - iocg->last_stat_abs_vusage;
        iocg->last_stat_abs_vusage = abs_vusage;
 
-       iocg->local_stat.usage_us += div64_u64(vusage_delta, now->vrate);
+       iocg->usage_delta_us = div64_u64(vusage_delta, now->vrate);
+       iocg->local_stat.usage_us += iocg->usage_delta_us;
 
        new_stat.usage_us =
                iocg->local_stat.usage_us + iocg->desc_stat.usage_us;
@@ -1558,8 +1559,9 @@ static void ioc_timer_fn(struct timer_list *timer)
 
        /* calc usages and see whether some weights need to be moved around */
        list_for_each_entry(iocg, &ioc->active_iocgs, active_list) {
-               u64 vdone, vtime, vusage, vmin;
+               u64 vdone, vtime, usage_us, vmin;
                u32 hw_active, hw_inuse, usage;
+               int uidx;
 
                /*
                 * Collect unused and wind vtime closer to vnow to prevent
@@ -1583,27 +1585,44 @@ static void ioc_timer_fn(struct timer_list *timer)
                    time_before64(vdone, now.vnow - period_vtime))
                        nr_lagging++;
 
-               if (waitqueue_active(&iocg->waitq))
-                       vusage = now.vnow - iocg->last_vtime;
-               else if (time_before64(iocg->last_vtime, vtime))
-                       vusage = vtime - iocg->last_vtime;
-               else
-                       vusage = 0;
-
-               iocg->last_vtime += vusage;
                /*
-                * Factor in in-flight vtime into vusage to avoid
-                * high-latency completions appearing as idle.  This should
-                * be done after the above ->last_time adjustment.
+                * Determine absolute usage factoring in pending and in-flight
+                * IOs to avoid stalls and high-latency completions appearing as
+                * idle.
                 */
-               vusage = max(vusage, vtime - vdone);
-
-               /* calculate hweight based usage ratio and record */
-               if (vusage) {
-                       usage = DIV64_U64_ROUND_UP(vusage * hw_inuse,
-                                                  period_vtime);
-                       iocg->usage_idx = (iocg->usage_idx + 1) % NR_USAGE_SLOTS;
-                       iocg->usages[iocg->usage_idx] = usage;
+               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),
+                               now.vrate);
+                       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);
+                       usage = clamp_t(u32,
+                               DIV64_U64_ROUND_UP(usage_us * WEIGHT_ONE, dur),
+                               1, WEIGHT_ONE);
+
+                       iocg->usage_idx = uidx;
+                       iocg->usages[uidx] = usage;
                } else {
                        usage = 0;
                }
@@ -1620,7 +1639,6 @@ static void ioc_timer_fn(struct timer_list *timer)
                        /* throw away surplus vtime */
                        atomic64_add(delta, &iocg->vtime);
                        atomic64_add(delta, &iocg->done_vtime);
-                       iocg->last_vtime += delta;
                        /* if usage is sufficiently low, maybe it can donate */
                        if (surplus_adjusted_hweight_inuse(usage, hw_inuse)) {
                                iocg->has_surplus = true;

diff --git a/include/trace/events/iocost.h b/include/trace/events/iocost.h
index c2f580fd371b11e117be68238c8530bd88a84d1f..a905ecc0342fdb295e04832fa88ed1e351ee3480 100644 (file)
--- a/include/trace/events/iocost.h
+++ b/include/trace/events/iocost.h
@@ -26,7 +26,6 @@ TRACE_EVENT(iocost_iocg_activate,
                __field(u64, vrate)
                __field(u64, last_period)
                __field(u64, cur_period)
-               __field(u64, last_vtime)
                __field(u64, vtime)
                __field(u32, weight)
                __field(u32, inuse)
@@ -42,7 +41,6 @@ TRACE_EVENT(iocost_iocg_activate,
                __entry->vrate = now->vrate;
                __entry->last_period = last_period;
                __entry->cur_period = cur_period;
-               __entry->last_vtime = iocg->last_vtime;
                __entry->vtime = vtime;
                __entry->weight = iocg->weight;
                __entry->inuse = iocg->inuse;
@@ -51,13 +49,12 @@ TRACE_EVENT(iocost_iocg_activate,
        ),
 
        TP_printk("[%s:%s] now=%llu:%llu vrate=%llu "
-                 "period=%llu->%llu vtime=%llu->%llu "
+                 "period=%llu->%llu vtime=%llu "
                  "weight=%u/%u hweight=%llu/%llu",
                __get_str(devname), __get_str(cgroup),
                __entry->now, __entry->vnow, __entry->vrate,
                __entry->last_period, __entry->cur_period,
-               __entry->last_vtime, __entry->vtime,
-               __entry->inuse, __entry->weight,
+               __entry->vtime, __entry->inuse, __entry->weight,
                __entry->hweight_inuse, __entry->hweight_active
        )
 );