[RFC PATCH v7 3/4] blk-iocost: add iocost_ioc_tick tracepoint for per-period device summary

[Tao Cui <cui.tao@linux.dev>]

From: Tao Cui <cuitao@kylinos.cn>

Add iocost_ioc_tick, emitted once per period from the tail of
ioc_timer_fn() with the overall controller state: period_us, vrate,
busy_level, active iocg count, usage percentage and running state.
It fires every period the controller is running, including steady
states, plus one final tick before the controller goes idle, which
makes dormancy (e.g. a device saturated entirely by uncharged IO)
directly visible.

Unlike the existing iocost tracepoints, which are state-change
driven and silent in steady state, this allows a bound cost model's
behaviour to be evaluated without drgn.

Depending on the autop profile this is 2-100 events per second per
device; the added cost outside the tracepoint static key is one
increment per active cgroup per period.

Signed-off-by: Tao Cui <cuitao@kylinos.cn>
---
 block/blk-iocost.c            | 15 ++++++++++++
 include/trace/events/iocost.h | 45 +++++++++++++++++++++++++++++++++++
 2 files changed, 60 insertions(+)

diff --git a/block/blk-iocost.c b/block/blk-iocost.c
index 21e4f8cbd9f2..a48751b6dbdd 100644
--- a/block/blk-iocost.c
+++ b/block/blk-iocost.c
@@ -2252,6 +2252,7 @@ static void ioc_timer_fn(struct timer_list *timer)
 	struct ioc_now now;
 	LIST_HEAD(surpluses);
 	int nr_debtors, nr_shortages = 0, nr_lagging = 0;
+	int nr_active = 0;
 	u64 usage_us_sum = 0;
 	u32 ppm_rthr;
 	u32 ppm_wthr;
@@ -2288,6 +2289,8 @@ static void ioc_timer_fn(struct timer_list *timer)
 		u64 vdone, vtime, usage_us;
 		u32 hw_active, hw_inuse;
 
+		nr_active++;
+
 		/*
 		 * Collect unused and wind vtime closer to vnow to prevent
 		 * iocgs from accumulating a large amount of budget.
@@ -2449,6 +2452,18 @@ static void ioc_timer_fn(struct timer_list *timer)
 
 	ioc->busy_level = clamp(ioc->busy_level, -1000, 1000);
 
+	/*
+	 * Everything the tick reports is final here: busy_level was just
+	 * computed, running and cur_period haven't changed, nr_active and
+	 * usage_us_sum are complete, and vrate and period_us still hold
+	 * the values this period ran in.  Emit before the refresh below
+	 * so the event reads the completed period directly.
+	 */
+	trace_iocost_ioc_tick(ioc, nr_active, usage_us_sum,
+			      ioc->period_us, ioc->vtime_base_rate,
+			      ioc->busy_level, ioc->running,
+			      now.now - ioc->period_at);
+
 	ioc_adjust_base_vrate(ioc, rq_wait_pct, nr_lagging, nr_shortages,
 			      prev_busy_level, missed_ppm);
 
diff --git a/include/trace/events/iocost.h b/include/trace/events/iocost.h
index e772b1bc60d6..32f19861a78f 100644
--- a/include/trace/events/iocost.h
+++ b/include/trace/events/iocost.h
@@ -178,6 +178,51 @@ TRACE_EVENT(iocost_ioc_vrate_adj,
 	)
 );
 
+/*
+ * Periodic per-device summary, emitted once per period from the tail of
+ * ioc_timer_fn().  Unlike the state-change events above, this fires every
+ * period the controller is running, including steady states, and carries
+ * the overall controller state so basic monitoring doesn't require drgn.
+ */
+TRACE_EVENT(iocost_ioc_tick,
+
+	TP_PROTO(struct ioc *ioc, int nr_active, u64 usage_us_sum,
+		 u32 tick_period_us, u64 tick_vrate,
+		 int tick_busy, int tick_running, u64 tick_dur),
+
+	TP_ARGS(ioc, nr_active, usage_us_sum, tick_period_us, tick_vrate,
+		tick_busy, tick_running, tick_dur),
+
+	TP_STRUCT__entry (
+		__string(devname, ioc_name(ioc))
+		__field(u64, cur_period)
+		__field(u32, period_us)
+		__field(u64, vrate)
+		__field(int, busy_level)
+		__field(int, nr_active)
+		__field(u32, usage_pct)
+		__field(int, running)
+	),
+
+	TP_fast_assign(
+		__assign_str(devname);
+		__entry->cur_period = atomic64_read(&ioc->cur_period);
+		__entry->period_us = tick_period_us;
+		__entry->vrate = tick_vrate;
+		__entry->busy_level = tick_busy;
+		__entry->nr_active = nr_active;
+		__entry->usage_pct = tick_dur ?
+			div64_u64(usage_us_sum * 100, tick_dur) : 0;
+		__entry->running = tick_running;
+	),
+
+	TP_printk("[%s] period=%llu:%uus vrate=%llu busy=%d active=%d usage=%u%% running=%d",
+		__get_str(devname), __entry->cur_period, __entry->period_us,
+		__entry->vrate, __entry->busy_level, __entry->nr_active,
+		__entry->usage_pct, __entry->running
+	)
+);
+
 TRACE_EVENT(iocost_iocg_forgive_debt,
 
 	TP_PROTO(struct ioc_gq *iocg, const char *path, struct ioc_now *now,
-- 
2.43.0