blk-iocost: Fix error on iocost_ioc_vrate_adj

Systemtap 4.2 is unable to correctly interpret the "u32 (*missed_ppm)[2]"
argument of the iocost_ioc_vrate_adj trace entry defined in
include/trace/events/iocost.h leading to the following error:

  /tmp/stapAcz0G0/stap_c89c58b83cea1724e26395efa9ed4939_6321_aux_6.c:78:8:
  error: expected ‘;’, ‘,’ or ‘)’ before ‘*’ token
   , u32[]* __tracepoint_arg_missed_ppm

That argument type is indeed rather complex and hard to read. Looking
at block/blk-iocost.c. It is just a 2-entry u32 array. By simplifying
the argument to a simple "u32 *missed_ppm" and adjusting the trace
entry accordingly, the compilation error was gone.

Fixes: 7caa471 ("blkcg: implement blk-iocost")
Acked-by: Steven Rostedt (VMware) <[email protected]>
Acked-by: Tejun Heo <[email protected]>
Signed-off-by: Waiman Long <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

Author: Waiman-Long

block/blk-iocost.c

@@ -1591,7 +1591,7 @@ static void ioc_timer_fn(struct timer_list *timer)
 				      vrate_min, vrate_max);
 		}
 
-		trace_iocost_ioc_vrate_adj(ioc, vrate, &missed_ppm, rq_wait_pct,
+		trace_iocost_ioc_vrate_adj(ioc, vrate, missed_ppm, rq_wait_pct,
 					   nr_lagging, nr_shortages,
 					   nr_surpluses);
 
@@ -1600,7 +1600,7 @@ static void ioc_timer_fn(struct timer_list *timer)
 			ioc->period_us * vrate * INUSE_MARGIN_PCT, 100);
 	} else if (ioc->busy_level != prev_busy_level || nr_lagging) {
 		trace_iocost_ioc_vrate_adj(ioc, atomic64_read(&ioc->vtime_rate),
-					   &missed_ppm, rq_wait_pct, nr_lagging,
+					   missed_ppm, rq_wait_pct, nr_lagging,
 					   nr_shortages, nr_surpluses);
 	}
 

include/trace/events/iocost.h

@@ -130,7 +130,7 @@ DEFINE_EVENT(iocg_inuse_update, iocost_inuse_reset,
 
 TRACE_EVENT(iocost_ioc_vrate_adj,
 
-	TP_PROTO(struct ioc *ioc, u64 new_vrate, u32 (*missed_ppm)[2],
+	TP_PROTO(struct ioc *ioc, u64 new_vrate, u32 *missed_ppm,
 		u32 rq_wait_pct, int nr_lagging, int nr_shortages,
 		int nr_surpluses),
 
@@ -155,8 +155,8 @@ TRACE_EVENT(iocost_ioc_vrate_adj,
 		__entry->old_vrate = atomic64_read(&ioc->vtime_rate);;
 		__entry->new_vrate = new_vrate;
 		__entry->busy_level = ioc->busy_level;
-		__entry->read_missed_ppm = (*missed_ppm)[READ];
-		__entry->write_missed_ppm = (*missed_ppm)[WRITE];
+		__entry->read_missed_ppm = missed_ppm[READ];
+		__entry->write_missed_ppm = missed_ppm[WRITE];
 		__entry->rq_wait_pct = rq_wait_pct;
 		__entry->nr_lagging = nr_lagging;
 		__entry->nr_shortages = nr_shortages;