runqslower: create sufficiently large array table to avoid missing tasks

We have encountered an issue while running runqslower for long run test,
as you can check the log below. The workload of target platform is
running auto test script and launching lots of applications periodically
on Android TV. There are several abnormal records of run queue latency,
ex. 1311763801.

We have investgated this issue and found the root cuase of this issue is
the hash map to store en-queue start time of each task overflowed, and
it seems cpudist also encountered such issue before and fix by following
pull request.

iovisor#2568

Apply the similar fix to runqslower, getting the max_pid from
/proc/sys/kernel/max_pid and pass to BPF C in kernel to create enough
entries of BPF map to store en-queue start time of each task.

In the mean time, since we create the BPF map with max_pid entries, use
BPF_ARRAY instead of BPF_HASH to prevent collision.

```
(bcc)root@localhost:/# runqslower 100000 -P
Tracing run queue latency higher than 100000 us
TIME     COMM             TID           LAT(us) PREV COMM        PREV TID
23:42:45 glide-source-th  28986          161605 kworker/u9:3     12260
23:42:50 HTTPREQUEST_MAN  26249          107187 OMX_VideoInput   7152
23:42:50 migration/2      24             146799 OmxSideband      7147
23:42:50 rtk_post_worker  128            174145 migration/2      24
23:42:50 Binder:233_4     633            170457 runqslower       7086
23:42:58 GIBBON_SURFACE_  26233          103377 composer@2.4-se  457
23:43:14 Binder:581_1B    2202           113153 Binder:581_7     833
23:43:24 migration/0      15             107716 writer           7277
....
05:08:14 dumpLogs.sh      22242      1311763801 NBP_POOL-[1]     24233
05:08:14 dumpLogs.sh      22243      1311752373 appsrc7:src      24281
05:08:14 dumpLogs.sh      22248      1311760825 dumpLogs.sh      22193
```

Signed-off-by: Ism Hong <ism.hong@realtek.com>

Author: Ism Hong

tools/runqslower.py

@@ -74,7 +74,7 @@
 #include <linux/nsproxy.h>
 #include <linux/pid_namespace.h>
 
-BPF_HASH(start, u32);
+BPF_ARRAY(start, u64, MAX_PID);
 
 struct data_t {
     u32 pid;
@@ -132,7 +132,7 @@
 
     // fetch timestamp and calculate delta
     tsp = start.lookup(&pid);
-    if (tsp == 0) {
+    if ((tsp == 0) || (*tsp == 0)) {
         return 0;   // missed enqueue
     }
     delta_us = (bpf_ktime_get_ns() - *tsp) / 1000;
@@ -204,7 +204,7 @@
 
     // fetch timestamp and calculate delta
     tsp = start.lookup(&pid);
-    if (tsp == 0) {
+    if ((tsp == 0) || (*tsp == 0)) {
         return 0;   // missed enqueue
     }
     delta_us = (bpf_ktime_get_ns() - *tsp) / 1000;
@@ -262,12 +262,14 @@
 def print_event(cpu, data, size):
     event = b["events"].event(data)
     if args.previous:
-        print("%-8s %-16s %-6s %14s %-16s %-6s" % (strftime("%H:%M:%S"), event.task, event.pid, event.delta_us, event.prev_task, event.prev_pid))
+        print("%-8s %-16s %-6s %14s %-16s %-6s" % (strftime("%H:%M:%S"), event.task.decode('utf-8', 'replace'), event.pid, event.delta_us, event.prev_task.decode('utf-8', 'replace'), event.prev_pid))
     else:
-        print("%-8s %-16s %-6s %14s" % (strftime("%H:%M:%S"), event.task, event.pid, event.delta_us))
+        print("%-8s %-16s %-6s %14s" % (strftime("%H:%M:%S"), event.task.decode('utf-8', 'replace'), event.pid, event.delta_us))
+
+max_pid = int(open("/proc/sys/kernel/pid_max").read())
 
 # load BPF program
-b = BPF(text=bpf_text)
+b = BPF(text=bpf_text, cflags=["-DMAX_PID=%d" % max_pid])
 if not is_support_raw_tp:
     b.attach_kprobe(event="ttwu_do_wakeup", fn_name="trace_ttwu_do_wakeup")
     b.attach_kprobe(event="wake_up_new_task", fn_name="trace_wake_up_new_task")