fix runqlen.py with 4.15 kernel

The following kernel commit changes linux_src:kernel/sched/sched.h
struct cfs_rq structure:

```
commit 1ea6c46a23f1213d1972bfae220db5c165e27bba
Author: Peter Zijlstra <peterz@infradead.org>
Date:   Sat May 6 15:59:54 2017 +0200

    sched/fair: Propagate an effective runnable_load_avg

    The load balancer uses runnable_load_avg as load indicator. For
    !cgroup this is:

      runnable_load_avg = \Sum se->avg.load_avg ; where se->on_rq

    That is, a direct sum of all runnable tasks on that runqueue. As
    opposed to load_avg, which is a sum of all tasks on the runqueue,
    which includes a blocked component.
...
```

The commit is in kernel 4.15 release and will make current
runqlen.py internal cfs_rq_partial structure not syncing with the kernel one.
As a result, runqlen.py will produce incorrect results on 4.15.

This patch attempts to solve this issue by compiling a bpf program,
which accesses one of fields introduced by the above commit.
The successful compilation will indicate that we should amend
the cfs_rq_partial structure.
Signed-off-by: Yonghong Song <yhs@fb.com>

tools/runqlen.py

@@ -22,6 +22,8 @@
 from __future__ import print_function
 from bcc import BPF, PerfType, PerfSWConfig
 from time import sleep, strftime
+from tempfile import NamedTemporaryFile
+from os import open, close, dup, unlink, O_WRONLY
 import argparse
 
 # arguments
@@ -51,6 +53,66 @@ countdown = int(args.count)
 debug = 0
 frequency = 99
 
+# Linux 4.15 introduced a new field runnable_weight
+# in linux_src:kernel/sched/sched.h as
+#     struct cfs_rq {
+#         struct load_weight load;
+#         unsigned long runnable_weight;
+#         unsigned int nr_running, h_nr_running;
+#         ......
+#     }
+# and this tool requires to access nr_running to get
+# runqueue len information.
+#
+# The commit which introduces cfs_rq->runnable_weight
+# field also introduces the field sched_entity->runnable_weight
+# where sched_entity is defined in linux_src:include/linux/sched.h.
+#
+# To cope with pre-4.15 and 4.15/post-4.15 releases,
+# we run a simple BPF program to detect whether
+# field sched_entity->runnable_weight exists. The existence of
+# this field should infer the existence of cfs_rq->runnable_weight.
+#
+# This will need maintenance as the relationship between these
+# two fields may change in the future.
+#
+def check_runnable_weight_field():
+    # Define the bpf program for checking purpose
+    bpf_check_text = """
+#include <linux/sched.h>
+unsigned long dummy(struct sched_entity *entity)
+{
+    return entity->runnable_weight;
+}
+"""
+
+    # Get a temporary file name
+    tmp_file = NamedTemporaryFile(delete=False)
+    tmp_file.close();
+
+    # Duplicate and close stderr (fd = 2)
+    old_stderr = dup(2)
+    close(2)
+
+    # Open a new file, should get fd number 2
+    # This will avoid printing llvm errors on the screen
+    fd = open(tmp_file.name, O_WRONLY)
+    try:
+        t = BPF(text=bpf_check_text)
+        success_compile = True
+    except:
+        success_compile = False
+
+    # Release the fd 2, and next dup should restore old stderr
+    close(fd)
+    dup(old_stderr)
+    close(old_stderr)
+
+    # remove the temporary file and return
+    unlink(tmp_file.name)
+    return success_compile
+
+
 # define BPF program
 bpf_text = """
 #include <uapi/linux/ptrace.h>
@@ -60,6 +122,7 @@ bpf_text = """
 // header. This will need maintenance. It is from kernel/sched/sched.h:
 struct cfs_rq_partial {
     struct load_weight load;
+    RUNNABLE_WEIGHT_FIELD
     unsigned int nr_running, h_nr_running;
 };
 
@@ -106,6 +169,11 @@ else:
         'BPF_HISTOGRAM(dist, unsigned int);')
     bpf_text = bpf_text.replace('STORE', 'dist.increment(len);')
 
+if check_runnable_weight_field():
+    bpf_text = bpf_text.replace('RUNNABLE_WEIGHT_FIELD', 'unsigned long runnable_weight;')
+else:
+    bpf_text = bpf_text.replace('RUNNABLE_WEIGHT_FIELD', '')
+
 # code substitutions
 if debug:
     print(bpf_text)