fix cpuunclaimed.py with cfs_rq structure change (#2164)

Similar to runqlen.py, make proper adjustment for
cfs_rq_partial structure so it can align with
what the kernel expects.
Signed-off-by: Yonghong Song <yhs@fb.com>

tools/cpuunclaimed.py

@@ -62,8 +62,9 @@ from time import sleep, strftime
 from ctypes import c_int
 import argparse
 import multiprocessing
-from os import getpid, system
+from os import getpid, system, open, close, dup, unlink, O_WRONLY
 import ctypes as ct
+from tempfile import NamedTemporaryFile
 
 # arguments
 examples = """examples:
@@ -98,6 +99,66 @@ wakeup_s = float(1) / wakeup_hz
 ncpu = multiprocessing.cpu_count()  # assume all are online
 debug = 0
 
+# 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
+
+
 # process arguments
 if args.fullcsv:
     args.csv = True
@@ -128,6 +189,7 @@ BPF_PERF_OUTPUT(events);
 // 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;
 };
 
@@ -156,6 +218,11 @@ int do_perf_event(struct bpf_perf_event_data *ctx)
 }
 """
 
+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 or args.ebpf:
     print(bpf_text)