offcputime: Resolve stacks using a StackTrace table

The manual walking for kernel stacks in the eBPF code has been replaced
with a `BPF_STACK_TRACE` table; the stack ID is now stored as an integer
in the main key.

Thanks to the `StackTrace.walk` iterator, the printing code can also be
significantly simplified for both folded and full stack modes.

man/man8/offcputime.8

@@ -22,10 +22,8 @@ especially the -f format, can be used to generate an "off-CPU time flame graph".
 
 See http://www.brendangregg.com/FlameGraphs/offcpuflamegraphs.html
 
-The stack depth is currently limited to 20, and the stack traces are kernel
-mode only. Check for newer versions where either may be improved.
-
-This currently only works on x86_64. Check for future versions.
+This tool only works on Linux 4.6+. It uses the new `BPF_STACK_TRACE` table
+APIs to generate the in-kernel stack traces.
 .SH REQUIREMENTS
 CONFIG_BPF and bcc.
 .SH OPTIONS

tools/offcputime.py

@@ -9,8 +9,6 @@
 # as a proof of concept. This implementation should be replaced in the future
 # with an appropriate bpf_ call, when available.
 #
-# Currently limited to a stack trace depth of 21 (maxdepth + 1).
-#
 # Copyright 2016 Netflix, Inc.
 # Licensed under the Apache License, Version 2.0 (the "License")
 #
@@ -48,7 +46,6 @@ args = parser.parse_args()
 folded = args.folded
 duration = int(args.duration)
 debug = 0
-maxdepth = 20    # and MAXDEPTH
 if args.pid and args.useronly:
     print("ERROR: use either -p or -u.")
     exit()
@@ -62,31 +59,15 @@ bpf_text = """
 #include <uapi/linux/ptrace.h>
 #include <linux/sched.h>
 
-#define MAXDEPTH	20
 #define MINBLOCK_US	1
 
 struct key_t {
     char name[TASK_COMM_LEN];
-    // Skip saving the ip
-    u64 ret[MAXDEPTH];
+    int stack_id;
 };
 BPF_HASH(counts, struct key_t);
 BPF_HASH(start, u32);
-
-static u64 get_frame(u64 *bp) {
-    if (*bp) {
-        // The following stack walker is x86_64 specific
-        u64 ret = 0;
-        if (bpf_probe_read(&ret, sizeof(ret), (void *)(*bp+8)))
-            return 0;
-        if (bpf_probe_read(bp, sizeof(*bp), (void *)*bp))
-            *bp = 0;
-        if (ret < __START_KERNEL_map)
-            return 0;
-        return ret;
-    }
-    return 0;
-}
+BPF_STACK_TRACE(stack_traces, 1024)
 
 int oncpu(struct pt_regs *ctx, struct task_struct *prev) {
     u32 pid;
@@ -111,36 +92,12 @@ int oncpu(struct pt_regs *ctx, struct task_struct *prev) {
         return 0;
 
     // create map key
-    u64 zero = 0, *val, bp = 0;
-    int depth = 0;
+    u64 zero = 0, *val;
     struct key_t key = {};
+
     bpf_get_current_comm(&key.name, sizeof(key.name));
-    bp = ctx->bp;
-
-    // unrolled loop (MAXDEPTH):
-    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
-    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
-    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
-    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
-    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
-    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
-    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
-    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
-    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
-    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
-
-    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
-    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
-    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
-    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
-    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
-    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
-    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
-    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
-    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
-    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
-
-out:
+    key.stack_id = stack_traces.get_stackid(ctx, BPF_F_REUSE_STACKID);
+
     val = counts.lookup_or_init(&key, &zero);
     (*val) += delta;
     return 0;
@@ -183,24 +140,17 @@ while (1):
     if not folded:
         print()
     counts = b.get_table("counts")
+    stack_traces = b.get_table("stack_traces")
     for k, v in sorted(counts.items(), key=lambda counts: counts[1].value):
         if folded:
             # print folded stack output
-            line = k.name.decode() + ";"
-            for i in reversed(range(0, maxdepth)):
-                if k.ret[i] == 0:
-                    continue
-                line = line + b.ksym(k.ret[i])
-                if i != 0:
-                    line = line + ";"
-            print("%s %d" % (line, v.value))
+            stack = list(stack_traces.walk(k.stack_id))[1:]
+            line = [k.name.decode()] + [b.ksym(addr) for addr in reversed(stack)]
+            print("%s %d" % (";".join(line), v.value))
         else:
             # print default multi-line stack output
-            for i in range(0, maxdepth):
-                if k.ret[i] == 0:
-                    break
-                print("    %-16x %s" % (k.ret[i],
-                    b.ksym(k.ret[i])))
+            for addr in stack_traces.walk(k.stack_id):
+                print("    %-16x %s" % (addr, b.ksym(addr)))
             print("    %-16s %s" % ("-", k.name))
             print("        %d\n" % v.value)
     counts.clear()

tools/old/offcputime.py

+#!/usr/bin/python
+#
+# offcputime    Summarize off-CPU time by kernel stack trace
+#               For Linux, uses BCC, eBPF.
+#
+# USAGE: offcputime [-h] [-u] [-p PID] [-v] [-f] [duration]
+#
+# The current implementation uses an unrolled loop for x86_64, and was written
+# as a proof of concept. This implementation should be replaced in the future
+# with an appropriate bpf_ call, when available.
+#
+# Currently limited to a stack trace depth of 21 (maxdepth + 1).
+#
+# Copyright 2016 Netflix, Inc.
+# Licensed under the Apache License, Version 2.0 (the "License")
+#
+# 13-Jan-2016	Brendan Gregg	Created this.
+
+from __future__ import print_function
+from bcc import BPF
+from time import sleep, strftime
+import argparse
+import signal
+
+# arguments
+examples = """examples:
+    ./offcputime             # trace off-CPU stack time until Ctrl-C
+    ./offcputime 5           # trace for 5 seconds only
+    ./offcputime -f 5        # 5 seconds, and output in folded format
+    ./offcputime -u          # don't include kernel threads (user only)
+    ./offcputime -p 185      # trace fo PID 185 only
+"""
+parser = argparse.ArgumentParser(
+    description="Summarize off-CPU time by kernel stack trace",
+    formatter_class=argparse.RawDescriptionHelpFormatter,
+    epilog=examples)
+parser.add_argument("-u", "--useronly", action="store_true",
+    help="user threads only (no kernel threads)")
+parser.add_argument("-p", "--pid",
+    help="trace this PID only")
+parser.add_argument("-v", "--verbose", action="store_true",
+    help="show raw addresses")
+parser.add_argument("-f", "--folded", action="store_true",
+    help="output folded format")
+parser.add_argument("duration", nargs="?", default=99999999,
+    help="duration of trace, in seconds")
+args = parser.parse_args()
+folded = args.folded
+duration = int(args.duration)
+debug = 0
+maxdepth = 20    # and MAXDEPTH
+if args.pid and args.useronly:
+    print("ERROR: use either -p or -u.")
+    exit()
+
+# signal handler
+def signal_ignore(signal, frame):
+    print()
+
+# define BPF program
+bpf_text = """
+#include <uapi/linux/ptrace.h>
+#include <linux/sched.h>
+
+#define MAXDEPTH	20
+#define MINBLOCK_US	1
+
+struct key_t {
+    char name[TASK_COMM_LEN];
+    // Skip saving the ip
+    u64 ret[MAXDEPTH];
+};
+BPF_HASH(counts, struct key_t);
+BPF_HASH(start, u32);
+
+static u64 get_frame(u64 *bp) {
+    if (*bp) {
+        // The following stack walker is x86_64 specific
+        u64 ret = 0;
+        if (bpf_probe_read(&ret, sizeof(ret), (void *)(*bp+8)))
+            return 0;
+        if (bpf_probe_read(bp, sizeof(*bp), (void *)*bp))
+            *bp = 0;
+        if (ret < __START_KERNEL_map)
+            return 0;
+        return ret;
+    }
+    return 0;
+}
+
+int oncpu(struct pt_regs *ctx, struct task_struct *prev) {
+    u32 pid;
+    u64 ts, *tsp;
+
+    // record previous thread sleep time
+    if (FILTER) {
+        pid = prev->pid;
+        ts = bpf_ktime_get_ns();
+        start.update(&pid, &ts);
+    }
+
+    // calculate current thread's delta time
+    pid = bpf_get_current_pid_tgid();
+    tsp = start.lookup(&pid);
+    if (tsp == 0)
+        return 0;        // missed start or filtered
+    u64 delta = bpf_ktime_get_ns() - *tsp;
+    start.delete(&pid);
+    delta = delta / 1000;
+    if (delta < MINBLOCK_US)
+        return 0;
+
+    // create map key
+    u64 zero = 0, *val, bp = 0;
+    int depth = 0;
+    struct key_t key = {};
+    bpf_get_current_comm(&key.name, sizeof(key.name));
+    bp = ctx->bp;
+
+    // unrolled loop (MAXDEPTH):
+    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
+    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
+    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
+    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
+    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
+    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
+    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
+    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
+    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
+    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
+
+    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
+    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
+    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
+    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
+    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
+    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
+    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
+    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
+    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
+    if (!(key.ret[depth++] = get_frame(&bp))) goto out;
+
+out:
+    val = counts.lookup_or_init(&key, &zero);
+    (*val) += delta;
+    return 0;
+}
+"""
+if args.pid:
+    filter = 'pid == %s' % args.pid
+elif args.useronly:
+    filter = '!(prev->flags & PF_KTHREAD)'
+else:
+    filter = '1'
+bpf_text = bpf_text.replace('FILTER', filter)
+if debug:
+    print(bpf_text)
+
+# initialize BPF
+b = BPF(text=bpf_text)
+b.attach_kprobe(event="finish_task_switch", fn_name="oncpu")
+matched = b.num_open_kprobes()
+if matched == 0:
+    print("0 functions traced. Exiting.")
+    exit()
+
+# header
+if not folded:
+    print("Tracing off-CPU time (us) by kernel stack", end="")
+    if duration < 99999999:
+        print(" for %d secs." % duration)
+    else:
+        print("... Hit Ctrl-C to end.")
+
+# output
+while (1):
+    try:
+        sleep(duration)
+    except KeyboardInterrupt:
+        # as cleanup can take many seconds, trap Ctrl-C:
+        signal.signal(signal.SIGINT, signal_ignore)
+
+    if not folded:
+        print()
+    counts = b.get_table("counts")
+    for k, v in sorted(counts.items(), key=lambda counts: counts[1].value):
+        if folded:
+            # print folded stack output
+            line = k.name.decode() + ";"
+            for i in reversed(range(0, maxdepth)):
+                if k.ret[i] == 0:
+                    continue
+                line = line + b.ksym(k.ret[i])
+                if i != 0:
+                    line = line + ";"
+            print("%s %d" % (line, v.value))
+        else:
+            # print default multi-line stack output
+            for i in range(0, maxdepth):
+                if k.ret[i] == 0:
+                    break
+                print("    %-16x %s" % (k.ret[i],
+                    b.ksym(k.ret[i])))
+            print("    %-16s %s" % ("-", k.name))
+            print("        %d\n" % v.value)
+    counts.clear()
+
+    if not folded:
+        print("Detaching...")
+    exit()