cpudist: Support off-cpu time reports

Add -O switch, which directs cpudist to collect off-CPU time
statistics. Also restructure the code slightly and added examples
as appropriate.

README.md

@@ -78,7 +78,7 @@ Examples:
 - tools/[btrfsdist](tools/btrfsdist.py): Summarize btrfs operation latency distribution as a histogram. [Examples](tools/btrfsdist_example.txt).
 - tools/[btrfsslower](tools/btrfsslower.py): Trace slow btrfs operations. [Examples](tools/btrfsslower_example.txt).
 - tools/[cachestat](tools/cachestat.py): Trace page cache hit/miss ratio. [Examples](tools/cachestat_example.txt).
-- tools/[cpudist](tools/cpudist.py): Summarize on-CPU time per task as a histogram. [Examples](tools/cpudist_example.txt)
+- tools/[cpudist](tools/cpudist.py): Summarize on- and off-CPU time per task as a histogram. [Examples](tools/cpudist_example.txt)
 - tools/[dcsnoop](tools/dcsnoop.py): Trace directory entry cache (dcache) lookups. [Examples](tools/dcsnoop_example.txt).
 - tools/[dcstat](tools/dcstat.py): Directory entry cache (dcache) stats. [Examples](tools/dcstat_example.txt).
 - tools/[execsnoop](tools/execsnoop.py): Trace new processes via exec() syscalls. [Examples](tools/execsnoop_example.txt).

man/man8/cpudist.8

@@ -1,15 +1,20 @@
 .TH cpudist 8 "2016-06-28" "USER COMMANDS"
 .SH NAME
-cpudist \- On-CPU task time as a histogram.
+cpudist \- On- and off-CPU task time as a histogram.
 .SH SYNOPSIS
-.B cpudist [\-h] [\-T] [\-m] [\-P] [\-L] [\-p PID] [interval] [count]
+.B cpudist [\-h] [-O] [\-T] [\-m] [\-P] [\-L] [\-p PID] [interval] [count]
 .SH DESCRIPTION
 This measures the time a task spends on the CPU before being descheduled, and
 shows the times as a histogram. Tasks that spend a very short time on the CPU
 can be indicative of excessive context-switches and poor workload distribution,
 and possibly point to a shared source of contention that keeps tasks switching
 in and out as it becomes available (such as a mutex).
 
+Similarly, the tool can also measure the time a task spends off-CPU before it
+is scheduled again. This can be helpful in identifying long blocking and I/O
+operations, or alternatively very short descheduling times due to short-lived
+locks or timers.
+
 This tool uses in-kernel eBPF maps for storing timestamps and the histogram,
 for efficiency. Despite this, the overhead of this tool may become significant
 for some workloads: see the OVERHEAD section.
@@ -27,6 +32,9 @@ CONFIG_BPF and bcc.
 \-h
 Print usage message.
 .TP
+\-O
+Measure off-CPU time instead of on-CPU time.
+.TP
 \-T
 Include timestamps on output.
 .TP
@@ -53,6 +61,10 @@ Summarize task on-CPU time as a histogram:
 #
 .B cpudist
 .TP
+Summarize task off-CPU time as a histogram:
+#
+.B cpudist -O
+.TP
 Print 1 second summaries, 10 times:
 #
 .B cpudist 1 10

tools/cpudist.py

@@ -1,12 +1,12 @@
 #!/usr/bin/python
 # @lint-avoid-python-3-compatibility-imports
 #
-# cpudist Summarize on-CPU time per task as a histogram.
+# cpudist Summarize on- and off-CPU time per task as a histogram.
 #
-# USAGE: cpudist [-h] [-T] [-m] [-P] [-L] [-p PID] [interval] [count]
+# USAGE: cpudist [-h] [-O] [-T] [-m] [-P] [-L] [-p PID] [interval] [count]
 #
-# This measures the time a task spends on the CPU, and shows this time as a
-# histogram, optionally per-process.
+# This measures the time a task spends on or off the CPU, and shows this time
+# as a histogram, optionally per-process.
 #
 # Copyright 2016 Sasha Goldshtein
 # Licensed under the Apache License, Version 2.0 (the "License")
@@ -18,6 +18,7 @@
 
 examples = """examples:
  cpudist # summarize on-CPU time as a histogram
+ cpudist -O # summarize off-CPU time as a histogram
  cpudist 1 10 # print 1 second summaries, 10 times
  cpudist -mT 1 # 1s summaries, milliseconds, and timestamps
  cpudist -P # show each PID separately
@@ -27,6 +28,8 @@
  description="Summarize on-CPU time per task as a histogram.",
  formatter_class=argparse.RawDescriptionHelpFormatter,
  epilog=examples)
+parser.add_argument("-O", "--offcpu", action="store_true",
+ help="measure off-CPU time")
 parser.add_argument("-T", "--timestamp", action="store_true",
  help="include timestamp on output")
 parser.add_argument("-m", "--milliseconds", action="store_true",
@@ -52,60 +55,90 @@
 bpf_text += tp.generate_entry_probe()
 bpf_text += tp.generate_struct()
 
+if not args.offcpu:
+ bpf_text += "#define ONCPU\n"
+
 bpf_text += """
 typedef struct pid_key {
  u64 id;
  u64 slot;
 } pid_key_t;
 
-// We need to store the start time, which is when the thread got switched in,
-// and the tgid for the pid because the sched_switch tracepoint doesn't provide
-// that information.
+
 BPF_HASH(start, u32, u64);
 BPF_HASH(tgid_for_pid, u32, u32);
 STORAGE
 
+#define INVALID_TGID 0xffffffff
+
+static inline u32 get_tgid_if_missing(u32 tgid, u32 pid)
+{
+ if (tgid == INVALID_TGID) {
+ u32 *stored_tgid = tgid_for_pid.lookup(&pid);
+ if (stored_tgid != 0)
+ return *stored_tgid;
+ }
+ return tgid;
+}
+
+static inline void store_start(u32 tgid, u32 pid, u64 ts)
+{
+ tgid = get_tgid_if_missing(tgid, pid);
+
+ if (FILTER)
+ return;
+
+ start.update(&pid, &ts);
+}
+
+static inline void update_hist(u32 tgid, u32 pid, u64 ts)
+{
+ tgid = get_tgid_if_missing(tgid, pid);
+
+ if (FILTER)
+ return;
+
+ u64 *tsp = start.lookup(&pid);
+ if (tsp == 0)
+ return;
+
+ u64 delta = ts - *tsp;
+ FACTOR
+ STORE
+}
+
 int sched_switch(struct pt_regs *ctx)
 {
+ u64 ts = bpf_ktime_get_ns();
  u64 pid_tgid = bpf_get_current_pid_tgid();
+ u32 tgid = pid_tgid >> 32, pid = pid_tgid;
+ // Keep a mapping of tgid for pid because when sched_switch hits,
+ // we only have the tgid information for the *current* pid, but not
+ // for the previous one.
+ tgid_for_pid.update(&pid, &tgid);
+
  u64 *di = __trace_di.lookup(&pid_tgid);
  if (di == 0)
  return 0;
 
  struct sched_switch_trace_entry args = {};
  bpf_probe_read(&args, sizeof(args), (void *)*di);
-
- u32 tgid, pid;
- u64 ts = bpf_ktime_get_ns();
+ // TODO: Store the comm as well
 
  if (args.prev_state == TASK_RUNNING) {
- pid = args.prev_pid;
-
- u32 *stored_tgid = tgid_for_pid.lookup(&pid);
- if (stored_tgid == 0)
- goto BAIL;
- tgid = *stored_tgid;
-
- if (FILTER)
- goto BAIL;
-
- u64 *tsp = start.lookup(&pid);
- if (tsp == 0)
- goto BAIL;
-
- u64 delta = ts - *tsp;
- FACTOR
- STORE
+ u32 prev_pid = args.prev_pid;
+#ifdef ONCPU
+ update_hist(INVALID_TGID, prev_pid, ts);
+#else
+ store_start(INVALID_TGID, prev_pid, ts);
+#endif
  }
 
-BAIL:
- tgid = pid_tgid >> 32;
- pid = pid_tgid;
- if (FILTER)
- return 0;
-
- start.update(&pid, &ts);
- tgid_for_pid.update(&pid, &tgid);
+#ifdef ONCPU
+ store_start(tgid, pid, ts);
+#else
+ update_hist(tgid, pid, ts);
+#endif
 
  return 0;
 }
@@ -144,7 +177,8 @@
 Tracepoint.attach(b)
 b.attach_kprobe(event="perf_trace_sched_switch", fn_name="sched_switch")
 
-print("Tracing on-CPU time... Hit Ctrl-C to end.")
+print("Tracing %s-CPU time... Hit Ctrl-C to end." %
+ ("off" if args.offcpu else "on"))
 
 exiting = 0 if args.interval else 1
 dist = b.get_table("dist")

tools/cpudist_example.txt

@@ -6,6 +6,10 @@ that can indicate oversubscription (too many tasks for too few processors),
 overhead due to excessive context switching (e.g. a common shared lock for
 multiple threads), uneven workload distribution, too-granular tasks, and more.
 
+Alternatively, the same options are available for summarizing task off-CPU
+time, which helps understand how often threads are being descheduled and how
+long they spend waiting for I/O, locks, timers, and other causes of suspension.
+
 # ./cpudist.py
 Tracing on-CPU time... Hit Ctrl-C to end.
 ^C
@@ -155,6 +159,47 @@ pid = 5068
 This histogram was obtained while executing `dd if=/dev/zero of=/dev/null` with
 fairly large block sizes.
 
+You could also ask for an off-CPU report using the -O switch. Here's a
+histogram of task block times while the system is heavily loaded:
+
+# ./cpudist -O -p $(parprimes)
+Tracing off-CPU time... Hit Ctrl-C to end.
+^C
+ usecs : count distribution
+ 0 -> 1 : 0 | |
+ 2 -> 3 : 1 | |
+ 4 -> 7 : 0 | |
+ 8 -> 15 : 0 | |
+ 16 -> 31 : 0 | |
+ 32 -> 63 : 3 | |
+ 64 -> 127 : 1 | |
+ 128 -> 255 : 1 | |
+ 256 -> 511 : 0 | |
+ 512 -> 1023 : 2 | |
+ 1024 -> 2047 : 4 | |
+ 2048 -> 4095 : 3 | |
+ 4096 -> 8191 : 70 |*** |
+ 8192 -> 16383 : 867 |****************************************|
+ 16384 -> 32767 : 141 |****** |
+ 32768 -> 65535 : 8 | |
+ 65536 -> 131071 : 0 | |
+ 131072 -> 262143 : 1 | |
+ 262144 -> 524287 : 2 | |
+ 524288 -> 1048575 : 3 | |
+
+As you can see, threads are switching out for relatively long intervals, even
+though we know the workload doesn't have any significant blocking. This can be
+a result of over-subscription -- too many threads contending over too few CPUs.
+Indeed, there are four available CPUs and more than four runnable threads:
+
+# nproc
+4
+# cat /proc/loadavg
+0.04 0.11 0.06 9/147 7494
+
+(This shows we have 9 threads runnable out of 147 total. This is more than 4,
+the number of available CPUs.)
+
 Finally, let's ask for a per-thread report and values in milliseconds instead
 of microseconds:
 
@@ -235,7 +280,7 @@ USAGE message:
 
 # ./cpudist.py -h
 
-usage: cpudist.py [-h] [-T] [-m] [-P] [-L] [-p PID] [interval] [count]
+usage: cpudist.py [-h] [-O] [-T] [-m] [-P] [-L] [-p PID] [interval] [count]
 
 Summarize on-CPU time per task as a histogram.
 
@@ -245,6 +290,7 @@ positional arguments:
 
 optional arguments:
  -h, --help show this help message and exit
+ -O, --offcpu measure off-CPU time
  -T, --timestamp include timestamp on output
  -m, --milliseconds millisecond histogram
  -P, --pids print a histogram per process ID
@@ -253,6 +299,7 @@ optional arguments:
 
 examples:
  cpudist # summarize on-CPU time as a histogram
+ cpudist -O # summarize off-CPU time as a histogram
  cpudist 1 10 # print 1 second summaries, 10 times
  cpudist -mT 1 # 1s summaries, milliseconds, and timestamps
  cpudist -P # show each PID separately