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

 .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
@@ -10,6 +10,11 @@ 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

 #!/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 @@ import argparse
 
 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 @@ parser = argparse.ArgumentParser(
     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_decl()
 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 @@ b = BPF(text=bpf_text)
 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