dcstat.py

#!/usr/bin/env python
# @lint-avoid-python-3-compatibility-imports
#
# dcstat   Directory entry cache (dcache) stats.
#          For Linux, uses BCC, eBPF.
#
# USAGE: dcstat [interval [count]]
#
# This uses kernel dynamic tracing of kernel functions, lookup_fast() and
# d_lookup(), which will need to be modified to match kernel changes. See
# code comments.
#
# Copyright 2016 Netflix, Inc.
# Licensed under the Apache License, Version 2.0 (the "License")
#
# 09-Feb-2016   Brendan Gregg   Created this.

from __future__ import print_function
from bcc import BPF
from ctypes import c_int
from time import sleep, strftime
from sys import argv

def usage():
    print("USAGE: %s [interval [count]]" % argv[0])
    exit()

# arguments
interval = 1
count = -1
if len(argv) > 1:
    try:
        interval = int(argv[1])
        if interval == 0:
            raise
        if len(argv) > 2:
            count = int(argv[2])
    except:  # also catches -h, --help
        usage()

# define BPF program
bpf_text = """
#include <uapi/linux/ptrace.h>

enum stats {
    S_REFS = 1,
    S_SLOW,
    S_MISS,
    S_MAXSTAT
};

BPF_ARRAY(stats, u64, S_MAXSTAT);

/*
 * How this is instrumented, and how to interpret the statistics, is very much
 * tied to the current kernel implementation (this was written on Linux 4.4).
 * This will need maintenance to keep working as the implementation changes. To
 * aid future adventurers, this is is what the current code does, and why.
 *
 * First problem: the current implementation takes a path and then does a
 * lookup of each component. So how do we count a reference? Once for the path
 * lookup, or once for every component lookup? I've chosen the latter
 * since it seems to map more closely to actual dcache lookups (via
 * __d_lookup_rcu()). It's counted via calls to lookup_fast().
 *
 * The implementation tries different, progressively slower, approaches to
 * lookup a file. At what point do we call it a dcache miss? I've chosen when
 * a d_lookup() (which is called during lookup_slow()) returns zero.
 *
 * I've also included a "SLOW" statistic to show how often the fast lookup
 * failed. Whether this exists or is interesting is an implementation detail,
 * and the "SLOW" statistic may be removed in future versions.
 */
void count_fast(struct pt_regs *ctx) {
    int key = S_REFS;
    stats.atomic_increment(key);
}

void count_lookup(struct pt_regs *ctx) {
    int key = S_SLOW;
    stats.atomic_increment(key);
    if (PT_REGS_RC(ctx) == 0) {
        key = S_MISS;
        stats.atomic_increment(key);
    }
}
"""

# load BPF program
b = BPF(text=bpf_text)
b.attach_kprobe(event_re=r'^lookup_fast$|^lookup_fast.constprop.*.\d$', fn_name="count_fast")
b.attach_kretprobe(event="d_lookup", fn_name="count_lookup")

# stat column labels and indexes
stats = {
    "REFS": 1,
    "SLOW": 2,
    "MISS": 3
}

# header
print("%-8s  " % "TIME", end="")
for stype, idx in sorted(stats.items(), key=lambda k_v: (k_v[1], k_v[0])):
    print(" %8s" % (stype + "/s"), end="")
print(" %8s" % "HIT%")

# output
i = 0
while (1):
    if count > 0:
        i += 1
        if i > count:
            exit()
    try:
        sleep(interval)
    except KeyboardInterrupt:
        exit()

    print("%-8s: " % strftime("%H:%M:%S"), end="")

    # print each statistic as a column
    for stype, idx in sorted(stats.items(), key=lambda k_v: (k_v[1], k_v[0])):
        try:
            val = b["stats"][c_int(idx)].value / interval
            print(" %8d" % val, end="")
        except:
            print(" %8d" % 0, end="")

    # print hit ratio percentage
    try:
        ref = b["stats"][c_int(stats["REFS"])].value
        miss = b["stats"][c_int(stats["MISS"])].value
        hit = ref - miss
        pct = float(100) * hit / ref
        print(" %8.2f" % pct)
    except:
        print(" %7s%%" % "-")

    b["stats"].clear()