How the Busybox's chrt applet works
Introduction
This article dissects the chrt applet from Busybox 1.32.0. The command reads or modifies the scheduling attributes of a process.
chrt -m
SCHED_OTHER min/max priority : 0/0
SCHED_FIFO min/max priority : 1/99
SCHED_RR min/max priority : 1/99
SCHED_BATCH min/max priority : 0/0
SCHED_IDLE min/max priority : 0/0
SCHED_DEADLINE min/max priority : 0/0
pidof firefox
6987 6851 6825 6816 6800 6771 6767 6761 6720 6611
chrt -p 6987
pid 6987's current scheduling policy: SCHED_OTHER
pid 6987's current scheduling priority: 0
sudo chrt -f -p 1 6987
chrt -p 6987
pid 6987's current scheduling policy: SCHED_FIFO
pid 6987's current scheduling priority: 1
The Busybox applet is ten times smaller once compiled than the util-linux binary, with some limitations.
The dissection
The applet is implemented in util-linux/chrt.c, whose main function calls several helpers and has three parts:
- the first parses the command options
- the second prints the scheduler's information
- the last one, to apply scheduler changes in case of a set
At start of main, the character string containing the options are parsed to obtain a bitfield easier to use:
opt = getopt32(argv, "^"
"+" "mprfobi"
"\0"
/* only one policy accepted: */
"r--fobi:f--robi:o--rfbi:b--rfoi:i--rfob"
);
If the (-m) is set then the min and max valid priorities for each scheduling policies are shown and the command exits:
if (opt & OPT_m) { /* print min/max and exit */
show_min_max(SCHED_OTHER);
show_min_max(SCHED_FIFO);
show_min_max(SCHED_RR);
show_min_max(SCHED_BATCH);
show_min_max(SCHED_IDLE);
fflush_stdout_and_exit(EXIT_SUCCESS);
}
The function show_min_max uses the Posix functions sched_get_priority_max and sched_get_priority_min from the standard C library to send a syscall to the kernel in order to obtain the min and max values accepted by each policy:
max = sched_get_priority_max(pol);
min = sched_get_priority_min(pol);
if ((max|min) < 0)
fmt = "SCHED_%s not supported\n";
Otherwise the required options and arguments to show or to apply real-time attributes of a process:
//if (opt & OPT_r)
// policy = SCHED_RR; - default, already set
if (opt & OPT_f)
policy = SCHED_FIFO;
if (opt & OPT_o)
policy = SCHED_OTHER;
if (opt & OPT_b)
policy = SCHED_BATCH;
if (opt & OPT_i)
policy = SCHED_IDLE;
argv += optind;
if (!argv[0])
bb_show_usage();
if (opt & OPT_p) {
pid_str = *argv++;
if (*argv) { /* "-p PRIO PID [...]" */
priority = pid_str;
pid_str = *argv;
}
/* else "-p PID", and *argv == NULL */
pid = xatoul_range(pid_str, 1, ((unsigned)(pid_t)ULONG_MAX) >> 1);
} else {
priority = *argv++;
if (!*argv)
bb_show_usage();
}
Then the applet uses the Posix function sched_getscheduler provides by the standard C library to obtain the scheduling attributes of the process specified by the pid.
print_rt_info:
pol = sched_getscheduler(pid);
if (pol < 0)
bb_perror_msg_and_die("can't %cet pid %u's policy", 'g', (int)pid);
Finally, when chrt modifies scheduling attributes, the Posix function sched_setscheduler applies them and the new attributes are shown:
if (sched_setscheduler(pid, policy, &sp) < 0)
bb_perror_msg_and_die("can't %cet pid %u's policy", 's', (int)pid);
if (!argv[0]) /* "-p PRIO PID [...]" */
goto print_rt_info;
The function sched_setscheduler and sched_getscheduler will send a syscall to the scheduler subsystem of the kernel Linux. This subsystem also exposes this information from /proc:
cat /proc/6987/sched
WebExtensions (6987, #threads: 23)
-------------------------------------------------------------------
se.exec_start : 4421312.640001
se.vruntime : 344438.942254
se.sum_exec_runtime : 38238.466094
se.nr_migrations : 6811
nr_switches : 49452
nr_voluntary_switches : 21749
nr_involuntary_switches : 27703
se.load.weight : 1048576
se.runnable_weight : 1048576
se.avg.load_sum : 3415
se.avg.runnable_load_sum : 3415
se.avg.util_sum : 3497621
se.avg.load_avg : 74
se.avg.runnable_load_avg : 74
se.avg.util_avg : 74
se.avg.last_update_time : 4421312640000
se.avg.util_est.ewma : 75
se.avg.util_est.enqueued : 75
policy : 0
prio : 120
clock-delta : 89
mm->numa_scan_seq : 0
numa_pages_migrated : 0
numa_preferred_nid : -1
total_numa_faults : 0
current_node=0, numa_group_id=0
numa_faults node=0 task_private=0 task_shared=0 group_private=0 group_shared=0
Limitations
Limitations I observed in this applet:
Resetting scheduling policy
The chrt applet doesn't offer an option (-R) to specify if the scheduling policy should be applied or reset when a process forks to create children. This feature, introduced since Linux 2.6.32, can be only enabled or disabled at the build of busybox and it is applied on all scheduling attributes modifications done with this applet.
Deadline support
The chrt applet doesn't provide the required scheduling options (-d, -T, -P and -D) to set the deadline scheduling attributes of a process.