Hallo Michael,
enclosed my review comments.
Regards,
Loïc.
--
.\" Copyright (c) 2008 Linux Foundation, written by Michael Kerrisk
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.TH PTHREAD_SETSCHEDPARAM 3 2008-11-07 "Linux" "Linux Programmer's Manual"
.SH NAME
pthread_setschedparam, pthread_setschedparam \- set/get
scheduling policy and parameters of a thread
.SH SYNOPSIS
.nf
.B #include <pthread.h>
.BI "pthread_setschedparam(pthread_t " thread ", int " policy ,
.BI " const struct sched_param *" param );
.BI "pthread_getschedparam(pthread_t " thread ", int *" policy ,
.BI " struct sched_param *" param );
.sp
Compile and link with \fI\-pthread\fP.
One question regarding the synopsis. Is there a reason with the
"restrict" keyword is not used for pthread_getschedparam(). Accordingly
to /usr/include/pthread.h:
extern int pthread_getschedparam (pthread_t __target_thread,
int *__restrict __policy,
struct sched_param *__restrict __param);
.SH DESCRIPTION
The
.BR pthread_setschedparam ()
function sets the scheduling policy and parameters of the thread
.IR thread .
.I policy
specifies the new scheduling policy for
.IR thread .
The supported values for
.IR policy ,
and their semantics, are described in
.BR sched_setscheduler (2).
.\" FIXME . pthread_setschedparam() places no restriction on the policy,
.\" but pthread_attr_setschedpolicy() restricts policy to RR/FIFO/OTHER
.\" http://sourceware.org/bugzilla/show_bug.cgi?id=7013
Perhaps it's my poor english... But why "new scheduling policy"? I may
just want to change the priority, within the same scheduling policy...
The structure pointed to by
.I param
specifies the new scheduling parameters for
.IR thread .
Similar comment here. I may just want to change the scheduling policy,
without changing the priority. ( I admit, this one is seldom compared to
the previous one ).
Scheduling parameters are maintained in the following structure:
.in +4n
.nf
struct sched_param {
int sched_priority; /* Scheduling priority */
};
.fi
.in
As can be seen, only one scheduling parameter is supported
(this is the only parameter specified by POSIX.1-2001.)
No, it think this is not correct? If a system support SCHED_SPORADIC,
then the sched_param structure has additional fields.
For details of the permitted ranges for scheduling priorities
in each scheduling policy, see
.BR sched_setscheduler (2).
The
.BR pthread_getschedparam ()
function returns the scheduling policy and parameters of the thread
.IR thread ,
in the buffers pointed to by
.I policy
and
.IR param ,
respectively.
The returned priority value is that set by the most recent
.BR pthread_setschedparam (),
.BR pthread_setschedprio (3),
or
.BR pthread_create (3)
call that affected
.IR thread .
Hmm, that's perfectly right from a POSIX point of view. Knowing how
Linux implements threads, I have been interested about the effect of
sched_setscheduler() on a MT-process (since NPTL uses 1:1 model, this
should be a NOP).
I tested the following program against the stable glibc-2.7...
Apparently, it seems that sched_setscheduler() might affect the main
thread priority as well.
--
#include <stdio.h>
#include <pthread.h>
#include <sched.h>
#include <unistd.h>
void
print_schedinfo(const char* thread)
{
struct sched_param param;
int policy;
int rc;
rc = pthread_getschedparam(pthread_self(), &policy, ¶m);
if (rc!=0) printf("##%d\n", rc);
printf("%s > Policy=%s, prio=%d\n",
thread,
(policy==SCHED_FIFO) ? "FIFO" : "*NOT* FIFO",
param.sched_priority);
}
// dummy thread...
//
void*
thread(void* ignore)
{
sleep(3);
print_schedinfo("dummy thread");
pthread_exit(NULL);
}
int
main()
{
struct sched_param param;
int policy;
int rc;
pthread_t tid;
// create dummy thread
//
pthread_create(&tid, NULL, thread, NULL);
param.sched_priority=1;
// now we shall change the process policy/prio using
// sched_setscheduler().
// Normally: this should be a NOP. But due to the way Linux
// implements threads, I am suspecting that this shall affect
// the main thread
//
rc=sched_setscheduler(0, SCHED_FIFO, ¶m);
if (rc==-1) printf("sched_setscheduler FAILED\n");
// print my scheduling info
//
print_schedinfo("main");
// join dummy thread and terminate
//
pthread_join(tid, NULL);
return 0;
}
--
The returned priority does not reflect any temporary priority adjustments
as a result of calls to any priority inheritance or
priority ceiling functions (see, for example,
.BR pthread_mutexattr_setprioceiling (3)
and
.BR pthread_mutexattr_setprotocol (3)).
.\" FIXME . nptl/pthread_setschedparam.c has the following
.\" /* If the thread should have higher priority because of some
.\" PTHREAD_PRIO_PROTECT mutexes it holds, adjust the priority. */
.\" Eventually (perhaps after writing the mutexattr pages), we
.\" may want to add something on the topic to this page.
If I understood correctly, the thread can't lower the priority that he
has resulting from priority inheritance/ceiling. That makes perfectly
sense to me, otherwise this could defeat the purpose of such schemes,
among others avoiding priority inversion.
.SH RETURN VALUE
On success, these functions return 0;
on error, they return a non-zero error number.
If
.BR pthread_setschedparam ()
fails, the scheduling policy and parameters of
.I thread
are not changed.
.SH ERRORS
Both of these functions may fail with the following error:
.TP
.B ESRCH
No thread with the ID
.I thread
could be found.
Should the ERSCH error description be consistent across pthread_* linux
man-pages? See for instance pthread_attr_setaffinity_np().
.PP
.BR pthread_setschedparam ()
may additionally fail with the following errors:
.TP
.B EINVAL
.I policy
is not a recognized policy, or
.I param
does not make sense for the
.IR policy .
I got troubled by the "may additionally", as "may" has a particular
meaning in POSIX.1...
But I guess, you just want to express that pthread_setschedparam() shall
fail if the policy or the param is invalid, right?
.TP
.B EPERM
The caller does not have appropriate privileges
.RB ( CAP_SYS_NICE )
to set the specified scheduling policy and parameters.
See
.BR sched_setscheduler (2)
for more details on what scheduling changes require privileges.
.PP
POSIX.1-2001 also documents an
.B ENOTSUP
("attempt was made to set the policy or scheduling parameters
to an unsupported value") error for
.BR pthread_setschedparam ().
... but it doesn't seem to be used by Linux/Glibc...
.\" .SH VERSIONS
.\" Available since glibc 2.0
.SH CONFORMING TO
POSIX.1-2001.
.SH NOTES
See
.BR sched_setscheduler (2)
for a description of the effect of changing a thread's
scheduling policy and priority.
.SH EXAMPLE
The program below demonstrates the use of
.BR pthread_setschedparam ()
and
.BR pthread_getschedparam (),
as well as the use of a number of other scheduling-related
pthreads functions.
In the following run, the main thread sets its scheduling policy to
.BR SCHED_FIFO
with a priority of 10,
and initializes a thread attributes object with
a scheduling policy attribute of
.BR SCHED_RR
and a scheduling priority attribute of 20.
The program then sets (using
.BR pthread_attr_setinheritsched (3))
the inherit scheduler attribute of the thread attributes object to
.BR PTHREAD_EXPLICIT_SCHED ,
meaning that threads created using this attributes object should
take their scheduling attributes from the thread attributes object.
The program then creates a thread using the thread attributes object,
and that thread displays its scheduling policy and priority.
.in +4n
.nf
$ \fBsu\fP # Need privilege to set real-time scheduling policies
Password:
# \fB./a.out \-mf10 \-ar20 \-i e\fP
Scheduler settings of main thread
policy=SCHED_FIFO, priority=10
Scheduler settings in \(aqattr\(aq
policy=SCHED_RR, priority=20
inheritsched is EXPLICIT
Scheduler attributes of new thread
policy=SCHED_RR, priority=20
.fi
.in
In the above output, one can see that the scheduling policy and priority
were taken from the values specified in the thread attributes object.
The next run is the same as the previous,
except that the inherit scheduler attribute is set to
.BR PTHREAD_INHERIT_SCHED ,
meaning that threads created using the thread attributes object should
ignore the scheduling attributes specified in the attributes object
and instead take their scheduling attributes from the creating thread.
.in +4n
.nf
# \fB./a.out \-mf10 \-ar20 \-i i\fP
Scheduler settings of main thread
policy=SCHED_FIFO, priority=10
Scheduler settings in \(aqattr\(aq
policy=SCHED_RR, priority=20
inheritsched is INHERIT
Scheduler attributes of new thread
policy=SCHED_FIFO, priority=10
.fi
.in
In the above output, one can see that the scheduling policy and priority
were taken from the creating thread,
rather than the thread attributes object.
A classical trap is that people don't set inheritsched to explicit, and
by default it is inherit... You could illustrate this by the following
example: ./a.out -mf10 -ar20.
( I don't know however if it is the right place to speak about such
things ).
.SS Program source
\&
.nf
/* pthreads_sched_test.c */
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
/* Simple error handling functions */
#define errExit(msg) { perror(msg); exit(EXIT_FAILURE); }
#define errExitEN(en, msg) { errno = en; perror(msg); \\
exit(EXIT_FAILURE); }
static void
usage(char *prog_name, char *msg)
{
if (msg != NULL)
fputs(msg, stderr);
fprintf(stderr, "Usage: %s [options]\\n", prog_name);
fprintf(stderr, "Options are:\\n");
#define fpe(msg) fprintf(stderr, "\\t%s", msg); /* Shorter */
fpe("\-a<policy><prio> Set scheduling policy and priority in\\n");
fpe(" thread attributes object\\n");
fpe(" <policy> can be\\n");
fpe(" f SCHED_FIFO\\n");
fpe(" r SCHED_RR\\n");
fpe(" o SCHED_OTHER\\n");
fpe("\-A Use default thread attributes object\\n");
fpe("\-i {e|s} Set inherit scheduler attribute to\\n");
fpe(" \(aqexplicit\(aq or \(aqinherit\(aq\\n");
fpe("\-m<policy><prio> Set scheduling policy and priority on\\n");
fpe(" main thread before pthread_create() call\\n");
exit(EXIT_FAILURE);
} /* usage */
static int
get_policy(char p, int *policy)
{
switch (p) {
case \(aqf\(aq: *policy = SCHED_FIFO; return 1;
case \(aqr\(aq: *policy = SCHED_RR; return 1;
case \(aqo\(aq: *policy = SCHED_OTHER; return 1;
default: return 0;
}
} /* get_policy */
static void
display_sched_attr(int policy, struct sched_param *param)
{
printf(" policy=%s, priority=%d\\n",
(policy == SCHED_FIFO) ? "SCHED_FIFO" :
(policy == SCHED_RR) ? "SCHED_RR" :
(policy == SCHED_OTHER) ? "SCHED_OTHER" :
"???",
param\->sched_priority);
} /* display_sched_attr */
static void
display_thread_sched_attr(char *msg)
{
int policy, s;
struct sched_param param;
s = pthread_getschedparam(pthread_self(), &policy, ¶m);
if (s != 0)
errExitEN(s, "pthread_getschedparam");
printf("%s\\n", msg);
display_sched_attr(policy, ¶m);
} /* display_thread_sched_attr */
static void *
thread_start(void *arg)
{
display_thread_sched_attr("Scheduler attributes of new thread");
return NULL;
} /* thread_start */
int
main(int argc, char *argv[])
{
int s, opt, inheritsched, use_null_attrib, policy;
pthread_t thread;
pthread_attr_t attr;
pthread_attr_t *attrp;
char *attr_sched_str, *main_sched_str, *inheritsched_str;
struct sched_param param;
/* Process command\-line options */
use_null_attrib = 0;
attr_sched_str = NULL;
main_sched_str = NULL;
inheritsched_str = NULL;
while ((opt = getopt(argc, argv, "a:Ai:m:")) != \-1) {
switch (opt) {
case \(aqa\(aq: attr_sched_str = optarg; break;
case \(aqA\(aq: use_null_attrib = 1; break;
case \(aqi\(aq: inheritsched_str = optarg; break;
case \(aqm\(aq: main_sched_str = optarg; break;
default: usage(argv[0], "Unrecognized option\\n");
}
}
if (use_null_attrib &&
(inheritsched_str != NULL || attr_sched_str != NULL))
usage(argv[0], "Can\(aqt specify \-A with \-i or \-a\\n");
/* Optionally set scheduling attributes of main thread,
and display the attributes */
if (main_sched_str != NULL) {
if (!get_policy(main_sched_str[0], &policy))
usage(argv[0], "Bad policy for main thread (\-s)\\n");
param.sched_priority = strtol(&main_sched_str[1], NULL, 0);
s = pthread_setschedparam(pthread_self(), policy, ¶m);
if (s != 0)
errExitEN(s, "pthread_setschedparam");
}
display_thread_sched_attr("Scheduler settings of main thread");
printf("\\n");
/* Initialize thread attributes object according to options */
attrp = NULL;
if (!use_null_attrib) {
s = pthread_attr_init(&attr);
if (s != 0)
errExitEN(s, "pthread_attr_init");
attrp = &attr;
}
if (inheritsched_str != NULL) {
if (inheritsched_str[0] == \(aqe\(aq)
inheritsched = PTHREAD_EXPLICIT_SCHED;
else if (inheritsched_str[0] == \(aqi\(aq)
inheritsched = PTHREAD_INHERIT_SCHED;
else
usage(argv[0], "Value for \-i must be \(aqe\(aq or \(aqi\(aq\\n");
s = pthread_attr_setinheritsched(&attr, inheritsched);
if (s != 0)
errExitEN(s, "pthread_attr_setinheritsched");
}
if (attr_sched_str != NULL) {
if (!get_policy(attr_sched_str[0], &policy))
usage(argv[0],
"Bad policy for \(aqattr\(aq (\-a)\\n");
param.sched_priority = strtol(&attr_sched_str[1], NULL, 0);
s = pthread_attr_setschedpolicy(&attr, policy);
if (s != 0)
errExitEN(s, "pthread_attr_setschedpolicy");
s = pthread_attr_setschedparam(&attr, ¶m);
if (s != 0)
errExitEN(s, "pthread_attr_setschedparam");
}
/* If we initialized a thread attributes object, display
the scheduling attributes that were set in the object */
if (attrp != NULL) {
s = pthread_attr_getschedparam(&attr, ¶m);
if (s != 0)
errExitEN(s, "pthread_attr_getschedparam");
s = pthread_attr_getschedpolicy(&attr, &policy);
if (s != 0)
errExitEN(s, "pthread_attr_getschedpolicy");
printf("Scheduler settings in \(aqattr\(aq\\n");
display_sched_attr(policy, ¶m);
s = pthread_attr_getinheritsched(&attr, &inheritsched);
printf(" inheritsched is %s\\n",
(inheritsched == PTHREAD_INHERIT_SCHED) ? "INHERIT" :
(inheritsched == PTHREAD_EXPLICIT_SCHED) ? "EXPLICIT" :
"???");
printf("\\n");
}
/* Create a thread that will display its scheduling attributes */
s = pthread_create(&thread, attrp, &thread_start, NULL);
if (s != 0)
errExitEN(s, "pthread_create");
/* Destroy unneeded thread attributes object */
s = pthread_attr_destroy(&attr);
if (s != 0)
errExitEN(s, "pthread_attr_destroy");
s = pthread_join(thread, NULL);
if (s != 0)
errExitEN(s, "pthread_join");
exit(EXIT_SUCCESS);
} /* main */
.fi
.SH SEE ALSO
.BR sched_get_priority_min (2),
.BR sched_setscheduler (2),
.BR pthread_attr_init (3),
.BR pthread_attr_setinheritsched (3),
.BR pthread_attr_setschedparam (3),
.BR pthread_attr_setschedpolicy (3),
.BR pthread_setschedprio (3),
.BR pthread_create (3),
.BR pthread_self (3),
.BR pthreads (7)
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