Ubuntu Manpage: sched_setattr, sched_getattr - set and get scheduling policy and attributes

NAME

       sched_setattr, sched_getattr - set and get scheduling policy and attributes

SYNOPSIS

       #include <sched.h>

       int sched_setattr(pid_t pid, struct sched_attr *attr,
                         unsigned int flags);

       int sched_getattr(pid_t pid, struct sched_attr *attr,
                         unsigned int size, unsigned int flags);

DESCRIPTION

sched_setattr()
The sched_setattr() system call sets the scheduling policy and associated attributes for the thread whose
ID is specified in pid. If pid equals zero, the scheduling policy and attributes of the calling thread
will be set.

Currently, Linux supports the following "normal" (i.e., non-real-time) scheduling policies as values that
may be specified in policy:

SCHED_OTHER the standard round-robin time-sharing policy;

SCHED_BATCH for "batch" style execution of processes; and

SCHED_IDLE for running very low priority background jobs.

Various "real-time" policies are also supported, for special time-critical applications that need precise
control over the way in which runnable threads are selected for execution. For the rules governing when
a process may use these policies, see sched(7). The real-time policies that may be specified in policy
are:

SCHED_FIFO a first-in, first-out policy; and

SCHED_RR a round-robin policy.

Linux also provides the following policy:

SCHED_DEADLINE
a deadline scheduling policy; see sched(7) for details.

The attr argument is a pointer to a structure that defines the new scheduling policy and attributes for
the specified thread. This structure has the following form:

struct sched_attr {
u32 size; /* Size of this structure */
u32 sched_policy; /* Policy (SCHED_*) */
u64 sched_flags; /* Flags */
s32 sched_nice; /* Nice value (SCHED_OTHER,
SCHED_BATCH) */
u32 sched_priority; /* Static priority (SCHED_FIFO,
SCHED_RR) */
/* Remaining fields are for SCHED_DEADLINE */
u64 sched_runtime;
u64 sched_deadline;
u64 sched_period;
};

The fields of this structure are as follows:

size This field should be set to the size of the structure in bytes, as in sizeof(struct sched_attr).
If the provided structure is smaller than the kernel structure, any additional fields are assumed
to be '0'. If the provided structure is larger than the kernel structure, the kernel verifies
that all additional fields are 0; if they are not, sched_setattr() fails with the error E2BIG and
updates size to contain the size of the kernel structure.

The above behavior when the size of the user-space sched_attr structure does not match the size of
the kernel structure allows for future extensibility of the interface. Malformed applications
that pass oversize structures won't break in the future if the size of the kernel sched_attr
structure is increased. In the future, it could also allow applications that know about a larger
user-space sched_attr structure to determine whether they are running on an older kernel that does
not support the larger structure.

sched_policy
This field specifies the scheduling policy, as one of the SCHED_* values listed above.

sched_flags
This field contains flags controlling scheduling behavior. Only one such flag is currently
defined: SCHED_FLAG_RESET_ON_FORK. As a result of including this flag, children created by
fork(2) do not inherit privileged scheduling policies. See sched(7) for details.

sched_nice
This field specifies the nice value to be set when specifying sched_policy as SCHED_OTHER or
SCHED_BATCH. The nice value is a number in the range -20 (high priority) to +19 (low priority);
see setpriority(2).

sched_priority
This field specifies the static priority to be set when specifying sched_policy as SCHED_FIFO or
SCHED_RR. The allowed range of priorities for these policies can be determined using
sched_get_priority_min(2) and sched_get_priority_max(2). For other policies, this field must be
specified as 0.

sched_runtime
This field specifies the "Runtime" parameter for deadline scheduling. The value is expressed in
nanoseconds. This field, and the next two fields, are used only for SCHED_DEADLINE scheduling;
for further details, see sched(7).

sched_deadline
This field specifies the "Deadline" parameter for deadline scheduling. The value is expressed in
nanoseconds.

sched_period
This field specifies the "Period" parameter for deadline scheduling. The value is expressed in
nanoseconds.

The flags argument is provided to allow for future extensions to the interface; in the current
implementation it must be specified as 0.

sched_getattr()
The sched_getattr() system call fetches the scheduling policy and the associated attributes for the
thread whose ID is specified in pid. If pid equals zero, the scheduling policy and attributes of the
calling thread will be retrieved.

The size argument should be set to the size of the sched_attr structure as known to user space. The
value must be at least as large as the size of the initially published sched_attr structure, or the call
fails with the error EINVAL.

The retrieved scheduling attributes are placed in the fields of the sched_attr structure pointed to by
attr. The kernel sets attr.size to the size of its sched_attr structure.

If the caller-provided attr buffer is larger than the kernel's sched_attr structure, the additional bytes
in the user-space structure are not touched. If the caller-provided structure is smaller than the kernel
sched_attr structure and the kernel needs to return values outside the provided space, sched_getattr()
fails with the error E2BIG. As with sched_setattr(), these semantics allow for future extensibility of
the interface.

The flags argument is provided to allow for future extensions to the interface; in the current
implementation it must be specified as 0.

RETURN VALUE

       On success, sched_setattr() and sched_getattr() return 0.  On error, -1 is returned, and errno is set  to
       indicate the cause of the error.

ERRORS

       sched_getattr() and sched_setattr() can both fail for the following reasons:

       EINVAL attr is NULL; or pid is negative; or flags is not zero.

       ESRCH  The thread whose ID is pid could not be found.

       In addition, sched_getattr() can fail for the following reasons:

       E2BIG  The buffer specified by size and attr is too small.

       EINVAL size  is  invalid; that is, it is smaller than the initial version of the sched_attr structure (48
              bytes) or larger than the system page size.

       In addition, sched_setattr() can fail for the following reasons:

       E2BIG  The buffer specified by size and attr is larger than the kernel structure, and one or more of  the
              excess bytes is nonzero.

       EBUSY  SCHED_DEADLINE admission control failure, see sched(7).

       EINVAL attr.sched_policy  is  not  one of the recognized policies; attr.sched_flags contains a flag other
              than  SCHED_FLAG_RESET_ON_FORK;  or  attr.sched_priority  is  invalid;  or  attr.sched_policy   is
              SCHED_DEADLINE and the deadline scheduling parameters in attr are invalid.

       EPERM  The caller does not have appropriate privileges.

       EPERM  The caller's CPU affinity mask does not include all CPUs in the system (see sched_setaffinity(2)).

VERSIONS

       These system calls first appeared in Linux 3.14.

CONFORMING TO

       These system calls are nonstandard Linux extensions.

NOTES

       sched_setattr()  provides  a  superset  of the functionality of sched_setscheduler(2), sched_setparam(2),
       nice(2), and (other than the ability to set the priority of all processes belonging to a  specified  user
       or  all processes in a specified group) setpriority(2).  Analogously, sched_getattr() provides a superset
       of the functionality of sched_getscheduler(2), sched_getparam(2), and (partially) getpriority(2).

BUGS

       In Linux versions up to 3.15, sched_settattr() failed with the error EFAULT instead of E2BIG for the case
       described in ERRORS.

COLOPHON

       This page is part of release 4.04 of  the  Linux  man-pages  project.   A  description  of  the  project,
       information   about   reporting   bugs,   and   the  latest  version  of  this  page,  can  be  found  at
       http://www.kernel.org/doc/man-pages/.

Linux                                              2015-07-23                                   SCHED_SETATTR(2)