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

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sched.h>            /* Definition of SCHED_* constants */
       #include <sys/syscall.h>      /* Definition of SYS_* constants */
       #include <unistd.h>

       int syscall(SYS_sched_setattr, pid_t pid, struct sched_attr *attr,
                   unsigned int flags);
       int syscall(SYS_sched_getattr, pid_t pid, struct sched_attr *attr,
                   unsigned int size, unsigned int flags);

       Note: glibc provides no wrappers for these system calls, necessitating the use of syscall(2).

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 the sched_attr 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 zero or more of the following flags that are ORed together to control
scheduling behavior:

SCHED_FLAG_RESET_ON_FORK
Children created by fork(2) do not inherit privileged scheduling policies. See sched(7)
for details.

SCHED_FLAG_RECLAIM (since Linux 4.13)
This flag allows a SCHED_DEADLINE thread to reclaim bandwidth unused by other real-time
threads.

SCHED_FLAG_DL_OVERRUN (since Linux 4.16)
This flag allows an application to get informed about run-time overruns in SCHED_DEADLINE
threads. Such overruns may be caused by (for example) coarse execution time accounting or
incorrect parameter assignment. Notification takes the form of a SIGXCPU signal which is
generated on each overrun.

This SIGXCPU signal is process-directed (see signal(7)) rather than thread-directed. This
is probably a bug. On the one hand, sched_setattr() is being used to set a per-thread
attribute. On the other hand, if the process-directed signal is delivered to a thread
inside the process other than the one that had a run-time overrun, the application has no
way of knowing which thread overran.

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 sched(7).

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, the kernel will silently not return any values which would be stored outside the
provided space. 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 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  CPU affinity mask of the thread specified by pid does not include all CPUs in the system (see
              sched_setaffinity(2)).

STANDARDS

       Linux.

HISTORY

       Linux 3.14.

NOTES

       glibc does not provide wrappers for these system calls; call them using syscall(2).

       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_setattr() failed with the error EFAULT instead of E2BIG for the case
       described in ERRORS.

       Up to Linux 5.3, sched_getattr() failed with the error EFBIG if the in-kernel  sched_attr  structure  was
       larger than the size passed by user space.