sched_setattr, sched_getattr - set and get scheduling policy and attributes
#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);
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.
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.
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.
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)).
These system calls first appeared in Linux 3.14.
These system calls are nonstandard Linux extensions.
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).
In Linux versions up to 3.15,
sched_settattr() failed with the error
EFAULT instead of
E2BIG for the case described in ERRORS.
chrt(1),
nice(2),
sched_get_priority_max(2),
sched_get_priority_min(2),
sched_getaffinity(2),
sched_getparam(2),
sched_getscheduler(2),
sched_rr_get_interval(2),
sched_setaffinity(2),
sched_setparam(2),
sched_setscheduler(2),
sched_yield(2),
setpriority(2),
pthread_getschedparam(3),
pthread_setschedparam(3),
pthread_setschedprio(3),
capabilities(7),
cpuset(7),
sched(7)