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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 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 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  CPU  affinity mask of the thread specified by pid 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.

SEE ALSO

       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)

COLOPHON

       This  page  is  part of release 4.16 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 https://www.kernel.org/doc/man-pages/.