SCHED_SETATTR(2)          (2020-11-01)           SCHED_SETATTR(2)

     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 speci-
          fied 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 execu-
          tion.  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:

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              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 pro-
               vided 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 ker-
               nel 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 privi-
                    leged scheduling policies.  See sched(7) for
                    details.

               SCHED_FLAG_RECLAIM (since Linux 4.13)
                    This flag allows a SCHED_DEADLINE thread to

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                    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 pri-
               ority) 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 dead-
               line scheduling.  The value is expressed in nanosec-
               onds.  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 dead-
               line scheduling.  The value is expressed in nanosec-
               onds.

          sched_period
               This field specifies the "Period" parameter for dead-
               line scheduling.  The value is expressed in nanosec-
               onds.

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

        sched_getattr()
          The sched_getattr() system call fetches the scheduling pol-
          icy 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 extensi-
          bility of the interface.

          The flags argument is provided to allow for future exten-
          sions 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 rea-
          sons:

          E2BIG

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               The buffer specified by size and attr is too small.

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

          In addition, sched_setattr() can fail for the following rea-
          sons:

          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 poli-
               cies; 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 speci-
          fied 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

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          ERRORS.

          In Linux versions up to 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.

     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 5.10  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/.

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