plucky (2) sched_setattr.2.gz

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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) */
               /* For SCHED_DEADLINE */
               u64 sched_runtime;
               u64 sched_deadline;
               u64 sched_period;

               /* Utilization hints */
               u32 sched_util_min;
               u32 sched_util_max;
           };

       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_FLAG_UTIL_CLAMP_MIN
              SCHED_FLAG_UTIL_CLAMP_MAX (both since Linux 5.3)
                     These  flags  indicate  that the sched_util_min or sched_util_max fields, respectively, are
                     present, representing the expected minimum and maximum utilization of the thread.

                     The utilization attributes provide the scheduler with boundaries  within  which  it  should
                     schedule  the  thread,  potentially  informing  its  decisions regarding task placement and
                     frequency selection.

       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.

       sched_util_min
       sched_util_max (both since Linux 5.3)
              These fields specify the expected minimum and maximum utilization, respectively.  They are ignored
              unless their  corresponding  SCHED_FLAG_UTIL_CLAMP_MIN  or  SCHED_FLAG_UTIL_CLAMP_MAX  is  set  in
              sched_flags.

              Utilization  is  a value in the range [0, 1024], representing the percentage of CPU time used by a
              task when running at the maximum frequency on the highest capacity CPU of the system.  This  is  a
              fixed point representation, where 1024 corresponds to 100%, and 0 corresponds to 0%.  For example,
              a 20% utilization task is a  task  running  for  2ms  every  10ms  at  maximum  frequency  and  is
              represented by a utilization value of 0.2 * 1024 = 205.

              A  task  with  a  minimum utilization value larger than 0 is more likely scheduled on a CPU with a
              capacity big enough to fit the specified value.  A task with a maximum utilization  value  smaller
              than 1024 is more likely scheduled on a CPU with no more capacity than the specified value.

              A task utilization boundary can be reset by setting its field to UINT32_MAX (since Linux 5.11).

       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.

       EINVAL attr.sched_flags contains a flag other than SCHED_FLAG_RESET_ON_FORK.

       EINVAL attr.sched_priority is invalid.

       EINVAL attr.sched_policy is SCHED_DEADLINE, and the deadline scheduling parameters in attr are invalid.

       EINVAL attr.sched_flags    contains    SCHED_FLAG_UTIL_CLAMP_MIN    or   SCHED_FLAG_UTIL_CLAMP_MAX,   and
              attr.sched_util_min or attr.sched_util_max are out of bounds.

       EOPNOTSUPP
              SCHED_FLAG_UTIL_CLAMP was provided, but the kernel was not built with CONFIG_UCLAMP_TASK support.

       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.

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)