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1 .\" Copyright (C) 2014 Michael Kerrisk <mtk.manpages@gmail.com>
2 .\" and Copyright (C) 2014 Peter Zijlstra <peterz@infradead.org>
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25 .\"
26 .TH SCHED_SETATTR 2 2019-03-06 "Linux" "Linux Programmer's Manual"
27 .SH NAME
28 sched_setattr, sched_getattr \-
29 set and get scheduling policy and attributes
30 .SH SYNOPSIS
31 .nf
32 .B #include <sched.h>
33 .PP
34 .BI "int sched_setattr(pid_t " pid ", struct sched_attr *" attr ,
35 .BI " unsigned int " flags );
36 .PP
37 .BI "int sched_getattr(pid_t " pid ", struct sched_attr *" attr ,
38 .BI " unsigned int " size ", unsigned int " flags );
39 .fi
40 .\" FIXME . Add feature test macro requirements
41 .SH DESCRIPTION
42 .SS sched_setattr()
43 The
44 .BR sched_setattr ()
45 system call sets the scheduling policy and
46 associated attributes for the thread whose ID is specified in
47 .IR pid .
48 If
49 .I pid
50 equals zero,
51 the scheduling policy and attributes of the calling thread will be set.
52 .PP
53 Currently, Linux supports the following "normal"
54 (i.e., non-real-time) scheduling policies as values that may be specified in
55 .IR policy :
56 .TP 14
57 .BR SCHED_OTHER
58 the standard round-robin time-sharing policy;
59 .\" In the 2.6 kernel sources, SCHED_OTHER is actually called
60 .\" SCHED_NORMAL.
61 .TP
62 .BR SCHED_BATCH
63 for "batch" style execution of processes; and
64 .TP
65 .BR SCHED_IDLE
66 for running
67 .I very
68 low priority background jobs.
69 .PP
70 Various "real-time" policies are also supported,
71 for special time-critical applications that need precise control over
72 the way in which runnable threads are selected for execution.
73 For the rules governing when a process may use these policies, see
74 .BR sched (7).
75 The real-time policies that may be specified in
76 .IR policy
77 are:
78 .TP 14
79 .BR SCHED_FIFO
80 a first-in, first-out policy; and
81 .TP
82 .BR SCHED_RR
83 a round-robin policy.
84 .PP
85 Linux also provides the following policy:
86 .TP 14
87 .B SCHED_DEADLINE
88 a deadline scheduling policy; see
89 .BR sched (7)
90 for details.
91 .PP
92 The
93 .I attr
94 argument is a pointer to a structure that defines
95 the new scheduling policy and attributes for the specified thread.
96 This structure has the following form:
97 .PP
98 .in +4n
99 .EX
100 struct sched_attr {
101 u32 size; /* Size of this structure */
102 u32 sched_policy; /* Policy (SCHED_*) */
103 u64 sched_flags; /* Flags */
104 s32 sched_nice; /* Nice value (SCHED_OTHER,
105 SCHED_BATCH) */
106 u32 sched_priority; /* Static priority (SCHED_FIFO,
107 SCHED_RR) */
108 /* Remaining fields are for SCHED_DEADLINE */
109 u64 sched_runtime;
110 u64 sched_deadline;
111 u64 sched_period;
112 };
113 .EE
114 .in
115 .PP
116 The fields of this structure are as follows:
117 .TP
118 .B size
119 This field should be set to the size of the structure in bytes, as in
120 .IR "sizeof(struct sched_attr)" .
121 If the provided structure is smaller than the kernel structure,
122 any additional fields are assumed to be '0'.
123 If the provided structure is larger than the kernel structure,
124 the kernel verifies that all additional fields are 0;
125 if they are not,
126 .BR sched_setattr ()
127 fails with the error
128 .BR E2BIG
129 and updates
130 .I size
131 to contain the size of the kernel structure.
132 .IP
133 The above behavior when the size of the user-space
134 .I sched_attr
135 structure does not match the size of the kernel structure
136 allows for future extensibility of the interface.
137 Malformed applications that pass oversize structures
138 won't break in the future if the size of the kernel
139 .I sched_attr
140 structure is increased.
141 In the future,
142 it could also allow applications that know about a larger user-space
143 .I sched_attr
144 structure to determine whether they are running on an older kernel
145 that does not support the larger structure.
146 .TP
147 .I sched_policy
148 This field specifies the scheduling policy, as one of the
149 .BR SCHED_*
150 values listed above.
151 .TP
152 .I sched_flags
153 This field contains contains zero or more of the following flags
154 that are ORed together to control scheduling behavior:
155 .RS
156 .TP
157 .BR SCHED_FLAG_RESET_ON_FORK
158 Children created by
159 .BR fork (2)
160 do not inherit privileged scheduling policies.
161 See
162 .BR sched (7)
163 for details.
164 .TP
165 .BR SCHED_FLAG_RECLAIM " (since Linux 4.13)"
166 .\" 2d4283e9d583a3ee8cfb1cbb9c1270614df4c29d
167 This flag allows a
168 .BR SCHED_DEADLINE
169 thread to reclaim bandwidth unused by other real-time threads.
170 .\" Bandwidth reclaim is done via the GRUB algorithm; see
171 .\" Documentation/scheduler/sched-deadline.txt
172 .TP
173 .BR SCHED_FLAG_DL_OVERRUN " (since Linux 4.16)"
174 .\" commit 34be39305a77b8b1ec9f279163c7cdb6cc719b91
175 This flag allows an application to get informed about run-time overruns in
176 .BR SCHED_DEADLINE
177 threads.
178 Such overruns may be caused by (for example) coarse execution time accounting
179 or incorrect parameter assignment.
180 Notification takes the form of a
181 .B SIGXCPU
182 signal which is generated on each overrun.
183 .IP
184 This
185 .BR SIGXCPU
186 signal is
187 .I process-directed
188 (see
189 .BR signal (7))
190 rather than thread-directed.
191 This is probably a bug.
192 On the one hand,
193 .BR sched_setattr ()
194 is being used to set a per-thread attribute.
195 On the other hand, if the process-directed signal is delivered to
196 a thread inside the process other than the one that had a run-time overrun,
197 the application has no way of knowing which thread overran.
198 .RE
199 .TP
200 .I sched_nice
201 This field specifies the nice value to be set when specifying
202 .IR sched_policy
203 as
204 .BR SCHED_OTHER
205 or
206 .BR SCHED_BATCH .
207 The nice value is a number in the range \-20 (high priority)
208 to +19 (low priority); see
209 .BR sched (7).
210 .TP
211 .I sched_priority
212 This field specifies the static priority to be set when specifying
213 .IR sched_policy
214 as
215 .BR SCHED_FIFO
216 or
217 .BR SCHED_RR .
218 The allowed range of priorities for these policies can be determined using
219 .BR sched_get_priority_min (2)
220 and
221 .BR sched_get_priority_max (2).
222 For other policies, this field must be specified as 0.
223 .TP
224 .I sched_runtime
225 This field specifies the "Runtime" parameter for deadline scheduling.
226 The value is expressed in nanoseconds.
227 This field, and the next two fields,
228 are used only for
229 .BR SCHED_DEADLINE
230 scheduling; for further details, see
231 .BR sched (7).
232 .TP
233 .I sched_deadline
234 This field specifies the "Deadline" parameter for deadline scheduling.
235 The value is expressed in nanoseconds.
236 .TP
237 .I sched_period
238 This field specifies the "Period" parameter for deadline scheduling.
239 The value is expressed in nanoseconds.
240 .PP
241 The
242 .I flags
243 argument is provided to allow for future extensions to the interface;
244 in the current implementation it must be specified as 0.
245 .\"
246 .\"
247 .SS sched_getattr()
248 The
249 .BR sched_getattr ()
250 system call fetches the scheduling policy and the
251 associated attributes for the thread whose ID is specified in
252 .IR pid .
253 If
254 .I pid
255 equals zero,
256 the scheduling policy and attributes of the calling thread
257 will be retrieved.
258 .PP
259 The
260 .I size
261 argument should be set to the size of the
262 .I sched_attr
263 structure as known to user space.
264 The value must be at least as large as the size of the initially published
265 .I sched_attr
266 structure, or the call fails with the error
267 .BR EINVAL .
268 .PP
269 The retrieved scheduling attributes are placed in the fields of the
270 .I sched_attr
271 structure pointed to by
272 .IR attr .
273 The kernel sets
274 .I attr.size
275 to the size of its
276 .I sched_attr
277 structure.
278 .PP
279 If the caller-provided
280 .I attr
281 buffer is larger than the kernel's
282 .I sched_attr
283 structure,
284 the additional bytes in the user-space structure are not touched.
285 If the caller-provided structure is smaller than the kernel
286 .I sched_attr
287 structure and the kernel needs to return values outside the provided space,
288 .BR sched_getattr ()
289 fails with the error
290 .BR E2BIG .
291 As with
292 .BR sched_setattr (),
293 these semantics allow for future extensibility of the interface.
294 .PP
295 The
296 .I flags
297 argument is provided to allow for future extensions to the interface;
298 in the current implementation it must be specified as 0.
299 .SH RETURN VALUE
300 On success,
301 .BR sched_setattr ()
302 and
303 .BR sched_getattr ()
304 return 0.
305 On error, \-1 is returned, and
306 .I errno
307 is set to indicate the cause of the error.
308 .SH ERRORS
309 .BR sched_getattr ()
310 and
311 .BR sched_setattr ()
312 can both fail for the following reasons:
313 .TP
314 .B EINVAL
315 .I attr
316 is NULL; or
317 .I pid
318 is negative; or
319 .I flags
320 is not zero.
321 .TP
322 .B ESRCH
323 The thread whose ID is
324 .I pid
325 could not be found.
326 .PP
327 In addition,
328 .BR sched_getattr ()
329 can fail for the following reasons:
330 .TP
331 .B E2BIG
332 The buffer specified by
333 .I size
334 and
335 .I attr
336 is too small.
337 .TP
338 .B EINVAL
339 .I size
340 is invalid; that is, it is smaller than the initial version of the
341 .I sched_attr
342 structure (48 bytes) or larger than the system page size.
343 .PP
344 In addition,
345 .BR sched_setattr ()
346 can fail for the following reasons:
347 .TP
348 .B E2BIG
349 The buffer specified by
350 .I size
351 and
352 .I attr
353 is larger than the kernel structure,
354 and one or more of the excess bytes is nonzero.
355 .TP
356 .B EBUSY
357 .B SCHED_DEADLINE
358 admission control failure, see
359 .BR sched (7).
360 .TP
361 .B EINVAL
362 .I attr.sched_policy
363 is not one of the recognized policies;
364 .I attr.sched_flags
365 contains a flag other than
366 .BR SCHED_FLAG_RESET_ON_FORK ;
367 or
368 .I attr.sched_priority
369 is invalid; or
370 .I attr.sched_policy
371 is
372 .BR SCHED_DEADLINE
373 and the deadline scheduling parameters in
374 .I attr
375 are invalid.
376 .TP
377 .B EPERM
378 The caller does not have appropriate privileges.
379 .TP
380 .B EPERM
381 The CPU affinity mask of the thread specified by
382 .I pid
383 does not include all CPUs in the system
384 (see
385 .BR sched_setaffinity (2)).
386 .SH VERSIONS
387 These system calls first appeared in Linux 3.14.
388 .\" FIXME . Add glibc version
389 .SH CONFORMING TO
390 These system calls are nonstandard Linux extensions.
391 .SH NOTES
392 .BR sched_setattr ()
393 provides a superset of the functionality of
394 .BR sched_setscheduler (2),
395 .BR sched_setparam (2),
396 .BR nice (2),
397 and (other than the ability to set the priority of all processes
398 belonging to a specified user or all processes in a specified group)
399 .BR setpriority (2).
400 Analogously,
401 .BR sched_getattr ()
402 provides a superset of the functionality of
403 .BR sched_getscheduler (2),
404 .BR sched_getparam (2),
405 and (partially)
406 .BR getpriority (2).
407 .SH BUGS
408 In Linux versions up to
409 .\" FIXME . patch sent to Peter Zijlstra
410 3.15,
411 .BR sched_settattr ()
412 failed with the error
413 .BR EFAULT
414 instead of
415 .BR E2BIG
416 for the case described in ERRORS.
417 .\" In Linux versions up to up 3.15,
418 .\" FIXME . patch from Peter Zijlstra pending
419 .\" .BR sched_setattr ()
420 .\" allowed a negative
421 .\" .I attr.sched_policy
422 .\" value.
423 .SH SEE ALSO
424 .ad l
425 .nh
426 .BR chrt (1),
427 .BR nice (2),
428 .BR sched_get_priority_max (2),
429 .BR sched_get_priority_min (2),
430 .BR sched_getaffinity (2),
431 .BR sched_getparam (2),
432 .BR sched_getscheduler (2),
433 .BR sched_rr_get_interval (2),
434 .BR sched_setaffinity (2),
435 .BR sched_setparam (2),
436 .BR sched_setscheduler (2),
437 .BR sched_yield (2),
438 .BR setpriority (2),
439 .BR pthread_getschedparam (3),
440 .BR pthread_setschedparam (3),
441 .BR pthread_setschedprio (3),
442 .BR capabilities (7),
443 .BR cpuset (7),
444 .BR sched (7)
445 .ad