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1 | CGROUPS |
2 | ------- | |
3 | ||
4 | Written by Paul Menage <menage@google.com> based on Documentation/cpusets.txt | |
5 | ||
6 | Original copyright statements from cpusets.txt: | |
7 | Portions Copyright (C) 2004 BULL SA. | |
8 | Portions Copyright (c) 2004-2006 Silicon Graphics, Inc. | |
9 | Modified by Paul Jackson <pj@sgi.com> | |
10 | Modified by Christoph Lameter <clameter@sgi.com> | |
11 | ||
12 | CONTENTS: | |
13 | ========= | |
14 | ||
15 | 1. Control Groups | |
16 | 1.1 What are cgroups ? | |
17 | 1.2 Why are cgroups needed ? | |
18 | 1.3 How are cgroups implemented ? | |
19 | 1.4 What does notify_on_release do ? | |
20 | 1.5 How do I use cgroups ? | |
21 | 2. Usage Examples and Syntax | |
22 | 2.1 Basic Usage | |
23 | 2.2 Attaching processes | |
24 | 3. Kernel API | |
25 | 3.1 Overview | |
26 | 3.2 Synchronization | |
27 | 3.3 Subsystem API | |
28 | 4. Questions | |
29 | ||
30 | 1. Control Groups | |
31 | ========== | |
32 | ||
33 | 1.1 What are cgroups ? | |
34 | ---------------------- | |
35 | ||
36 | Control Groups provide a mechanism for aggregating/partitioning sets of | |
37 | tasks, and all their future children, into hierarchical groups with | |
38 | specialized behaviour. | |
39 | ||
40 | Definitions: | |
41 | ||
42 | A *cgroup* associates a set of tasks with a set of parameters for one | |
43 | or more subsystems. | |
44 | ||
45 | A *subsystem* is a module that makes use of the task grouping | |
46 | facilities provided by cgroups to treat groups of tasks in | |
47 | particular ways. A subsystem is typically a "resource controller" that | |
48 | schedules a resource or applies per-cgroup limits, but it may be | |
49 | anything that wants to act on a group of processes, e.g. a | |
50 | virtualization subsystem. | |
51 | ||
52 | A *hierarchy* is a set of cgroups arranged in a tree, such that | |
53 | every task in the system is in exactly one of the cgroups in the | |
54 | hierarchy, and a set of subsystems; each subsystem has system-specific | |
55 | state attached to each cgroup in the hierarchy. Each hierarchy has | |
56 | an instance of the cgroup virtual filesystem associated with it. | |
57 | ||
58 | At any one time there may be multiple active hierachies of task | |
59 | cgroups. Each hierarchy is a partition of all tasks in the system. | |
60 | ||
61 | User level code may create and destroy cgroups by name in an | |
62 | instance of the cgroup virtual file system, specify and query to | |
63 | which cgroup a task is assigned, and list the task pids assigned to | |
64 | a cgroup. Those creations and assignments only affect the hierarchy | |
65 | associated with that instance of the cgroup file system. | |
66 | ||
67 | On their own, the only use for cgroups is for simple job | |
68 | tracking. The intention is that other subsystems hook into the generic | |
69 | cgroup support to provide new attributes for cgroups, such as | |
70 | accounting/limiting the resources which processes in a cgroup can | |
71 | access. For example, cpusets (see Documentation/cpusets.txt) allows | |
72 | you to associate a set of CPUs and a set of memory nodes with the | |
73 | tasks in each cgroup. | |
74 | ||
75 | 1.2 Why are cgroups needed ? | |
76 | ---------------------------- | |
77 | ||
78 | There are multiple efforts to provide process aggregations in the | |
79 | Linux kernel, mainly for resource tracking purposes. Such efforts | |
80 | include cpusets, CKRM/ResGroups, UserBeanCounters, and virtual server | |
81 | namespaces. These all require the basic notion of a | |
82 | grouping/partitioning of processes, with newly forked processes ending | |
83 | in the same group (cgroup) as their parent process. | |
84 | ||
85 | The kernel cgroup patch provides the minimum essential kernel | |
86 | mechanisms required to efficiently implement such groups. It has | |
87 | minimal impact on the system fast paths, and provides hooks for | |
88 | specific subsystems such as cpusets to provide additional behaviour as | |
89 | desired. | |
90 | ||
91 | Multiple hierarchy support is provided to allow for situations where | |
92 | the division of tasks into cgroups is distinctly different for | |
93 | different subsystems - having parallel hierarchies allows each | |
94 | hierarchy to be a natural division of tasks, without having to handle | |
95 | complex combinations of tasks that would be present if several | |
96 | unrelated subsystems needed to be forced into the same tree of | |
97 | cgroups. | |
98 | ||
99 | At one extreme, each resource controller or subsystem could be in a | |
100 | separate hierarchy; at the other extreme, all subsystems | |
101 | would be attached to the same hierarchy. | |
102 | ||
103 | As an example of a scenario (originally proposed by vatsa@in.ibm.com) | |
104 | that can benefit from multiple hierarchies, consider a large | |
105 | university server with various users - students, professors, system | |
106 | tasks etc. The resource planning for this server could be along the | |
107 | following lines: | |
108 | ||
109 | CPU : Top cpuset | |
110 | / \ | |
111 | CPUSet1 CPUSet2 | |
112 | | | | |
113 | (Profs) (Students) | |
114 | ||
115 | In addition (system tasks) are attached to topcpuset (so | |
116 | that they can run anywhere) with a limit of 20% | |
117 | ||
118 | Memory : Professors (50%), students (30%), system (20%) | |
119 | ||
120 | Disk : Prof (50%), students (30%), system (20%) | |
121 | ||
122 | Network : WWW browsing (20%), Network File System (60%), others (20%) | |
123 | / \ | |
124 | Prof (15%) students (5%) | |
125 | ||
126 | Browsers like firefox/lynx go into the WWW network class, while (k)nfsd go | |
127 | into NFS network class. | |
128 | ||
129 | At the same time firefox/lynx will share an appropriate CPU/Memory class | |
130 | depending on who launched it (prof/student). | |
131 | ||
132 | With the ability to classify tasks differently for different resources | |
133 | (by putting those resource subsystems in different hierarchies) then | |
134 | the admin can easily set up a script which receives exec notifications | |
135 | and depending on who is launching the browser he can | |
136 | ||
137 | # echo browser_pid > /mnt/<restype>/<userclass>/tasks | |
138 | ||
139 | With only a single hierarchy, he now would potentially have to create | |
140 | a separate cgroup for every browser launched and associate it with | |
141 | approp network and other resource class. This may lead to | |
142 | proliferation of such cgroups. | |
143 | ||
144 | Also lets say that the administrator would like to give enhanced network | |
145 | access temporarily to a student's browser (since it is night and the user | |
146 | wants to do online gaming :) OR give one of the students simulation | |
147 | apps enhanced CPU power, | |
148 | ||
149 | With ability to write pids directly to resource classes, its just a | |
150 | matter of : | |
151 | ||
152 | # echo pid > /mnt/network/<new_class>/tasks | |
153 | (after some time) | |
154 | # echo pid > /mnt/network/<orig_class>/tasks | |
155 | ||
156 | Without this ability, he would have to split the cgroup into | |
157 | multiple separate ones and then associate the new cgroups with the | |
158 | new resource classes. | |
159 | ||
160 | ||
161 | ||
162 | 1.3 How are cgroups implemented ? | |
163 | --------------------------------- | |
164 | ||
165 | Control Groups extends the kernel as follows: | |
166 | ||
167 | - Each task in the system has a reference-counted pointer to a | |
168 | css_set. | |
169 | ||
170 | - A css_set contains a set of reference-counted pointers to | |
171 | cgroup_subsys_state objects, one for each cgroup subsystem | |
172 | registered in the system. There is no direct link from a task to | |
173 | the cgroup of which it's a member in each hierarchy, but this | |
174 | can be determined by following pointers through the | |
175 | cgroup_subsys_state objects. This is because accessing the | |
176 | subsystem state is something that's expected to happen frequently | |
177 | and in performance-critical code, whereas operations that require a | |
178 | task's actual cgroup assignments (in particular, moving between | |
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179 | cgroups) are less common. A linked list runs through the cg_list |
180 | field of each task_struct using the css_set, anchored at | |
181 | css_set->tasks. | |
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182 | |
183 | - A cgroup hierarchy filesystem can be mounted for browsing and | |
184 | manipulation from user space. | |
185 | ||
186 | - You can list all the tasks (by pid) attached to any cgroup. | |
187 | ||
188 | The implementation of cgroups requires a few, simple hooks | |
189 | into the rest of the kernel, none in performance critical paths: | |
190 | ||
191 | - in init/main.c, to initialize the root cgroups and initial | |
192 | css_set at system boot. | |
193 | ||
194 | - in fork and exit, to attach and detach a task from its css_set. | |
195 | ||
196 | In addition a new file system, of type "cgroup" may be mounted, to | |
197 | enable browsing and modifying the cgroups presently known to the | |
198 | kernel. When mounting a cgroup hierarchy, you may specify a | |
199 | comma-separated list of subsystems to mount as the filesystem mount | |
200 | options. By default, mounting the cgroup filesystem attempts to | |
201 | mount a hierarchy containing all registered subsystems. | |
202 | ||
203 | If an active hierarchy with exactly the same set of subsystems already | |
204 | exists, it will be reused for the new mount. If no existing hierarchy | |
205 | matches, and any of the requested subsystems are in use in an existing | |
206 | hierarchy, the mount will fail with -EBUSY. Otherwise, a new hierarchy | |
207 | is activated, associated with the requested subsystems. | |
208 | ||
209 | It's not currently possible to bind a new subsystem to an active | |
210 | cgroup hierarchy, or to unbind a subsystem from an active cgroup | |
211 | hierarchy. This may be possible in future, but is fraught with nasty | |
212 | error-recovery issues. | |
213 | ||
214 | When a cgroup filesystem is unmounted, if there are any | |
215 | child cgroups created below the top-level cgroup, that hierarchy | |
216 | will remain active even though unmounted; if there are no | |
217 | child cgroups then the hierarchy will be deactivated. | |
218 | ||
219 | No new system calls are added for cgroups - all support for | |
220 | querying and modifying cgroups is via this cgroup file system. | |
221 | ||
222 | Each task under /proc has an added file named 'cgroup' displaying, | |
223 | for each active hierarchy, the subsystem names and the cgroup name | |
224 | as the path relative to the root of the cgroup file system. | |
225 | ||
226 | Each cgroup is represented by a directory in the cgroup file system | |
227 | containing the following files describing that cgroup: | |
228 | ||
229 | - tasks: list of tasks (by pid) attached to that cgroup | |
230 | - notify_on_release flag: run /sbin/cgroup_release_agent on exit? | |
231 | ||
232 | Other subsystems such as cpusets may add additional files in each | |
233 | cgroup dir | |
234 | ||
235 | New cgroups are created using the mkdir system call or shell | |
236 | command. The properties of a cgroup, such as its flags, are | |
237 | modified by writing to the appropriate file in that cgroups | |
238 | directory, as listed above. | |
239 | ||
240 | The named hierarchical structure of nested cgroups allows partitioning | |
241 | a large system into nested, dynamically changeable, "soft-partitions". | |
242 | ||
243 | The attachment of each task, automatically inherited at fork by any | |
244 | children of that task, to a cgroup allows organizing the work load | |
245 | on a system into related sets of tasks. A task may be re-attached to | |
246 | any other cgroup, if allowed by the permissions on the necessary | |
247 | cgroup file system directories. | |
248 | ||
249 | When a task is moved from one cgroup to another, it gets a new | |
250 | css_set pointer - if there's an already existing css_set with the | |
251 | desired collection of cgroups then that group is reused, else a new | |
252 | css_set is allocated. Note that the current implementation uses a | |
253 | linear search to locate an appropriate existing css_set, so isn't | |
254 | very efficient. A future version will use a hash table for better | |
255 | performance. | |
256 | ||
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257 | To allow access from a cgroup to the css_sets (and hence tasks) |
258 | that comprise it, a set of cg_cgroup_link objects form a lattice; | |
259 | each cg_cgroup_link is linked into a list of cg_cgroup_links for | |
260 | a single cgroup on its cont_link_list field, and a list of | |
261 | cg_cgroup_links for a single css_set on its cg_link_list. | |
262 | ||
263 | Thus the set of tasks in a cgroup can be listed by iterating over | |
264 | each css_set that references the cgroup, and sub-iterating over | |
265 | each css_set's task set. | |
266 | ||
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267 | The use of a Linux virtual file system (vfs) to represent the |
268 | cgroup hierarchy provides for a familiar permission and name space | |
269 | for cgroups, with a minimum of additional kernel code. | |
270 | ||
271 | 1.4 What does notify_on_release do ? | |
272 | ------------------------------------ | |
273 | ||
274 | *** notify_on_release is disabled in the current patch set. It will be | |
275 | *** reactivated in a future patch in a less-intrusive manner | |
276 | ||
277 | If the notify_on_release flag is enabled (1) in a cgroup, then | |
278 | whenever the last task in the cgroup leaves (exits or attaches to | |
279 | some other cgroup) and the last child cgroup of that cgroup | |
280 | is removed, then the kernel runs the command specified by the contents | |
281 | of the "release_agent" file in that hierarchy's root directory, | |
282 | supplying the pathname (relative to the mount point of the cgroup | |
283 | file system) of the abandoned cgroup. This enables automatic | |
284 | removal of abandoned cgroups. The default value of | |
285 | notify_on_release in the root cgroup at system boot is disabled | |
286 | (0). The default value of other cgroups at creation is the current | |
287 | value of their parents notify_on_release setting. The default value of | |
288 | a cgroup hierarchy's release_agent path is empty. | |
289 | ||
290 | 1.5 How do I use cgroups ? | |
291 | -------------------------- | |
292 | ||
293 | To start a new job that is to be contained within a cgroup, using | |
294 | the "cpuset" cgroup subsystem, the steps are something like: | |
295 | ||
296 | 1) mkdir /dev/cgroup | |
297 | 2) mount -t cgroup -ocpuset cpuset /dev/cgroup | |
298 | 3) Create the new cgroup by doing mkdir's and write's (or echo's) in | |
299 | the /dev/cgroup virtual file system. | |
300 | 4) Start a task that will be the "founding father" of the new job. | |
301 | 5) Attach that task to the new cgroup by writing its pid to the | |
302 | /dev/cgroup tasks file for that cgroup. | |
303 | 6) fork, exec or clone the job tasks from this founding father task. | |
304 | ||
305 | For example, the following sequence of commands will setup a cgroup | |
306 | named "Charlie", containing just CPUs 2 and 3, and Memory Node 1, | |
307 | and then start a subshell 'sh' in that cgroup: | |
308 | ||
309 | mount -t cgroup cpuset -ocpuset /dev/cgroup | |
310 | cd /dev/cgroup | |
311 | mkdir Charlie | |
312 | cd Charlie | |
313 | /bin/echo 2-3 > cpus | |
314 | /bin/echo 1 > mems | |
315 | /bin/echo $$ > tasks | |
316 | sh | |
317 | # The subshell 'sh' is now running in cgroup Charlie | |
318 | # The next line should display '/Charlie' | |
319 | cat /proc/self/cgroup | |
320 | ||
321 | 2. Usage Examples and Syntax | |
322 | ============================ | |
323 | ||
324 | 2.1 Basic Usage | |
325 | --------------- | |
326 | ||
327 | Creating, modifying, using the cgroups can be done through the cgroup | |
328 | virtual filesystem. | |
329 | ||
330 | To mount a cgroup hierarchy will all available subsystems, type: | |
331 | # mount -t cgroup xxx /dev/cgroup | |
332 | ||
333 | The "xxx" is not interpreted by the cgroup code, but will appear in | |
334 | /proc/mounts so may be any useful identifying string that you like. | |
335 | ||
336 | To mount a cgroup hierarchy with just the cpuset and numtasks | |
337 | subsystems, type: | |
338 | # mount -t cgroup -o cpuset,numtasks hier1 /dev/cgroup | |
339 | ||
340 | To change the set of subsystems bound to a mounted hierarchy, just | |
341 | remount with different options: | |
342 | ||
343 | # mount -o remount,cpuset,ns /dev/cgroup | |
344 | ||
345 | Note that changing the set of subsystems is currently only supported | |
346 | when the hierarchy consists of a single (root) cgroup. Supporting | |
347 | the ability to arbitrarily bind/unbind subsystems from an existing | |
348 | cgroup hierarchy is intended to be implemented in the future. | |
349 | ||
350 | Then under /dev/cgroup you can find a tree that corresponds to the | |
351 | tree of the cgroups in the system. For instance, /dev/cgroup | |
352 | is the cgroup that holds the whole system. | |
353 | ||
354 | If you want to create a new cgroup under /dev/cgroup: | |
355 | # cd /dev/cgroup | |
356 | # mkdir my_cgroup | |
357 | ||
358 | Now you want to do something with this cgroup. | |
359 | # cd my_cgroup | |
360 | ||
361 | In this directory you can find several files: | |
362 | # ls | |
363 | notify_on_release release_agent tasks | |
364 | (plus whatever files are added by the attached subsystems) | |
365 | ||
366 | Now attach your shell to this cgroup: | |
367 | # /bin/echo $$ > tasks | |
368 | ||
369 | You can also create cgroups inside your cgroup by using mkdir in this | |
370 | directory. | |
371 | # mkdir my_sub_cs | |
372 | ||
373 | To remove a cgroup, just use rmdir: | |
374 | # rmdir my_sub_cs | |
375 | ||
376 | This will fail if the cgroup is in use (has cgroups inside, or | |
377 | has processes attached, or is held alive by other subsystem-specific | |
378 | reference). | |
379 | ||
380 | 2.2 Attaching processes | |
381 | ----------------------- | |
382 | ||
383 | # /bin/echo PID > tasks | |
384 | ||
385 | Note that it is PID, not PIDs. You can only attach ONE task at a time. | |
386 | If you have several tasks to attach, you have to do it one after another: | |
387 | ||
388 | # /bin/echo PID1 > tasks | |
389 | # /bin/echo PID2 > tasks | |
390 | ... | |
391 | # /bin/echo PIDn > tasks | |
392 | ||
393 | 3. Kernel API | |
394 | ============= | |
395 | ||
396 | 3.1 Overview | |
397 | ------------ | |
398 | ||
399 | Each kernel subsystem that wants to hook into the generic cgroup | |
400 | system needs to create a cgroup_subsys object. This contains | |
401 | various methods, which are callbacks from the cgroup system, along | |
402 | with a subsystem id which will be assigned by the cgroup system. | |
403 | ||
404 | Other fields in the cgroup_subsys object include: | |
405 | ||
406 | - subsys_id: a unique array index for the subsystem, indicating which | |
407 | entry in cgroup->subsys[] this subsystem should be | |
408 | managing. Initialized by cgroup_register_subsys(); prior to this | |
409 | it should be initialized to -1 | |
410 | ||
411 | - hierarchy: an index indicating which hierarchy, if any, this | |
412 | subsystem is currently attached to. If this is -1, then the | |
413 | subsystem is not attached to any hierarchy, and all tasks should be | |
414 | considered to be members of the subsystem's top_cgroup. It should | |
415 | be initialized to -1. | |
416 | ||
417 | - name: should be initialized to a unique subsystem name prior to | |
418 | calling cgroup_register_subsystem. Should be no longer than | |
419 | MAX_CGROUP_TYPE_NAMELEN | |
420 | ||
421 | Each cgroup object created by the system has an array of pointers, | |
422 | indexed by subsystem id; this pointer is entirely managed by the | |
423 | subsystem; the generic cgroup code will never touch this pointer. | |
424 | ||
425 | 3.2 Synchronization | |
426 | ------------------- | |
427 | ||
428 | There is a global mutex, cgroup_mutex, used by the cgroup | |
429 | system. This should be taken by anything that wants to modify a | |
430 | cgroup. It may also be taken to prevent cgroups from being | |
431 | modified, but more specific locks may be more appropriate in that | |
432 | situation. | |
433 | ||
434 | See kernel/cgroup.c for more details. | |
435 | ||
436 | Subsystems can take/release the cgroup_mutex via the functions | |
437 | cgroup_lock()/cgroup_unlock(), and can | |
438 | take/release the callback_mutex via the functions | |
439 | cgroup_lock()/cgroup_unlock(). | |
440 | ||
441 | Accessing a task's cgroup pointer may be done in the following ways: | |
442 | - while holding cgroup_mutex | |
443 | - while holding the task's alloc_lock (via task_lock()) | |
444 | - inside an rcu_read_lock() section via rcu_dereference() | |
445 | ||
446 | 3.3 Subsystem API | |
447 | -------------------------- | |
448 | ||
449 | Each subsystem should: | |
450 | ||
451 | - add an entry in linux/cgroup_subsys.h | |
452 | - define a cgroup_subsys object called <name>_subsys | |
453 | ||
454 | Each subsystem may export the following methods. The only mandatory | |
455 | methods are create/destroy. Any others that are null are presumed to | |
456 | be successful no-ops. | |
457 | ||
458 | struct cgroup_subsys_state *create(struct cgroup *cont) | |
8dc4f3e1 | 459 | (cgroup_mutex held by caller) |
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460 | |
461 | Called to create a subsystem state object for a cgroup. The | |
462 | subsystem should allocate its subsystem state object for the passed | |
463 | cgroup, returning a pointer to the new object on success or a | |
464 | negative error code. On success, the subsystem pointer should point to | |
465 | a structure of type cgroup_subsys_state (typically embedded in a | |
466 | larger subsystem-specific object), which will be initialized by the | |
467 | cgroup system. Note that this will be called at initialization to | |
468 | create the root subsystem state for this subsystem; this case can be | |
469 | identified by the passed cgroup object having a NULL parent (since | |
470 | it's the root of the hierarchy) and may be an appropriate place for | |
471 | initialization code. | |
472 | ||
473 | void destroy(struct cgroup *cont) | |
8dc4f3e1 | 474 | (cgroup_mutex held by caller) |
ddbcc7e8 | 475 | |
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476 | The cgroup system is about to destroy the passed cgroup; the subsystem |
477 | should do any necessary cleanup and free its subsystem state | |
478 | object. By the time this method is called, the cgroup has already been | |
479 | unlinked from the file system and from the child list of its parent; | |
480 | cgroup->parent is still valid. (Note - can also be called for a | |
481 | newly-created cgroup if an error occurs after this subsystem's | |
482 | create() method has been called for the new cgroup). | |
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483 | |
484 | int can_attach(struct cgroup_subsys *ss, struct cgroup *cont, | |
485 | struct task_struct *task) | |
8dc4f3e1 | 486 | (cgroup_mutex held by caller) |
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487 | |
488 | Called prior to moving a task into a cgroup; if the subsystem | |
489 | returns an error, this will abort the attach operation. If a NULL | |
490 | task is passed, then a successful result indicates that *any* | |
491 | unspecified task can be moved into the cgroup. Note that this isn't | |
492 | called on a fork. If this method returns 0 (success) then this should | |
493 | remain valid while the caller holds cgroup_mutex. | |
494 | ||
495 | void attach(struct cgroup_subsys *ss, struct cgroup *cont, | |
496 | struct cgroup *old_cont, struct task_struct *task) | |
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497 | |
498 | Called after the task has been attached to the cgroup, to allow any | |
499 | post-attachment activity that requires memory allocations or blocking. | |
500 | ||
501 | void fork(struct cgroup_subsy *ss, struct task_struct *task) | |
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502 | |
503 | Called when a task is forked into a cgroup. Also called during | |
504 | registration for all existing tasks. | |
505 | ||
506 | void exit(struct cgroup_subsys *ss, struct task_struct *task) | |
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507 | |
508 | Called during task exit | |
509 | ||
510 | int populate(struct cgroup_subsys *ss, struct cgroup *cont) | |
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511 | |
512 | Called after creation of a cgroup to allow a subsystem to populate | |
513 | the cgroup directory with file entries. The subsystem should make | |
514 | calls to cgroup_add_file() with objects of type cftype (see | |
515 | include/linux/cgroup.h for details). Note that although this | |
516 | method can return an error code, the error code is currently not | |
517 | always handled well. | |
518 | ||
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519 | void post_clone(struct cgroup_subsys *ss, struct cgroup *cont) |
520 | ||
521 | Called at the end of cgroup_clone() to do any paramater | |
522 | initialization which might be required before a task could attach. For | |
523 | example in cpusets, no task may attach before 'cpus' and 'mems' are set | |
524 | up. | |
525 | ||
ddbcc7e8 | 526 | void bind(struct cgroup_subsys *ss, struct cgroup *root) |
8dc4f3e1 | 527 | (cgroup_mutex held by caller) |
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528 | |
529 | Called when a cgroup subsystem is rebound to a different hierarchy | |
530 | and root cgroup. Currently this will only involve movement between | |
531 | the default hierarchy (which never has sub-cgroups) and a hierarchy | |
532 | that is being created/destroyed (and hence has no sub-cgroups). | |
533 | ||
534 | 4. Questions | |
535 | ============ | |
536 | ||
537 | Q: what's up with this '/bin/echo' ? | |
538 | A: bash's builtin 'echo' command does not check calls to write() against | |
539 | errors. If you use it in the cgroup file system, you won't be | |
540 | able to tell whether a command succeeded or failed. | |
541 | ||
542 | Q: When I attach processes, only the first of the line gets really attached ! | |
543 | A: We can only return one error code per call to write(). So you should also | |
544 | put only ONE pid. | |
545 |