Based on kernel version 4.10.8. Page generated on 2017-04-01 14:44 EST.
1 CONFIG_RCU_TRACE debugfs Files and Formats 2 3 4 The rcutree and rcutiny implementations of RCU provide debugfs trace 5 output that summarizes counters and state. This information is useful for 6 debugging RCU itself, and can sometimes also help to debug abuses of RCU. 7 The following sections describe the debugfs files and formats, first 8 for rcutree and next for rcutiny. 9 10 11 CONFIG_TREE_RCU and CONFIG_PREEMPT_RCU debugfs Files and Formats 12 13 These implementations of RCU provide several debugfs directories under the 14 top-level directory "rcu": 15 16 rcu/rcu_bh 17 rcu/rcu_preempt 18 rcu/rcu_sched 19 20 Each directory contains files for the corresponding flavor of RCU. 21 Note that rcu/rcu_preempt is only present for CONFIG_PREEMPT_RCU. 22 For CONFIG_TREE_RCU, the RCU flavor maps onto the RCU-sched flavor, 23 so that activity for both appears in rcu/rcu_sched. 24 25 In addition, the following file appears in the top-level directory: 26 rcu/rcutorture. This file displays rcutorture test progress. The output 27 of "cat rcu/rcutorture" looks as follows: 28 29 rcutorture test sequence: 0 (test in progress) 30 rcutorture update version number: 615 31 32 The first line shows the number of rcutorture tests that have completed 33 since boot. If a test is currently running, the "(test in progress)" 34 string will appear as shown above. The second line shows the number of 35 update cycles that the current test has started, or zero if there is 36 no test in progress. 37 38 39 Within each flavor directory (rcu/rcu_bh, rcu/rcu_sched, and possibly 40 also rcu/rcu_preempt) the following files will be present: 41 42 rcudata: 43 Displays fields in struct rcu_data. 44 rcuexp: 45 Displays statistics for expedited grace periods. 46 rcugp: 47 Displays grace-period counters. 48 rcuhier: 49 Displays the struct rcu_node hierarchy. 50 rcu_pending: 51 Displays counts of the reasons rcu_pending() decided that RCU had 52 work to do. 53 rcuboost: 54 Displays RCU boosting statistics. Only present if 55 CONFIG_RCU_BOOST=y. 56 57 The output of "cat rcu/rcu_preempt/rcudata" looks as follows: 58 59 0!c=30455 g=30456 cnq=1/0:1 dt=126535/140000000000000/0 df=2002 of=4 ql=0/0 qs=N... b=10 ci=74572 nci=0 co=1131 ca=716 60 1!c=30719 g=30720 cnq=1/0:0 dt=132007/140000000000000/0 df=1874 of=10 ql=0/0 qs=N... b=10 ci=123209 nci=0 co=685 ca=982 61 2!c=30150 g=30151 cnq=1/1:1 dt=138537/140000000000000/0 df=1707 of=8 ql=0/0 qs=N... b=10 ci=80132 nci=0 co=1328 ca=1458 62 3 c=31249 g=31250 cnq=1/1:0 dt=107255/140000000000000/0 df=1749 of=6 ql=0/450 qs=NRW. b=10 ci=151700 nci=0 co=509 ca=622 63 4!c=29502 g=29503 cnq=1/0:1 dt=83647/140000000000000/0 df=965 of=5 ql=0/0 qs=N... b=10 ci=65643 nci=0 co=1373 ca=1521 64 5 c=31201 g=31202 cnq=1/0:1 dt=70422/0/0 df=535 of=7 ql=0/0 qs=.... b=10 ci=58500 nci=0 co=764 ca=698 65 6!c=30253 g=30254 cnq=1/0:1 dt=95363/140000000000000/0 df=780 of=5 ql=0/0 qs=N... b=10 ci=100607 nci=0 co=1414 ca=1353 66 7 c=31178 g=31178 cnq=1/0:0 dt=91536/0/0 df=547 of=4 ql=0/0 qs=.... b=10 ci=109819 nci=0 co=1115 ca=969 67 68 This file has one line per CPU, or eight for this 8-CPU system. 69 The fields are as follows: 70 71 o The number at the beginning of each line is the CPU number. 72 CPUs numbers followed by an exclamation mark are offline, 73 but have been online at least once since boot. There will be 74 no output for CPUs that have never been online, which can be 75 a good thing in the surprisingly common case where NR_CPUS is 76 substantially larger than the number of actual CPUs. 77 78 o "c" is the count of grace periods that this CPU believes have 79 completed. Offlined CPUs and CPUs in dynticks idle mode may lag 80 quite a ways behind, for example, CPU 4 under "rcu_sched" above, 81 which has been offline through 16 RCU grace periods. It is not 82 unusual to see offline CPUs lagging by thousands of grace periods. 83 Note that although the grace-period number is an unsigned long, 84 it is printed out as a signed long to allow more human-friendly 85 representation near boot time. 86 87 o "g" is the count of grace periods that this CPU believes have 88 started. Again, offlined CPUs and CPUs in dynticks idle mode 89 may lag behind. If the "c" and "g" values are equal, this CPU 90 has already reported a quiescent state for the last RCU grace 91 period that it is aware of, otherwise, the CPU believes that it 92 owes RCU a quiescent state. 93 94 o "pq" indicates that this CPU has passed through a quiescent state 95 for the current grace period. It is possible for "pq" to be 96 "1" and "c" different than "g", which indicates that although 97 the CPU has passed through a quiescent state, either (1) this 98 CPU has not yet reported that fact, (2) some other CPU has not 99 yet reported for this grace period, or (3) both. 100 101 o "qp" indicates that RCU still expects a quiescent state from 102 this CPU. Offlined CPUs and CPUs in dyntick idle mode might 103 well have qp=1, which is OK: RCU is still ignoring them. 104 105 o "dt" is the current value of the dyntick counter that is incremented 106 when entering or leaving idle, either due to a context switch or 107 due to an interrupt. This number is even if the CPU is in idle 108 from RCU's viewpoint and odd otherwise. The number after the 109 first "/" is the interrupt nesting depth when in idle state, 110 or a large number added to the interrupt-nesting depth when 111 running a non-idle task. Some architectures do not accurately 112 count interrupt nesting when running in non-idle kernel context, 113 which can result in interesting anomalies such as negative 114 interrupt-nesting levels. The number after the second "/" 115 is the NMI nesting depth. 116 117 o "df" is the number of times that some other CPU has forced a 118 quiescent state on behalf of this CPU due to this CPU being in 119 idle state. 120 121 o "of" is the number of times that some other CPU has forced a 122 quiescent state on behalf of this CPU due to this CPU being 123 offline. In a perfect world, this might never happen, but it 124 turns out that offlining and onlining a CPU can take several grace 125 periods, and so there is likely to be an extended period of time 126 when RCU believes that the CPU is online when it really is not. 127 Please note that erring in the other direction (RCU believing a 128 CPU is offline when it is really alive and kicking) is a fatal 129 error, so it makes sense to err conservatively. 130 131 o "ql" is the number of RCU callbacks currently residing on 132 this CPU. The first number is the number of "lazy" callbacks 133 that are known to RCU to only be freeing memory, and the number 134 after the "/" is the total number of callbacks, lazy or not. 135 These counters count callbacks regardless of what phase of 136 grace-period processing that they are in (new, waiting for 137 grace period to start, waiting for grace period to end, ready 138 to invoke). 139 140 o "qs" gives an indication of the state of the callback queue 141 with four characters: 142 143 "N" Indicates that there are callbacks queued that are not 144 ready to be handled by the next grace period, and thus 145 will be handled by the grace period following the next 146 one. 147 148 "R" Indicates that there are callbacks queued that are 149 ready to be handled by the next grace period. 150 151 "W" Indicates that there are callbacks queued that are 152 waiting on the current grace period. 153 154 "D" Indicates that there are callbacks queued that have 155 already been handled by a prior grace period, and are 156 thus waiting to be invoked. Note that callbacks in 157 the process of being invoked are not counted here. 158 Callbacks in the process of being invoked are those 159 that have been removed from the rcu_data structures 160 queues by rcu_do_batch(), but which have not yet been 161 invoked. 162 163 If there are no callbacks in a given one of the above states, 164 the corresponding character is replaced by ".". 165 166 o "b" is the batch limit for this CPU. If more than this number 167 of RCU callbacks is ready to invoke, then the remainder will 168 be deferred. 169 170 o "ci" is the number of RCU callbacks that have been invoked for 171 this CPU. Note that ci+nci+ql is the number of callbacks that have 172 been registered in absence of CPU-hotplug activity. 173 174 o "nci" is the number of RCU callbacks that have been offloaded from 175 this CPU. This will always be zero unless the kernel was built 176 with CONFIG_RCU_NOCB_CPU=y and the "rcu_nocbs=" kernel boot 177 parameter was specified. 178 179 o "co" is the number of RCU callbacks that have been orphaned due to 180 this CPU going offline. These orphaned callbacks have been moved 181 to an arbitrarily chosen online CPU. 182 183 o "ca" is the number of RCU callbacks that have been adopted by this 184 CPU due to other CPUs going offline. Note that ci+co-ca+ql is 185 the number of RCU callbacks registered on this CPU. 186 187 188 Kernels compiled with CONFIG_RCU_BOOST=y display the following from 189 /debug/rcu/rcu_preempt/rcudata: 190 191 0!c=12865 g=12866 cnq=1/0:1 dt=83113/140000000000000/0 df=288 of=11 ql=0/0 qs=N... kt=0/O ktl=944 b=10 ci=60709 nci=0 co=748 ca=871 192 1 c=14407 g=14408 cnq=1/0:0 dt=100679/140000000000000/0 df=378 of=7 ql=0/119 qs=NRW. kt=0/W ktl=9b6 b=10 ci=109740 nci=0 co=589 ca=485 193 2 c=14407 g=14408 cnq=1/0:0 dt=105486/0/0 df=90 of=9 ql=0/89 qs=NRW. kt=0/W ktl=c0c b=10 ci=83113 nci=0 co=533 ca=490 194 3 c=14407 g=14408 cnq=1/0:0 dt=107138/0/0 df=142 of=8 ql=0/188 qs=NRW. kt=0/W ktl=b96 b=10 ci=121114 nci=0 co=426 ca=290 195 4 c=14405 g=14406 cnq=1/0:1 dt=50238/0/0 df=706 of=7 ql=0/0 qs=.... kt=0/W ktl=812 b=10 ci=34929 nci=0 co=643 ca=114 196 5!c=14168 g=14169 cnq=1/0:0 dt=45465/140000000000000/0 df=161 of=11 ql=0/0 qs=N... kt=0/O ktl=b4d b=10 ci=47712 nci=0 co=677 ca=722 197 6 c=14404 g=14405 cnq=1/0:0 dt=59454/0/0 df=94 of=6 ql=0/0 qs=.... kt=0/W ktl=e57 b=10 ci=55597 nci=0 co=701 ca=811 198 7 c=14407 g=14408 cnq=1/0:1 dt=68850/0/0 df=31 of=8 ql=0/0 qs=.... kt=0/W ktl=14bd b=10 ci=77475 nci=0 co=508 ca=1042 199 200 This is similar to the output discussed above, but contains the following 201 additional fields: 202 203 o "kt" is the per-CPU kernel-thread state. The digit preceding 204 the first slash is zero if there is no work pending and 1 205 otherwise. The character between the first pair of slashes is 206 as follows: 207 208 "S" The kernel thread is stopped, in other words, all 209 CPUs corresponding to this rcu_node structure are 210 offline. 211 212 "R" The kernel thread is running. 213 214 "W" The kernel thread is waiting because there is no work 215 for it to do. 216 217 "O" The kernel thread is waiting because it has been 218 forced off of its designated CPU or because its 219 ->cpus_allowed mask permits it to run on other than 220 its designated CPU. 221 222 "Y" The kernel thread is yielding to avoid hogging CPU. 223 224 "?" Unknown value, indicates a bug. 225 226 The number after the final slash is the CPU that the kthread 227 is actually running on. 228 229 This field is displayed only for CONFIG_RCU_BOOST kernels. 230 231 o "ktl" is the low-order 16 bits (in hexadecimal) of the count of 232 the number of times that this CPU's per-CPU kthread has gone 233 through its loop servicing invoke_rcu_cpu_kthread() requests. 234 235 This field is displayed only for CONFIG_RCU_BOOST kernels. 236 237 238 The output of "cat rcu/rcu_preempt/rcuexp" looks as follows: 239 240 s=21872 wd1=0 wd2=0 wd3=5 n=0 enq=0 sc=21872 241 242 These fields are as follows: 243 244 o "s" is the sequence number, with an odd number indicating that 245 an expedited grace period is in progress. 246 247 o "wd1", "wd2", and "wd3" are the number of times that an attempt 248 to start an expedited grace period found that someone else had 249 completed an expedited grace period that satisfies the attempted 250 request. "Our work is done." 251 252 o "n" is number of times that a concurrent CPU-hotplug operation 253 forced a fallback to a normal grace period. 254 255 o "enq" is the number of quiescent states still outstanding. 256 257 o "sc" is the number of times that the attempt to start a 258 new expedited grace period succeeded. 259 260 261 The output of "cat rcu/rcu_preempt/rcugp" looks as follows: 262 263 completed=31249 gpnum=31250 age=1 max=18 264 265 These fields are taken from the rcu_state structure, and are as follows: 266 267 o "completed" is the number of grace periods that have completed. 268 It is comparable to the "c" field from rcu/rcudata in that a 269 CPU whose "c" field matches the value of "completed" is aware 270 that the corresponding RCU grace period has completed. 271 272 o "gpnum" is the number of grace periods that have started. It is 273 similarly comparable to the "g" field from rcu/rcudata in that 274 a CPU whose "g" field matches the value of "gpnum" is aware that 275 the corresponding RCU grace period has started. 276 277 If these two fields are equal, then there is no grace period 278 in progress, in other words, RCU is idle. On the other hand, 279 if the two fields differ (as they are above), then an RCU grace 280 period is in progress. 281 282 o "age" is the number of jiffies that the current grace period 283 has extended for, or zero if there is no grace period currently 284 in effect. 285 286 o "max" is the age in jiffies of the longest-duration grace period 287 thus far. 288 289 The output of "cat rcu/rcu_preempt/rcuhier" looks as follows: 290 291 c=14407 g=14408 s=0 jfq=2 j=c863 nfqs=12040/nfqsng=0(12040) fqlh=1051 oqlen=0/0 292 3/3 ..>. 0:7 ^0 293 e/e ..>. 0:3 ^0 d/d ..>. 4:7 ^1 294 295 The fields are as follows: 296 297 o "c" is exactly the same as "completed" under rcu/rcu_preempt/rcugp. 298 299 o "g" is exactly the same as "gpnum" under rcu/rcu_preempt/rcugp. 300 301 o "s" is the current state of the force_quiescent_state() 302 state machine. 303 304 o "jfq" is the number of jiffies remaining for this grace period 305 before force_quiescent_state() is invoked to help push things 306 along. Note that CPUs in idle mode throughout the grace period 307 will not report on their own, but rather must be check by some 308 other CPU via force_quiescent_state(). 309 310 o "j" is the low-order four hex digits of the jiffies counter. 311 Yes, Paul did run into a number of problems that turned out to 312 be due to the jiffies counter no longer counting. Why do you ask? 313 314 o "nfqs" is the number of calls to force_quiescent_state() since 315 boot. 316 317 o "nfqsng" is the number of useless calls to force_quiescent_state(), 318 where there wasn't actually a grace period active. This can 319 no longer happen due to grace-period processing being pushed 320 into a kthread. The number in parentheses is the difference 321 between "nfqs" and "nfqsng", or the number of times that 322 force_quiescent_state() actually did some real work. 323 324 o "fqlh" is the number of calls to force_quiescent_state() that 325 exited immediately (without even being counted in nfqs above) 326 due to contention on ->fqslock. 327 328 o Each element of the form "3/3 ..>. 0:7 ^0" represents one rcu_node 329 structure. Each line represents one level of the hierarchy, 330 from root to leaves. It is best to think of the rcu_data 331 structures as forming yet another level after the leaves. 332 Note that there might be either one, two, three, or even four 333 levels of rcu_node structures, depending on the relationship 334 between CONFIG_RCU_FANOUT, CONFIG_RCU_FANOUT_LEAF (possibly 335 adjusted using the rcu_fanout_leaf kernel boot parameter), and 336 CONFIG_NR_CPUS (possibly adjusted using the nr_cpu_ids count of 337 possible CPUs for the booting hardware). 338 339 o The numbers separated by the "/" are the qsmask followed 340 by the qsmaskinit. The qsmask will have one bit 341 set for each entity in the next lower level that has 342 not yet checked in for the current grace period ("e" 343 indicating CPUs 5, 6, and 7 in the example above). 344 The qsmaskinit will have one bit for each entity that is 345 currently expected to check in during each grace period. 346 The value of qsmaskinit is assigned to that of qsmask 347 at the beginning of each grace period. 348 349 o The characters separated by the ">" indicate the state 350 of the blocked-tasks lists. A "G" preceding the ">" 351 indicates that at least one task blocked in an RCU 352 read-side critical section blocks the current grace 353 period, while a "E" preceding the ">" indicates that 354 at least one task blocked in an RCU read-side critical 355 section blocks the current expedited grace period. 356 A "T" character following the ">" indicates that at 357 least one task is blocked within an RCU read-side 358 critical section, regardless of whether any current 359 grace period (expedited or normal) is inconvenienced. 360 A "." character appears if the corresponding condition 361 does not hold, so that "..>." indicates that no tasks 362 are blocked. In contrast, "GE>T" indicates maximal 363 inconvenience from blocked tasks. CONFIG_TREE_RCU 364 builds of the kernel will always show "..>.". 365 366 o The numbers separated by the ":" are the range of CPUs 367 served by this struct rcu_node. This can be helpful 368 in working out how the hierarchy is wired together. 369 370 For example, the example rcu_node structure shown above 371 has "0:7", indicating that it covers CPUs 0 through 7. 372 373 o The number after the "^" indicates the bit in the 374 next higher level rcu_node structure that this rcu_node 375 structure corresponds to. For example, the "d/d ..>. 4:7 376 ^1" has a "1" in this position, indicating that it 377 corresponds to the "1" bit in the "3" shown in the 378 "3/3 ..>. 0:7 ^0" entry on the next level up. 379 380 381 The output of "cat rcu/rcu_sched/rcu_pending" looks as follows: 382 383 0!np=26111 qsp=29 rpq=5386 cbr=1 cng=570 gpc=3674 gps=577 nn=15903 ndw=0 384 1!np=28913 qsp=35 rpq=6097 cbr=1 cng=448 gpc=3700 gps=554 nn=18113 ndw=0 385 2!np=32740 qsp=37 rpq=6202 cbr=0 cng=476 gpc=4627 gps=546 nn=20889 ndw=0 386 3 np=23679 qsp=22 rpq=5044 cbr=1 cng=415 gpc=3403 gps=347 nn=14469 ndw=0 387 4!np=30714 qsp=4 rpq=5574 cbr=0 cng=528 gpc=3931 gps=639 nn=20042 ndw=0 388 5 np=28910 qsp=2 rpq=5246 cbr=0 cng=428 gpc=4105 gps=709 nn=18422 ndw=0 389 6!np=38648 qsp=5 rpq=7076 cbr=0 cng=840 gpc=4072 gps=961 nn=25699 ndw=0 390 7 np=37275 qsp=2 rpq=6873 cbr=0 cng=868 gpc=3416 gps=971 nn=25147 ndw=0 391 392 The fields are as follows: 393 394 o The leading number is the CPU number, with "!" indicating 395 an offline CPU. 396 397 o "np" is the number of times that __rcu_pending() has been invoked 398 for the corresponding flavor of RCU. 399 400 o "qsp" is the number of times that the RCU was waiting for a 401 quiescent state from this CPU. 402 403 o "rpq" is the number of times that the CPU had passed through 404 a quiescent state, but not yet reported it to RCU. 405 406 o "cbr" is the number of times that this CPU had RCU callbacks 407 that had passed through a grace period, and were thus ready 408 to be invoked. 409 410 o "cng" is the number of times that this CPU needed another 411 grace period while RCU was idle. 412 413 o "gpc" is the number of times that an old grace period had 414 completed, but this CPU was not yet aware of it. 415 416 o "gps" is the number of times that a new grace period had started, 417 but this CPU was not yet aware of it. 418 419 o "ndw" is the number of times that a wakeup of an rcuo 420 callback-offload kthread had to be deferred in order to avoid 421 deadlock. 422 423 o "nn" is the number of times that this CPU needed nothing. 424 425 426 The output of "cat rcu/rcuboost" looks as follows: 427 428 0:3 tasks=.... kt=W ntb=0 neb=0 nnb=0 j=c864 bt=c894 429 balk: nt=0 egt=4695 bt=0 nb=0 ny=56 nos=0 430 4:7 tasks=.... kt=W ntb=0 neb=0 nnb=0 j=c864 bt=c894 431 balk: nt=0 egt=6541 bt=0 nb=0 ny=126 nos=0 432 433 This information is output only for rcu_preempt. Each two-line entry 434 corresponds to a leaf rcu_node structure. The fields are as follows: 435 436 o "n:m" is the CPU-number range for the corresponding two-line 437 entry. In the sample output above, the first entry covers 438 CPUs zero through three and the second entry covers CPUs four 439 through seven. 440 441 o "tasks=TNEB" gives the state of the various segments of the 442 rnp->blocked_tasks list: 443 444 "T" This indicates that there are some tasks that blocked 445 while running on one of the corresponding CPUs while 446 in an RCU read-side critical section. 447 448 "N" This indicates that some of the blocked tasks are preventing 449 the current normal (non-expedited) grace period from 450 completing. 451 452 "E" This indicates that some of the blocked tasks are preventing 453 the current expedited grace period from completing. 454 455 "B" This indicates that some of the blocked tasks are in 456 need of RCU priority boosting. 457 458 Each character is replaced with "." if the corresponding 459 condition does not hold. 460 461 o "kt" is the state of the RCU priority-boosting kernel 462 thread associated with the corresponding rcu_node structure. 463 The state can be one of the following: 464 465 "S" The kernel thread is stopped, in other words, all 466 CPUs corresponding to this rcu_node structure are 467 offline. 468 469 "R" The kernel thread is running. 470 471 "W" The kernel thread is waiting because there is no work 472 for it to do. 473 474 "Y" The kernel thread is yielding to avoid hogging CPU. 475 476 "?" Unknown value, indicates a bug. 477 478 o "ntb" is the number of tasks boosted. 479 480 o "neb" is the number of tasks boosted in order to complete an 481 expedited grace period. 482 483 o "nnb" is the number of tasks boosted in order to complete a 484 normal (non-expedited) grace period. When boosting a task 485 that was blocking both an expedited and a normal grace period, 486 it is counted against the expedited total above. 487 488 o "j" is the low-order 16 bits of the jiffies counter in 489 hexadecimal. 490 491 o "bt" is the low-order 16 bits of the value that the jiffies 492 counter will have when we next start boosting, assuming that 493 the current grace period does not end beforehand. This is 494 also in hexadecimal. 495 496 o "balk: nt" counts the number of times we didn't boost (in 497 other words, we balked) even though it was time to boost because 498 there were no blocked tasks to boost. This situation occurs 499 when there is one blocked task on one rcu_node structure and 500 none on some other rcu_node structure. 501 502 o "egt" counts the number of times we balked because although 503 there were blocked tasks, none of them were blocking the 504 current grace period, whether expedited or otherwise. 505 506 o "bt" counts the number of times we balked because boosting 507 had already been initiated for the current grace period. 508 509 o "nb" counts the number of times we balked because there 510 was at least one task blocking the current non-expedited grace 511 period that never had blocked. If it is already running, it 512 just won't help to boost its priority! 513 514 o "ny" counts the number of times we balked because it was 515 not yet time to start boosting. 516 517 o "nos" counts the number of times we balked for other 518 reasons, e.g., the grace period ended first. 519 520 521 CONFIG_TINY_RCU debugfs Files and Formats 522 523 These implementations of RCU provides a single debugfs file under the 524 top-level directory RCU, namely rcu/rcudata, which displays fields in 525 rcu_bh_ctrlblk and rcu_sched_ctrlblk. 526 527 The output of "cat rcu/rcudata" is as follows: 528 529 rcu_sched: qlen: 0 530 rcu_bh: qlen: 0 531 532 This is split into rcu_sched and rcu_bh sections. The field is as 533 follows: 534 535 o "qlen" is the number of RCU callbacks currently waiting either 536 for an RCU grace period or waiting to be invoked. This is the 537 only field present for rcu_sched and rcu_bh, due to the 538 short-circuiting of grace period in those two cases.