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Linux/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.rst

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  1 =================================================
  2 A Tour Through TREE_RCU's Expedited Grace Periods
  3 =================================================
  4 
  5 Introduction
  6 ============
  7 
  8 This document describes RCU's expedited grace periods.
  9 Unlike RCU's normal grace periods, which accept long latencies to attain
 10 high efficiency and minimal disturbance, expedited grace periods accept
 11 lower efficiency and significant disturbance to attain shorter latencies.
 12 
 13 There are two flavors of RCU (RCU-preempt and RCU-sched), with an earlier
 14 third RCU-bh flavor having been implemented in terms of the other two.
 15 Each of the two implementations is covered in its own section.
 16 
 17 Expedited Grace Period Design
 18 =============================
 19 
 20 The expedited RCU grace periods cannot be accused of being subtle,
 21 given that they for all intents and purposes hammer every CPU that
 22 has not yet provided a quiescent state for the current expedited
 23 grace period.
 24 The one saving grace is that the hammer has grown a bit smaller
 25 over time:  The old call to ``try_stop_cpus()`` has been
 26 replaced with a set of calls to ``smp_call_function_single()``,
 27 each of which results in an IPI to the target CPU.
 28 The corresponding handler function checks the CPU's state, motivating
 29 a faster quiescent state where possible, and triggering a report
 30 of that quiescent state.
 31 As always for RCU, once everything has spent some time in a quiescent
 32 state, the expedited grace period has completed.
 33 
 34 The details of the ``smp_call_function_single()`` handler's
 35 operation depend on the RCU flavor, as described in the following
 36 sections.
 37 
 38 RCU-preempt Expedited Grace Periods
 39 ===================================
 40 
 41 ``CONFIG_PREEMPTION=y`` kernels implement RCU-preempt.
 42 The overall flow of the handling of a given CPU by an RCU-preempt
 43 expedited grace period is shown in the following diagram:
 44 
 45 .. kernel-figure:: ExpRCUFlow.svg
 46 
 47 The solid arrows denote direct action, for example, a function call.
 48 The dotted arrows denote indirect action, for example, an IPI
 49 or a state that is reached after some time.
 50 
 51 If a given CPU is offline or idle, ``synchronize_rcu_expedited()``
 52 will ignore it because idle and offline CPUs are already residing
 53 in quiescent states.
 54 Otherwise, the expedited grace period will use
 55 ``smp_call_function_single()`` to send the CPU an IPI, which
 56 is handled by ``rcu_exp_handler()``.
 57 
 58 However, because this is preemptible RCU, ``rcu_exp_handler()``
 59 can check to see if the CPU is currently running in an RCU read-side
 60 critical section.
 61 If not, the handler can immediately report a quiescent state.
 62 Otherwise, it sets flags so that the outermost ``rcu_read_unlock()``
 63 invocation will provide the needed quiescent-state report.
 64 This flag-setting avoids the previous forced preemption of all
 65 CPUs that might have RCU read-side critical sections.
 66 In addition, this flag-setting is done so as to avoid increasing
 67 the overhead of the common-case fastpath through the scheduler.
 68 
 69 Again because this is preemptible RCU, an RCU read-side critical section
 70 can be preempted.
 71 When that happens, RCU will enqueue the task, which will the continue to
 72 block the current expedited grace period until it resumes and finds its
 73 outermost ``rcu_read_unlock()``.
 74 The CPU will report a quiescent state just after enqueuing the task because
 75 the CPU is no longer blocking the grace period.
 76 It is instead the preempted task doing the blocking.
 77 The list of blocked tasks is managed by ``rcu_preempt_ctxt_queue()``,
 78 which is called from ``rcu_preempt_note_context_switch()``, which
 79 in turn is called from ``rcu_note_context_switch()``, which in
 80 turn is called from the scheduler.
 81 
 82 
 83 +-----------------------------------------------------------------------+
 84 | **Quick Quiz**:                                                       |
 85 +-----------------------------------------------------------------------+
 86 | Why not just have the expedited grace period check the state of all   |
 87 | the CPUs? After all, that would avoid all those real-time-unfriendly  |
 88 | IPIs.                                                                 |
 89 +-----------------------------------------------------------------------+
 90 | **Answer**:                                                           |
 91 +-----------------------------------------------------------------------+
 92 | Because we want the RCU read-side critical sections to run fast,      |
 93 | which means no memory barriers. Therefore, it is not possible to      |
 94 | safely check the state from some other CPU. And even if it was        |
 95 | possible to safely check the state, it would still be necessary to    |
 96 | IPI the CPU to safely interact with the upcoming                      |
 97 | ``rcu_read_unlock()`` invocation, which means that the remote state   |
 98 | testing would not help the worst-case latency that real-time          |
 99 | applications care about.                                              |
100 |                                                                       |
101 | One way to prevent your real-time application from getting hit with   |
102 | these IPIs is to build your kernel with ``CONFIG_NO_HZ_FULL=y``. RCU  |
103 | would then perceive the CPU running your application as being idle,   |
104 | and it would be able to safely detect that state without needing to   |
105 | IPI the CPU.                                                          |
106 +-----------------------------------------------------------------------+
107 
108 Please note that this is just the overall flow: Additional complications
109 can arise due to races with CPUs going idle or offline, among other
110 things.
111 
112 RCU-sched Expedited Grace Periods
113 ---------------------------------
114 
115 ``CONFIG_PREEMPTION=n`` kernels implement RCU-sched. The overall flow of
116 the handling of a given CPU by an RCU-sched expedited grace period is
117 shown in the following diagram:
118 
119 .. kernel-figure:: ExpSchedFlow.svg
120 
121 As with RCU-preempt, RCU-sched's ``synchronize_rcu_expedited()`` ignores
122 offline and idle CPUs, again because they are in remotely detectable
123 quiescent states. However, because the ``rcu_read_lock_sched()`` and
124 ``rcu_read_unlock_sched()`` leave no trace of their invocation, in
125 general it is not possible to tell whether or not the current CPU is in
126 an RCU read-side critical section. The best that RCU-sched's
127 ``rcu_exp_handler()`` can do is to check for idle, on the off-chance
128 that the CPU went idle while the IPI was in flight. If the CPU is idle,
129 then ``rcu_exp_handler()`` reports the quiescent state.
130 
131 Otherwise, the handler forces a future context switch by setting the
132 NEED_RESCHED flag of the current task's thread flag and the CPU preempt
133 counter. At the time of the context switch, the CPU reports the
134 quiescent state. Should the CPU go offline first, it will report the
135 quiescent state at that time.
136 
137 Expedited Grace Period and CPU Hotplug
138 --------------------------------------
139 
140 The expedited nature of expedited grace periods require a much tighter
141 interaction with CPU hotplug operations than is required for normal
142 grace periods. In addition, attempting to IPI offline CPUs will result
143 in splats, but failing to IPI online CPUs can result in too-short grace
144 periods. Neither option is acceptable in production kernels.
145 
146 The interaction between expedited grace periods and CPU hotplug
147 operations is carried out at several levels:
148 
149 #. The number of CPUs that have ever been online is tracked by the
150    ``rcu_state`` structure's ``->ncpus`` field. The ``rcu_state``
151    structure's ``->ncpus_snap`` field tracks the number of CPUs that
152    have ever been online at the beginning of an RCU expedited grace
153    period. Note that this number never decreases, at least in the
154    absence of a time machine.
155 #. The identities of the CPUs that have ever been online is tracked by
156    the ``rcu_node`` structure's ``->expmaskinitnext`` field. The
157    ``rcu_node`` structure's ``->expmaskinit`` field tracks the
158    identities of the CPUs that were online at least once at the
159    beginning of the most recent RCU expedited grace period. The
160    ``rcu_state`` structure's ``->ncpus`` and ``->ncpus_snap`` fields are
161    used to detect when new CPUs have come online for the first time,
162    that is, when the ``rcu_node`` structure's ``->expmaskinitnext``
163    field has changed since the beginning of the last RCU expedited grace
164    period, which triggers an update of each ``rcu_node`` structure's
165    ``->expmaskinit`` field from its ``->expmaskinitnext`` field.
166 #. Each ``rcu_node`` structure's ``->expmaskinit`` field is used to
167    initialize that structure's ``->expmask`` at the beginning of each
168    RCU expedited grace period. This means that only those CPUs that have
169    been online at least once will be considered for a given grace
170    period.
171 #. Any CPU that goes offline will clear its bit in its leaf ``rcu_node``
172    structure's ``->qsmaskinitnext`` field, so any CPU with that bit
173    clear can safely be ignored. However, it is possible for a CPU coming
174    online or going offline to have this bit set for some time while
175    ``cpu_online`` returns ``false``.
176 #. For each non-idle CPU that RCU believes is currently online, the
177    grace period invokes ``smp_call_function_single()``. If this
178    succeeds, the CPU was fully online. Failure indicates that the CPU is
179    in the process of coming online or going offline, in which case it is
180    necessary to wait for a short time period and try again. The purpose
181    of this wait (or series of waits, as the case may be) is to permit a
182    concurrent CPU-hotplug operation to complete.
183 #. In the case of RCU-sched, one of the last acts of an outgoing CPU is
184    to invoke ``rcutree_report_cpu_dead()``, which reports a quiescent state for
185    that CPU. However, this is likely paranoia-induced redundancy.
186 
187 +-----------------------------------------------------------------------+
188 | **Quick Quiz**:                                                       |
189 +-----------------------------------------------------------------------+
190 | Why all the dancing around with multiple counters and masks tracking  |
191 | CPUs that were once online? Why not just have a single set of masks   |
192 | tracking the currently online CPUs and be done with it?               |
193 +-----------------------------------------------------------------------+
194 | **Answer**:                                                           |
195 +-----------------------------------------------------------------------+
196 | Maintaining single set of masks tracking the online CPUs *sounds*     |
197 | easier, at least until you try working out all the race conditions    |
198 | between grace-period initialization and CPU-hotplug operations. For   |
199 | example, suppose initialization is progressing down the tree while a  |
200 | CPU-offline operation is progressing up the tree. This situation can  |
201 | result in bits set at the top of the tree that have no counterparts   |
202 | at the bottom of the tree. Those bits will never be cleared, which    |
203 | will result in grace-period hangs. In short, that way lies madness,   |
204 | to say nothing of a great many bugs, hangs, and deadlocks.            |
205 | In contrast, the current multi-mask multi-counter scheme ensures that |
206 | grace-period initialization will always see consistent masks up and   |
207 | down the tree, which brings significant simplifications over the      |
208 | single-mask method.                                                   |
209 |                                                                       |
210 | This is an instance of `deferring work in order to avoid              |
211 | synchronization <http://www.cs.columbia.edu/~library/TR-repository/re |
212 | ports/reports-1992/cucs-039-92.ps.gz>`__.                             |
213 | Lazily recording CPU-hotplug events at the beginning of the next      |
214 | grace period greatly simplifies maintenance of the CPU-tracking       |
215 | bitmasks in the ``rcu_node`` tree.                                    |
216 +-----------------------------------------------------------------------+
217 
218 Expedited Grace Period Refinements
219 ----------------------------------
220 
221 Idle-CPU Checks
222 ~~~~~~~~~~~~~~~
223 
224 Each expedited grace period checks for idle CPUs when initially forming
225 the mask of CPUs to be IPIed and again just before IPIing a CPU (both
226 checks are carried out by ``sync_rcu_exp_select_cpus()``). If the CPU is
227 idle at any time between those two times, the CPU will not be IPIed.
228 Instead, the task pushing the grace period forward will include the idle
229 CPUs in the mask passed to ``rcu_report_exp_cpu_mult()``.
230 
231 For RCU-sched, there is an additional check: If the IPI has interrupted
232 the idle loop, then ``rcu_exp_handler()`` invokes
233 ``rcu_report_exp_rdp()`` to report the corresponding quiescent state.
234 
235 For RCU-preempt, there is no specific check for idle in the IPI handler
236 (``rcu_exp_handler()``), but because RCU read-side critical sections are
237 not permitted within the idle loop, if ``rcu_exp_handler()`` sees that
238 the CPU is within RCU read-side critical section, the CPU cannot
239 possibly be idle. Otherwise, ``rcu_exp_handler()`` invokes
240 ``rcu_report_exp_rdp()`` to report the corresponding quiescent state,
241 regardless of whether or not that quiescent state was due to the CPU
242 being idle.
243 
244 In summary, RCU expedited grace periods check for idle when building the
245 bitmask of CPUs that must be IPIed, just before sending each IPI, and
246 (either explicitly or implicitly) within the IPI handler.
247 
248 Batching via Sequence Counter
249 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
250 
251 If each grace-period request was carried out separately, expedited grace
252 periods would have abysmal scalability and problematic high-load
253 characteristics. Because each grace-period operation can serve an
254 unlimited number of updates, it is important to *batch* requests, so
255 that a single expedited grace-period operation will cover all requests
256 in the corresponding batch.
257 
258 This batching is controlled by a sequence counter named
259 ``->expedited_sequence`` in the ``rcu_state`` structure. This counter
260 has an odd value when there is an expedited grace period in progress and
261 an even value otherwise, so that dividing the counter value by two gives
262 the number of completed grace periods. During any given update request,
263 the counter must transition from even to odd and then back to even, thus
264 indicating that a grace period has elapsed. Therefore, if the initial
265 value of the counter is ``s``, the updater must wait until the counter
266 reaches at least the value ``(s+3)&~0x1``. This counter is managed by
267 the following access functions:
268 
269 #. ``rcu_exp_gp_seq_start()``, which marks the start of an expedited
270    grace period.
271 #. ``rcu_exp_gp_seq_end()``, which marks the end of an expedited grace
272    period.
273 #. ``rcu_exp_gp_seq_snap()``, which obtains a snapshot of the counter.
274 #. ``rcu_exp_gp_seq_done()``, which returns ``true`` if a full expedited
275    grace period has elapsed since the corresponding call to
276    ``rcu_exp_gp_seq_snap()``.
277 
278 Again, only one request in a given batch need actually carry out a
279 grace-period operation, which means there must be an efficient way to
280 identify which of many concurrent requests will initiate the grace
281 period, and that there be an efficient way for the remaining requests to
282 wait for that grace period to complete. However, that is the topic of
283 the next section.
284 
285 Funnel Locking and Wait/Wakeup
286 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
287 
288 The natural way to sort out which of a batch of updaters will initiate
289 the expedited grace period is to use the ``rcu_node`` combining tree, as
290 implemented by the ``exp_funnel_lock()`` function. The first updater
291 corresponding to a given grace period arriving at a given ``rcu_node``
292 structure records its desired grace-period sequence number in the
293 ``->exp_seq_rq`` field and moves up to the next level in the tree.
294 Otherwise, if the ``->exp_seq_rq`` field already contains the sequence
295 number for the desired grace period or some later one, the updater
296 blocks on one of four wait queues in the ``->exp_wq[]`` array, using the
297 second-from-bottom and third-from bottom bits as an index. An
298 ``->exp_lock`` field in the ``rcu_node`` structure synchronizes access
299 to these fields.
300 
301 An empty ``rcu_node`` tree is shown in the following diagram, with the
302 white cells representing the ``->exp_seq_rq`` field and the red cells
303 representing the elements of the ``->exp_wq[]`` array.
304 
305 .. kernel-figure:: Funnel0.svg
306 
307 The next diagram shows the situation after the arrival of Task A and
308 Task B at the leftmost and rightmost leaf ``rcu_node`` structures,
309 respectively. The current value of the ``rcu_state`` structure's
310 ``->expedited_sequence`` field is zero, so adding three and clearing the
311 bottom bit results in the value two, which both tasks record in the
312 ``->exp_seq_rq`` field of their respective ``rcu_node`` structures:
313 
314 .. kernel-figure:: Funnel1.svg
315 
316 Each of Tasks A and B will move up to the root ``rcu_node`` structure.
317 Suppose that Task A wins, recording its desired grace-period sequence
318 number and resulting in the state shown below:
319 
320 .. kernel-figure:: Funnel2.svg
321 
322 Task A now advances to initiate a new grace period, while Task B moves
323 up to the root ``rcu_node`` structure, and, seeing that its desired
324 sequence number is already recorded, blocks on ``->exp_wq[1]``.
325 
326 +-----------------------------------------------------------------------+
327 | **Quick Quiz**:                                                       |
328 +-----------------------------------------------------------------------+
329 | Why ``->exp_wq[1]``? Given that the value of these tasks' desired     |
330 | sequence number is two, so shouldn't they instead block on            |
331 | ``->exp_wq[2]``?                                                      |
332 +-----------------------------------------------------------------------+
333 | **Answer**:                                                           |
334 +-----------------------------------------------------------------------+
335 | No.                                                                   |
336 | Recall that the bottom bit of the desired sequence number indicates   |
337 | whether or not a grace period is currently in progress. It is         |
338 | therefore necessary to shift the sequence number right one bit        |
339 | position to obtain the number of the grace period. This results in    |
340 | ``->exp_wq[1]``.                                                      |
341 +-----------------------------------------------------------------------+
342 
343 If Tasks C and D also arrive at this point, they will compute the same
344 desired grace-period sequence number, and see that both leaf
345 ``rcu_node`` structures already have that value recorded. They will
346 therefore block on their respective ``rcu_node`` structures'
347 ``->exp_wq[1]`` fields, as shown below:
348 
349 .. kernel-figure:: Funnel3.svg
350 
351 Task A now acquires the ``rcu_state`` structure's ``->exp_mutex`` and
352 initiates the grace period, which increments ``->expedited_sequence``.
353 Therefore, if Tasks E and F arrive, they will compute a desired sequence
354 number of 4 and will record this value as shown below:
355 
356 .. kernel-figure:: Funnel4.svg
357 
358 Tasks E and F will propagate up the ``rcu_node`` combining tree, with
359 Task F blocking on the root ``rcu_node`` structure and Task E wait for
360 Task A to finish so that it can start the next grace period. The
361 resulting state is as shown below:
362 
363 .. kernel-figure:: Funnel5.svg
364 
365 Once the grace period completes, Task A starts waking up the tasks
366 waiting for this grace period to complete, increments the
367 ``->expedited_sequence``, acquires the ``->exp_wake_mutex`` and then
368 releases the ``->exp_mutex``. This results in the following state:
369 
370 .. kernel-figure:: Funnel6.svg
371 
372 Task E can then acquire ``->exp_mutex`` and increment
373 ``->expedited_sequence`` to the value three. If new tasks G and H arrive
374 and moves up the combining tree at the same time, the state will be as
375 follows:
376 
377 .. kernel-figure:: Funnel7.svg
378 
379 Note that three of the root ``rcu_node`` structure's waitqueues are now
380 occupied. However, at some point, Task A will wake up the tasks blocked
381 on the ``->exp_wq`` waitqueues, resulting in the following state:
382 
383 .. kernel-figure:: Funnel8.svg
384 
385 Execution will continue with Tasks E and H completing their grace
386 periods and carrying out their wakeups.
387 
388 +-----------------------------------------------------------------------+
389 | **Quick Quiz**:                                                       |
390 +-----------------------------------------------------------------------+
391 | What happens if Task A takes so long to do its wakeups that Task E's  |
392 | grace period completes?                                               |
393 +-----------------------------------------------------------------------+
394 | **Answer**:                                                           |
395 +-----------------------------------------------------------------------+
396 | Then Task E will block on the ``->exp_wake_mutex``, which will also   |
397 | prevent it from releasing ``->exp_mutex``, which in turn will prevent |
398 | the next grace period from starting. This last is important in        |
399 | preventing overflow of the ``->exp_wq[]`` array.                      |
400 +-----------------------------------------------------------------------+
401 
402 Use of Workqueues
403 ~~~~~~~~~~~~~~~~~
404 
405 In earlier implementations, the task requesting the expedited grace
406 period also drove it to completion. This straightforward approach had
407 the disadvantage of needing to account for POSIX signals sent to user
408 tasks, so more recent implementations use the Linux kernel's
409 workqueues (see Documentation/core-api/workqueue.rst).
410 
411 The requesting task still does counter snapshotting and funnel-lock
412 processing, but the task reaching the top of the funnel lock does a
413 ``schedule_work()`` (from ``_synchronize_rcu_expedited()`` so that a
414 workqueue kthread does the actual grace-period processing. Because
415 workqueue kthreads do not accept POSIX signals, grace-period-wait
416 processing need not allow for POSIX signals. In addition, this approach
417 allows wakeups for the previous expedited grace period to be overlapped
418 with processing for the next expedited grace period. Because there are
419 only four sets of waitqueues, it is necessary to ensure that the
420 previous grace period's wakeups complete before the next grace period's
421 wakeups start. This is handled by having the ``->exp_mutex`` guard
422 expedited grace-period processing and the ``->exp_wake_mutex`` guard
423 wakeups. The key point is that the ``->exp_mutex`` is not released until
424 the first wakeup is complete, which means that the ``->exp_wake_mutex``
425 has already been acquired at that point. This approach ensures that the
426 previous grace period's wakeups can be carried out while the current
427 grace period is in process, but that these wakeups will complete before
428 the next grace period starts. This means that only three waitqueues are
429 required, guaranteeing that the four that are provided are sufficient.
430 
431 Stall Warnings
432 ~~~~~~~~~~~~~~
433 
434 Expediting grace periods does nothing to speed things up when RCU
435 readers take too long, and therefore expedited grace periods check for
436 stalls just as normal grace periods do.
437 
438 +-----------------------------------------------------------------------+
439 | **Quick Quiz**:                                                       |
440 +-----------------------------------------------------------------------+
441 | But why not just let the normal grace-period machinery detect the     |
442 | stalls, given that a given reader must block both normal and          |
443 | expedited grace periods?                                              |
444 +-----------------------------------------------------------------------+
445 | **Answer**:                                                           |
446 +-----------------------------------------------------------------------+
447 | Because it is quite possible that at a given time there is no normal  |
448 | grace period in progress, in which case the normal grace period       |
449 | cannot emit a stall warning.                                          |
450 +-----------------------------------------------------------------------+
451 
452 The ``synchronize_sched_expedited_wait()`` function loops waiting for
453 the expedited grace period to end, but with a timeout set to the current
454 RCU CPU stall-warning time. If this time is exceeded, any CPUs or
455 ``rcu_node`` structures blocking the current grace period are printed.
456 Each stall warning results in another pass through the loop, but the
457 second and subsequent passes use longer stall times.
458 
459 Mid-boot operation
460 ~~~~~~~~~~~~~~~~~~
461 
462 The use of workqueues has the advantage that the expedited grace-period
463 code need not worry about POSIX signals. Unfortunately, it has the
464 corresponding disadvantage that workqueues cannot be used until they are
465 initialized, which does not happen until some time after the scheduler
466 spawns the first task. Given that there are parts of the kernel that
467 really do want to execute grace periods during this mid-boot “dead
468 zone”, expedited grace periods must do something else during this time.
469 
470 What they do is to fall back to the old practice of requiring that the
471 requesting task drive the expedited grace period, as was the case before
472 the use of workqueues. However, the requesting task is only required to
473 drive the grace period during the mid-boot dead zone. Before mid-boot, a
474 synchronous grace period is a no-op. Some time after mid-boot,
475 workqueues are used.
476 
477 Non-expedited non-SRCU synchronous grace periods must also operate
478 normally during mid-boot. This is handled by causing non-expedited grace
479 periods to take the expedited code path during mid-boot.
480 
481 The current code assumes that there are no POSIX signals during the
482 mid-boot dead zone. However, if an overwhelming need for POSIX signals
483 somehow arises, appropriate adjustments can be made to the expedited
484 stall-warning code. One such adjustment would reinstate the
485 pre-workqueue stall-warning checks, but only during the mid-boot dead
486 zone.
487 
488 With this refinement, synchronous grace periods can now be used from
489 task context pretty much any time during the life of the kernel. That
490 is, aside from some points in the suspend, hibernate, or shutdown code
491 path.
492 
493 Summary
494 ~~~~~~~
495 
496 Expedited grace periods use a sequence-number approach to promote
497 batching, so that a single grace-period operation can serve numerous
498 requests. A funnel lock is used to efficiently identify the one task out
499 of a concurrent group that will request the grace period. All members of
500 the group will block on waitqueues provided in the ``rcu_node``
501 structure. The actual grace-period processing is carried out by a
502 workqueue.
503 
504 CPU-hotplug operations are noted lazily in order to prevent the need for
505 tight synchronization between expedited grace periods and CPU-hotplug
506 operations. The dyntick-idle counters are used to avoid sending IPIs to
507 idle CPUs, at least in the common case. RCU-preempt and RCU-sched use
508 different IPI handlers and different code to respond to the state
509 changes carried out by those handlers, but otherwise use common code.
510 
511 Quiescent states are tracked using the ``rcu_node`` tree, and once all
512 necessary quiescent states have been reported, all tasks waiting on this
513 expedited grace period are awakened. A pair of mutexes are used to allow
514 one grace period's wakeups to proceed concurrently with the next grace
515 period's processing.
516 
517 This combination of mechanisms allows expedited grace periods to run
518 reasonably efficiently. However, for non-time-critical tasks, normal
519 grace periods should be used instead because their longer duration
520 permits much higher degrees of batching, and thus much lower per-request
521 overheads.

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