Is Parallel Programming Hard, And, If So, What Can You Do About It?

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222 APPENDIX D. READ-COPY UPDATE IMPLEMENTATIONS−>levelspread[]−>levelcnt[]−>level[]−>node[][0]2[0]1[0][0]parent[1]3[1]2[1][2][3]60[1] parent [2]parentrcu_state−>rda[][0][1][2][3][4][5]mynodemynodemynodemynodemynodemynodeFigure D.42: Scanning rcu node Structures When Applying Quiescent Statesstates. In this case, line 24 updates the rcu_statestructure’s ->completed field to match the numberof the newly ended grace period, indicating that thegrace period has in fact ended. Line 24 then invokesrcu_process_gp_end() to advance the runningCPU’s RCU callbacks, and, finally, line 26 invokesrcu_start_gp() in order to start a new graceperiod should any remaining callbacks on the currentlyrunning CPU require one.Figure D.43 shows rcu_do_batch(), which invokesRCU callbacks whose grace periods haveended. Only callbacks on the running CPU will beinvoked—other CPUs must invoke their own callbacks.Quick Quiz D.49: How do RCU callbacks ondynticks-idle or offline CPUs get invoked?Line 7 invokes cpu_has_callbacks_ready_to_invoke() to see if this CPU has any RCU callbackswhose grace period has completed, and, if not, line 8returns. Lines 9 and 18 disable and re-enable interrupts,respectively. Lines 11-13 remove the readyto-invokecallbacks from ->nxtlist, and lines 14-17make any needed adjustments to the tail pointers.Quick Quiz D.50: Why would lines 14-17 inFigure D.43 need to adjust the tail pointers?Line 19 initializes local variable count to zero inpreparation for counting the number of callbacksthat will actually be invoked. Each pass through theloop spanning lines 20-27 invokes and counts a callback,with lines 25-26 exiting the loop if too manycallbacks are to be invoked at a time (thus preservingresponsiveness). The remainder of the functionthen requeues any callbacks that could not be invokeddue to this limit.Lines 28 and 41 disable and re-enable interrupts,respectively. Line 29 updates the ->qlen field,which maintains a count of the total number of RCUcallbacks for this CPU. Line 30 checks to see if therewere any ready-to-invoke callbacks that could not beinvoked at the moment due to the limit on the numberthat may be invoked at a given time. <strong>If</strong> suchcallbacks remain, lines 30-38 requeue them, againadjusting the tail pointers as needed. Lines 39-40restore the batch limit if it was increased due toexcessive callback backlog, and lines 42-43 cause additionalRCU processing to be scheduled if there areany ready-to-invoke callbacks remaining.D.3.7 Dyntick-Idle FunctionsThe functions in this section are defined only inCONFIG_NO_HZ builds of the Linux kernel, though insome cases, extended-no-op versions are present otherwise.These functions control whether or not RCUpays attention to a given CPU. CPUs in dynticksidlemode are ignored, but only if they are not currentlyinaninterruptorNMIhandler.Thefunctionsin this section communicate this CPU state to RCU.
D.3. HIERARCHICAL RCU CODE WALKTHROUGH 2231 static void rcu_do_batch(struct rcu_data *rdp)2 {3 unsigned long flags;4 struct rcu_head *next, *list, **tail;5 int count;67 if (!cpu_has_callbacks_ready_to_invoke(rdp))8 return;9 local_irq_save(flags);10 list = rdp->nxtlist;11 rdp->nxtlist = *rdp->nxttail[RCU_DONE_TAIL];12 *rdp->nxttail[RCU_DONE_TAIL] = NULL;13 tail = rdp->nxttail[RCU_DONE_TAIL];14 for (count = RCU_NEXT_SIZE - 1; count >= 0; count--)15 if (rdp->nxttail[count] ==16 rdp->nxttail[RCU_DONE_TAIL])17 rdp->nxttail[count] = &rdp->nxtlist;18 local_irq_restore(flags);19 count = 0;20 while (list) {21 next = list->next;22 prefetch(next);23 list->func(list);24 list = next;25 if (++count >= rdp->blimit)26 break;27 }28 local_irq_save(flags);29 rdp->qlen -= count;30 if (list != NULL) {31 *tail = rdp->nxtlist;32 rdp->nxtlist = list;33 for (count = 0; count < RCU_NEXT_SIZE; count++)34 if (&rdp->nxtlist == rdp->nxttail[count])35 rdp->nxttail[count] = tail;36 else37 break;38 }39 if (rdp->blimit == LONG_MAX && rdp->qlen blimit = blimit;41 local_irq_restore(flags);42 if (cpu_has_callbacks_ready_to_invoke(rdp))43 raise_softirq(RCU_SOFTIRQ);44 }Figure D.43: Code for rcu do batch()1 void rcu_enter_nohz(void)2 {3 unsigned long flags;4 struct rcu_dynticks *rdtp;56 smp_mb();7 local_irq_save(flags);8 rdtp = &__get_cpu_var(rcu_dynticks);9 rdtp->dynticks++;10 rdtp->dynticks_nesting--;11 local_irq_restore(flags);12 }1314 void rcu_exit_nohz(void)15 {16 unsigned long flags;17 struct rcu_dynticks *rdtp;1819 local_irq_save(flags);20 rdtp = &__get_cpu_var(rcu_dynticks);21 rdtp->dynticks++;22 rdtp->dynticks_nesting++;23 local_irq_restore(flags);24 smp_mb();25 }Figure D.44: Entering and Exiting Dyntick-IdleModeThissetoffunctionsisgreatlysimplifiedfromthatused in preemptable RCU, see Section E.7 for a descriptionof the earlier more-complex model. ManfredSpraulputforththeideaforthissimplifedinterfacein one of his state-based RCU patches [Spr08b,Spr08a].Section D.3.7.1 describes the functions that enterand exit dynticks-idle mode from process context,Section D.3.7.2 describes the handling of NM<strong>Is</strong>from dynticks-idle mode, Section D.3.7.3 covers handlingofinterruptsfromdynticks-idlemode,andSectionD.3.7.4 presents functions that check whethersome other CPU is currently in dynticks-idle mode.D.3.7.1 Entering and Exiting Dyntick-IdleModeFigure D.44 shows the rcu_enter_nohz() and rcu_exit_nohz() functions that allow the scheduler totransition to and from dynticks-idle mode. Therefore,after rcu_enter_nohz() has been call, RCUwill ignore it, at least until the next rcu_exit_nohz(), the next interrupt, or the next NMI.Line 6 of rcu_enter_nohz() executes a memorybarrier to ensure that any preceding RCU readsidecritical sections are seen to have occured beforethe following code that tells RCU to ignorethis CPU. Lines 7 and 11 disable and restore interruptsin order to avoid interference with the statechange. Line 8 picks up a pointer to the runningCPU’s rcu_dynticks structure, line 9 incrementsthe ->dynticks field (which now must be even to
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Is Parallel Programming Hard, And,
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Contents1 Introduction 11.1 Histori
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CONTENTSv6 Locking 676.1 Staying Al
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CONTENTSviiB Synchronization Primit
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CONTENTSixE.7.1 Introduction to Pre
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PrefaceThe purpose of this book is
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Chapter 1IntroductionParallel progr
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1.2. PARALLEL PROGRAMMING GOALS 3CP
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1.3. ALTERNATIVES TO PARALLEL PROGR
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1.4. WHAT MAKES PARALLEL PROGRAMMIN
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1.5. GUIDE TO THIS BOOK 9other hand
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Chapter 2Hardware and its HabitsMos
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2.1. OVERVIEW 13Therefore, as shown
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16 CHAPTER 2. HARDWARE AND ITS HABI
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18 CHAPTER 2. HARDWARE AND ITS HABI
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20 CHAPTER 3. TOOLS OF THE TRADE1 p
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22 CHAPTER 3. TOOLS OF THE TRADE1 p
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24 CHAPTER 3. TOOLS OF THE TRADE1.1
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26 CHAPTER 3. TOOLS OF THE TRADEQui
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28 CHAPTER 3. TOOLS OF THE TRADE
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30 CHAPTER 4. COUNTING1 atomic_t co
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32 CHAPTER 4. COUNTING4.2.3 Eventua
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34 CHAPTER 4. COUNTINGvanish when t
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36 CHAPTER 4. COUNTINGper-thread va
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38 CHAPTER 4. COUNTING1 unsigned lo
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42 CHAPTER 4. COUNTING1 #define THE
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46 CHAPTER 4. COUNTINGReadsAlgorith
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48 CHAPTER 5. PARTITIONING AND SYNC
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68 CHAPTER 6. LOCKING1 int delete(i
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70 CHAPTER 7. DATA OWNERSHIP
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72 CHAPTER 8. DEFERRED PROCESSINGfo
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74 CHAPTER 8. DEFERRED PROCESSINGth
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76 CHAPTER 8. DEFERRED PROCESSING
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108 CHAPTER 8. DEFERRED PROCESSING
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110 CHAPTER 9. APPLYING RCU1 struct
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112 CHAPTER 9. APPLYING RCU
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114 CHAPTER 10. VALIDATION: DEBUGGI
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116 CHAPTER 11. DATA STRUCTURES
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118 CHAPTER 12. ADVANCED SYNCHRONIZ
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138 CHAPTER 13. EASE OF USEFigure 1
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140 CHAPTER 13. EASE OF USE
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142 CHAPTER 14. TIME MANAGEMENT
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144 CHAPTER 15. CONFLICTING VISIONS
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154 APPENDIX A. IMPORTANT QUESTIONS
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156 APPENDIX A. IMPORTANT QUESTIONS
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158 APPENDIX B. SYNCHRONIZATION PRI
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160 APPENDIX B. SYNCHRONIZATION PRI
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162 APPENDIX C. WHY MEMORY BARRIERS
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272 APPENDIX F. ANSWERS TO QUICK QU
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330 APPENDIX G. GLOSSARY(2) A physi
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332 APPENDIX G. GLOSSARYnear by. Th
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334 APPENDIX G. GLOSSARY
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336 BIBLIOGRAPHY[But97]USA, March 2
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338 BIBLIOGRAPHY[HMB06][Hol03][HP95
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340 BIBLIOGRAPHY[McK06] Paul E. McK
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342 BIBLIOGRAPHYtor. Software - Pra
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344 BIBLIOGRAPHY[UoC08][VGS08]Berke
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346 APPENDIX H. CREDITSH.4 Original
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