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

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212 APPENDIX D. READ-COPY UPDATE IMPLEMENTATIONSact fanout (specified by CONFIG_RCU_FANOUT), andthe other on lines 11-25 that determines the numberof child nodes based indirectly on the specifiedfanout, but then balances the tree. The CONFIG_RCU_FANOUT_EXACT kernel parameter selects whichversion to use for a given kernel build.The exact-fanout version simply assigns all ofthe elements of the specified rcu_state structure’s->levelspread array to the CONFIG_RCU_FANOUTkernel parameter, as shown by the loop on lines 7and 8.The hierarchy-balancing version on lines 11-24uses a pair of local variables ccur and cprv whichtrack the number of rcu_node structures on the currentand previous levels, respectively. This functionworks from the leaf level up the hierarchy, so cprv isinitialized by line 18 to NR_CPUS, which correspondsto the number of rcu_data structures that feed intothe leaf level. Lines 19-23 iterate from the leaf to theroot. Within this loop, line 20 picking up the numberof rcu_node structures for the current level intoccur. Line 21 then rounds up the ratio of the numberof nodes on the previous (lower) level (be theyrcu_node or rcu_data) to the number of rcu_nodestructures on the current level, placing the result inthe specified rcu_state structure’s ->levelspreadarray. Line 22 then sets up for the next pass throughthe loop.Afteracalltoeitherfunction, the->levelspreadarray contains the number of children for each levelof the rcu_node hierarchy.D.3.3.2 rcu init one()Figure D.28 shows the code for rcu_init_one(),which does boot-time initialization for the specifiedrcu_state structure.Recall from Section D.3.1.4 that the ->levelcnt[] array in the rcu_state structure iscompile-time initialized to the number of nodes ateach level of the hierarchy starting from the root,with an additional element in the array initializedtothemaximumpossiblenumberofCPUs,NR_CPUS.In addition, the first element of the ->level[] arrayis compile-time initialized to reference to theroot rcu_node structure, which is in turn the firstelement of the ->node[] array in the rcu_statestructure. This array is further laid out in breadthfirstorder. Keeping all of this in mind, the loop atlines 8-10 initializes the rest of the ->level[] arrayto reference the first rcu_node structure of eachlevel of the rcu_node hierarchy.Line 11 then invokes rcu_init_levelspread(),which fills in the ->levelspread[] array, as was1 static void __init rcu_init_one(struct rcu_state *rsp)2 {3 int cpustride = 1;4 int i;5 int j;6 struct rcu_node *rnp;78 for (i = 1; i < NUM_RCU_LVLS; i++)9 rsp->level[i] = rsp->level[i - 1] +10 rsp->levelcnt[i - 1];11 rcu_init_levelspread(rsp);12 for (i = NUM_RCU_LVLS - 1; i >= 0; i--) {13 cpustride *= rsp->levelspread[i];14 rnp = rsp->level[i];15 for (j = 0; j < rsp->levelcnt[i]; j++, rnp++) {16 spin_lock_init(&rnp->lock);17 rnp->qsmask = 0;18 rnp->qsmaskinit = 0;19 rnp->grplo = j * cpustride;20 rnp->grphi = (j + 1) * cpustride - 1;21 if (rnp->grphi >= NR_CPUS)22 rnp->grphi = NR_CPUS - 1;23 if (i == 0) {24 rnp->grpnum = 0;25 rnp->grpmask = 0;26 rnp->parent = NULL;27 } else {28 rnp->grpnum = j % rsp->levelspread[i - 1];29 rnp->grpmask = 1UL grpnum;30 rnp->parent = rsp->level[i - 1] +31 j / rsp->levelspread[i - 1];32 }33 rnp->level = i;34 }35 }36 }Figure D.28: rcu init one() Codedescribed in Section D.3.3.1. The auxiliary arraysare then fully initialized, and thus ready for the loopfrom lines 15-35, each pass through which initializesone level of the rcu_node hierarchy, starting fromthe leaves.Line 13 computes the number of CPUs per rcu_node structure for the current level of the hierarchy,and line 14 obtains a pointer to the first rcu_nodestructure on the current level of the hierarchy, inpreparation for the loop from lines 15-34, each passthrough which initializes one rcu_node structure.Lines 16-18 initialize the rcu_node structure’sspinlock and its CPU masks. The qsmaskinit fieldwill have bits set as CPUs come online later in boot,and the qsmask field will have bits set when the firstgrace period starts. Line 19 sets the ->grplo fieldto the number of the this rcu_node structure’s firstCPU and line 20 sets the ->grphi to the numberof this rcu_node structure’s last CPU. If the lastrcu_node structure on a given level of the hierarchyis only partially full, lines 21 and 22 set its ->grphifield to the number of the last possible CPU in thesystem.Lines 24-26 initialize the ->grpnum, ->grpmask,and->parentfieldsfortherootrcu_nodestructure,which has no parent, hence the zeroes and NULL.
D.3. HIERARCHICAL RCU CODE WALKTHROUGH 2131 #define RCU_DATA_PTR_INIT(rsp, rcu_data) \2 do { \3 rnp = (rsp)->level[NUM_RCU_LVLS - 1]; \4 j = 0; \5 for_each_possible_cpu(i) { \6 if (i > rnp[j].grphi) \7 j++; \8 per_cpu(rcu_data, i).mynode = &rnp[j]; \9 (rsp)->rda[i] = &per_cpu(rcu_data, i); \10 } \11 } while (0)1213 void __init __rcu_init(void)14 {15 int i;16 int j;17 struct rcu_node *rnp;1819 rcu_init_one(&rcu_state);20 RCU_DATA_PTR_INIT(&rcu_state, rcu_data);21 rcu_init_one(&rcu_bh_state);22 RCU_DATA_PTR_INIT(&rcu_bh_state, rcu_bh_data);2324 for_each_online_cpu(i)25 rcu_cpu_notify(&rcu_nb, CPU_UP_PREPARE,26 (void *)(long)i);27 register_cpu_notifier(&rcu_nb);28 }Figure D.29: rcu init() CodeLines 28-31 initialize these same fields for the restof the rcu_node structures in the hierarchy. Line 28computes the ->grpnum field as the index of thisrcu_node structure within the set having the sameparent, and line 29 sets the corresponding bit in the->grpmaskfield. Finally, lines30-31placesapointerto the parent node into the ->parent field. Thesethree fields will used to propagate quiescent statesup the hierarchy.Finally, line 33 records the hierarchy level in ->level, which is used for tracing when traversing thefull hierarchy.D.3.3.3rcu init()Figure D.29 shows the __rcu_init() function andits RCU_DATA_PTR_INIT() helper macro. The __rcu_init() function is invoked during early boot,before the scheduler has initialized, and before morethan one CPU is running.The RCU_DATA_PTR_INIT() macro takes as argumentsa pointer to an rcu_state structure andthe name of a set of rcu_data per-CPU variables.This macro scans the per-CPU rcu_datastructures, assigning the ->mynode pointer of eachrcu_data structure to point to the correspondingleaf rcu_node structure. It also fills out the specifiedrcu_state structure’s ->rda[] array entries toeach point to the corresponding rcu_data structure.Line 3 picks up a pointer to the first leaf rcu_nodestructure in local variable rnp (which must be declaredby the invoker of this macro), and line 4 setslocal variable j to the corresponding leaf-node numberof zero. Each pass through the loop spanninglines 5-10 performs initialization for the correspondingpotentialCPU(asspecifiedbyNR_CPUS).Withinthis loop, line 6 checks to see if we have moved beyondthe bounds of the current leaf rcu_node structure,and, ifso, line7advancestothenextstructure.Then, still within the loop, line 8 sets the ->mynodepointer of the current CPU’s rcu_data structureto reference the current leaf rcu_node structure,and line 9 sets the current CPU’s ->rda[] element(within the rcu_state structure) to reference thecurrent CPU’s rcu_data structure.Quick Quiz D.34: C-preprocessor macros are so1990s! Why not get with the times and convertRCU_DATA_PTR_INIT() in Figure D.29 to be a function?The __rcu_init() function first invokes rcu_init_one() on the rcu_state structure on line 19,then invokes RCU_DATA_PTR_INIT() on the rcu_state structure and the rcu_data set of per-CPUvariables. It then repeats this for rcu_bh_stateand rcu_bh_data on lines 21-22. The loop spanninglines 24-26 invokes rcu_cpu_notify() for eachCPU that is currently online (which should be onlythe boot CPU), and line 27 registers a notifier sothat rcu_cpu_notify() will be invoked each timea CPU comes online, in order to inform RCU of itspresence.Quick Quiz D.35: What happens if a CPUcomes online between the time that the last onlineCPUisnotifiedonlines25-26ofFigureD.29andthetime that register_cpu_notifier() is invoked online 27?The rcu_cpu_notify() and related functions arediscussed in Section D.3.4 below.D.3.4 CPU HotplugThe CPU-hotplug functions described in the followingsections allow RCU to track which CPUs are andare not present, but also complete initialization ofeach CPU’s rcu_data structure as that CPU comesonline.D.3.4.1 rcu init percpu data()Figure D.30 shows the code for rcu_init_percpu_data(), which initializes the specified CPU’s rcu_data structure in response to booting up or to thatCPU coming online. It also sets up the rcu_nodehierarchy so that this CPU will participate in futuregrace periods.
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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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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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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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262 APPENDIX E. FORMAL VERIFICATION
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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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