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

D.3. HIERARCHICAL RCU CODE WALKTHROUGH 211−>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.26: Initialized RCU Data Layouterarchy. In the figure, therefore, the root node hastwochildrenandthenodesattheleafleveleachhavethree children. Each element of the levelcnt[] arrayindicateshowmanynodesthereareonthecorrespondinglevel of the hierarchy: 1 at the root level,2 at the leaf level, and 6 at the rcu_data level—and any extra elements are unused and left as zero.Each element of the ->level[] array references thefirst node of the corresponding level of the rcu_nodehierarchy, and each element of the ->rda[] arrayreferences the corresponding CPU’s rcu_data structure.The->parentfieldofeachrcu_nodestructurereferences its parent, except for the root rcu_nodestructure, which has a NULL ->parent pointer. Finally,the ->mynode field of each rcu_data structurereferences its parent rcu_node structure.Quick Quiz D.33: How does the code traversea given path through the rcu_node hierarchy fromroot to leaves?Again, the following sections walk through thecode that builds this structure.1 #ifdef CONFIG_RCU_FANOUT_EXACT2 static void __init3 rcu_init_levelspread(struct rcu_state *rsp)4 {5 int i;67 for (i = NUM_RCU_LVLS - 1; i >= 0; i--)8 rsp->levelspread[i] = CONFIG_RCU_FANOUT;9 }10 #else11 static void __init12 rcu_init_levelspread(struct rcu_state *rsp)13 {14 int ccur;15 int cprv;16 int i;1718 cprv = NR_CPUS;19 for (i = NUM_RCU_LVLS - 1; i >= 0; i--) {20 ccur = rsp->levelcnt[i];21 rsp->levelspread[i] = (cprv + ccur - 1) / ccur;22 cprv = ccur;23 }24 }25 #endifD.3.3.1 rcu init levelspread()Figure D.27 shows the code for the rcu_init_levelspread() function, which controls the fanout,or the number of children per parent, in the rcu_node hierarchy. There are two versions of this function,one shown on lines 2-9 that enforces the ex-Figure D.27: rcu init levelspread() Code