rcu: Merge preemptable-RCU functionality into hierarchical RCU

Create a kernel/rcutree_plugin.h file that contains definitions
for preemptable RCU (or, under the #else branch of the #ifdef,
empty definitions for the classic non-preemptable semantics).
These definitions fit into plugins defined in kernel/rcutree.c
for this purpose.

This variant of preemptable RCU uses a new algorithm whose
read-side expense is roughly that of classic hierarchical RCU
under CONFIG_PREEMPT. This new algorithm's update-side expense
is similar to that of classic hierarchical RCU, and, in absence
of read-side preemption or blocking, is exactly that of classic
hierarchical RCU.  Perhaps more important, this new algorithm
has a much simpler implementation, saving well over 1,000 lines
of code compared to mainline's implementation of preemptable
RCU, which will hopefully be retired in favor of this new
algorithm.

The simplifications are obtained by maintaining per-task
nesting state for running tasks, and using a simple
lock-protected algorithm to handle accounting when tasks block
within RCU read-side critical sections, making use of lessons
learned while creating numerous user-level RCU implementations
over the past 18 months.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: akpm@linux-foundation.org
Cc: mathieu.desnoyers@polymtl.ca
Cc: josht@linux.vnet.ibm.com
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
LKML-Reference: <12509746134003-git-send-email->
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 9 insertions, 0 deletions

diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 0024e5ddcc68..ca560364d8cd 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -80,6 +80,7 @@ struct rcu_dynticks {
  */
 struct rcu_node {
         spinlock_t lock;
+        long    gpnum;          /* Current grace period for this node. */
         unsigned long qsmask;   /* CPUs or groups that need to switch in */
                                 /*  order for current grace period to proceed.*/
         unsigned long qsmaskinit;
@@ -90,6 +91,8 @@ struct rcu_node {
         u8      grpnum;         /* CPU/group number for next level up. */
         u8      level;          /* root is at level 0. */
         struct rcu_node *parent;
+        struct list_head blocked_tasks[2];
+                                /* Tasks blocked in RCU read-side critsect. */
 } ____cacheline_internodealigned_in_smp;
 
 /* Index values for nxttail array in struct rcu_data. */
@@ -111,6 +114,7 @@ struct rcu_data {
         bool            passed_quiesc;  /* User-mode/idle loop etc. */
         bool            qs_pending;     /* Core waits for quiesc state. */
         bool            beenonline;     /* CPU online at least once. */
+        bool            preemptable;    /* Preemptable RCU? */
         struct rcu_node *mynode;        /* This CPU's leaf of hierarchy */
         unsigned long grpmask;          /* Mask to apply to leaf qsmask. */
 
@@ -244,5 +248,10 @@ DECLARE_PER_CPU(struct rcu_data, rcu_sched_data);
 extern struct rcu_state rcu_bh_state;
 DECLARE_PER_CPU(struct rcu_data, rcu_bh_data);
 
+#ifdef CONFIG_TREE_PREEMPT_RCU
+extern struct rcu_state rcu_preempt_state;
+DECLARE_PER_CPU(struct rcu_data, rcu_preempt_data);
+#endif /* #ifdef CONFIG_TREE_PREEMPT_RCU */
+
 #endif /* #ifdef RCU_TREE_NONCORE */