Merge branch 'core-rcu-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull RCU updates from Ingo Molnar:
 "The major changes:

  - Simplify RCU's grace-period and callback processing based on the new
    numbering for callbacks.

  - Removal of TINY_PREEMPT_RCU in favor of TREE_PREEMPT_RCU for
    single-CPU low-latency systems.

  - SRCU-related changes and fixes.

  - Miscellaneous fixes, including converting a few remaining printk()
    calls to pr_*().

  - Documentation updates"

* 'core-rcu-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (32 commits)
  rcu: Shrink TINY_RCU by reworking CPU-stall ifdefs
  rcu: Shrink TINY_RCU by moving exit_rcu()
  rcu: Remove TINY_PREEMPT_RCU tracing documentation
  rcu: Consolidate rcutiny_plugin.h ifdefs
  rcu: Remove rcu_preempt_note_context_switch()
  rcu: Remove the CONFIG_TINY_RCU ifdefs in rcutiny.h
  rcu: Remove check_cpu_stall_preempt()
  rcu: Simplify RCU_TINY RCU callback invocation
  rcu: Remove rcu_preempt_process_callbacks()
  rcu: Remove rcu_preempt_remove_callbacks()
  rcu: Remove rcu_preempt_check_callbacks()
  rcu: Remove show_tiny_preempt_stats()
  rcu: Remove TINY_PREEMPT_RCU
  powerpc,kvm: fix imbalance srcu_read_[un]lock()
  rcu: Remove srcu_read_lock_raw() and srcu_read_unlock_raw().
  rcu: Apply Dave Jones's NOCB Kconfig help feedback
  rcu: Merge adjacent identical ifdefs
  rcu: Drive quiescent-state-forcing delay from HZ
  rcu: Remove "Experimental" flags
  kthread: Add kworker kthreads to OS-jitter documentation
  ...

init/Kconfig

@@ -473,18 +473,10 @@ config TINY_RCU
 	  is not required.  This option greatly reduces the
 	  memory footprint of RCU.
 
-config TINY_PREEMPT_RCU
-	bool "Preemptible UP-only small-memory-footprint RCU"
-	depends on PREEMPT && !SMP
-	help
-	  This option selects the RCU implementation that is designed
-	  for real-time UP systems.  This option greatly reduces the
-	  memory footprint of RCU.
-
 endchoice
 
 config PREEMPT_RCU
-	def_bool ( TREE_PREEMPT_RCU || TINY_PREEMPT_RCU )
+	def_bool TREE_PREEMPT_RCU
 	help
 	  This option enables preemptible-RCU code that is common between
 	  the TREE_PREEMPT_RCU and TINY_PREEMPT_RCU implementations.
@@ -670,7 +662,7 @@ config RCU_BOOST_DELAY
 	  Accept the default if unsure.
 
 config RCU_NOCB_CPU
-	bool "Offload RCU callback processing from boot-selected CPUs (EXPERIMENTAL"
+	bool "Offload RCU callback processing from boot-selected CPUs"
 	depends on TREE_RCU || TREE_PREEMPT_RCU
 	default n
 	help
@@ -696,9 +688,10 @@ choice
 	prompt "Build-forced no-CBs CPUs"
 	default RCU_NOCB_CPU_NONE
 	help
-	  This option allows no-CBs CPUs to be specified at build time.
-	  Additional no-CBs CPUs may be specified by the rcu_nocbs=
-	  boot parameter.
+	  This option allows no-CBs CPUs (whose RCU callbacks are invoked
+	  from kthreads rather than from softirq context) to be specified
+	  at build time.  Additional no-CBs CPUs may be specified by
+	  the rcu_nocbs= boot parameter.
 
 config RCU_NOCB_CPU_NONE
 	bool "No build_forced no-CBs CPUs"
@@ -706,25 +699,40 @@ config RCU_NOCB_CPU_NONE
 	help
 	  This option does not force any of the CPUs to be no-CBs CPUs.
 	  Only CPUs designated by the rcu_nocbs= boot parameter will be
-	  no-CBs CPUs.
+	  no-CBs CPUs, whose RCU callbacks will be invoked by per-CPU
+	  kthreads whose names begin with "rcuo".  All other CPUs will
+	  invoke their own RCU callbacks in softirq context.
+
+	  Select this option if you want to choose no-CBs CPUs at
+	  boot time, for example, to allow testing of different no-CBs
+	  configurations without having to rebuild the kernel each time.
 
 config RCU_NOCB_CPU_ZERO
 	bool "CPU 0 is a build_forced no-CBs CPU"
 	depends on RCU_NOCB_CPU && !NO_HZ_FULL
 	help
-	  This option forces CPU 0 to be a no-CBs CPU.  Additional CPUs
-	  may be designated as no-CBs CPUs using the rcu_nocbs= boot
-	  parameter will be no-CBs CPUs.
+	  This option forces CPU 0 to be a no-CBs CPU, so that its RCU
+	  callbacks are invoked by a per-CPU kthread whose name begins
+	  with "rcuo".	Additional CPUs may be designated as no-CBs
+	  CPUs using the rcu_nocbs= boot parameter will be no-CBs CPUs.
+	  All other CPUs will invoke their own RCU callbacks in softirq
+	  context.
 
 	  Select this if CPU 0 needs to be a no-CBs CPU for real-time
-	  or energy-efficiency reasons.
+	  or energy-efficiency reasons, but the real reason it exists
+	  is to ensure that randconfig testing covers mixed systems.
 
 config RCU_NOCB_CPU_ALL
 	bool "All CPUs are build_forced no-CBs CPUs"
 	depends on RCU_NOCB_CPU
 	help
 	  This option forces all CPUs to be no-CBs CPUs.  The rcu_nocbs=
-	  boot parameter will be ignored.
+	  boot parameter will be ignored.  All CPUs' RCU callbacks will
+	  be executed in the context of per-CPU rcuo kthreads created for
+	  this purpose.  Assuming that the kthreads whose names start with
+	  "rcuo" are bound to "housekeeping" CPUs, this reduces OS jitter
+	  on the remaining CPUs, but might decrease memory locality during
+	  RCU-callback invocation, thus potentially degrading throughput.
 
 	  Select this if all CPUs need to be no-CBs CPUs for real-time
 	  or energy-efficiency reasons.