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
  ...

1 files changed, 27 insertions, 19 deletions

diff --git a/init/Kconfig b/init/Kconfig
index 1e825c299ea5..118895cc1f67 100644
--- a/init/Kconfig
+++ b/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.