1. 02 Jul, 2012 1 commit
  2. 07 Jun, 2012 1 commit
    • Paul E. McKenney's avatar
      rcu: Precompute RCU_FAST_NO_HZ timer offsets · aa9b1630
      Paul E. McKenney authored
      When a CPU is entering dyntick-idle mode, tick_nohz_stop_sched_tick()
      calls rcu_needs_cpu() see if RCU needs that CPU, and, if not, computes the
      next wakeup time based on the timer wheels.  Only later, when actually
      entering the idle loop, rcu_prepare_for_idle() will be invoked.  In some
      cases, rcu_prepare_for_idle() will post timers to wake the CPU back up.
      But all for naught: The next wakeup time for the CPU has already been
      computed, and posting a timer afterwards does not force that wakeup
      time to be recomputed.  This means that rcu_prepare_for_idle()'s have
      no effect.
      
      This is not a problem on a busy system because something else will wake
      up the CPU soon enough.  However, on lightly loaded systems, the CPU
      might stay asleep for a considerable length of time.  If that CPU has
      a callback that the rest of the system is waiting on, the system might
      run very slowly or (in theory) even hang.
      
      This commit avoids this problem by having rcu_needs_cpu() give
      tick_nohz_stop_sched_tick() an estimate of when RCU will need the CPU
      to wake back up, which tick_nohz_stop_sched_tick() takes into account
      when programming the CPU's wakeup time.  An alternative approach is
      for rcu_prepare_for_idle() to use hrtimers instead of normal timers,
      but timers are much more efficient than are hrtimers for frequently
      and repeatedly posting and cancelling a given timer, which is exactly
      what RCU_FAST_NO_HZ does.
      Reported-by: default avatarPascal Chapperon <pascal.chapperon@wanadoo.fr>
      Reported-by: default avatarHeiko Carstens <heiko.carstens@de.ibm.com>
      Signed-off-by: default avatarPaul E. McKenney <paul.mckenney@linaro.org>
      Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
      Tested-by: default avatarHeiko Carstens <heiko.carstens@de.ibm.com>
      Tested-by: default avatarPascal Chapperon <pascal.chapperon@wanadoo.fr>
      aa9b1630
  3. 02 May, 2012 2 commits
  4. 21 Feb, 2012 2 commits
    • Paul E. McKenney's avatar
      rcu: Prevent RCU callbacks from executing before scheduler initialized · 768dfffd
      Paul E. McKenney authored
      This is a port of commit #b0d30417 from TREE_RCU to TREE_PREEMPT_RCU.
      
      Under some rare but real combinations of configuration parameters, RCU
      callbacks are posted during early boot that use kernel facilities that are
      not yet initialized.  Therefore, when these callbacks are invoked, hard
      hangs and crashes ensue.  This commit therefore prevents RCU callbacks
      from being invoked until after the scheduler is fully up and running,
      as in after multiple tasks have been spawned.
      
      It might well turn out that a better approach is to identify the specific
      RCU callbacks that are causing this problem, but that discussion will
      wait until such time as someone really needs an RCU callback to be invoked
      (as opposed to merely registered) during early boot.
      Signed-off-by: default avatarPaul E. McKenney <paul.mckenney@linaro.org>
      Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
      768dfffd
    • Paul E. McKenney's avatar
      rcu: Avoid waking up CPUs having only kfree_rcu() callbacks · 486e2593
      Paul E. McKenney authored
      When CONFIG_RCU_FAST_NO_HZ is enabled, RCU will allow a given CPU to
      enter dyntick-idle mode even if it still has RCU callbacks queued.
      RCU avoids system hangs in this case by scheduling a timer for several
      jiffies in the future.  However, if all of the callbacks on that CPU
      are from kfree_rcu(), there is no reason to wake the CPU up, as it is
      not a problem to defer freeing of memory.
      
      This commit therefore tracks the number of callbacks on a given CPU
      that are from kfree_rcu(), and avoids scheduling the timer if all of
      a given CPU's callbacks are from kfree_rcu().
      Signed-off-by: default avatarPaul E. McKenney <paul.mckenney@linaro.org>
      Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
      486e2593
  5. 29 Sep, 2011 2 commits
  6. 06 May, 2011 1 commit
  7. 30 Nov, 2010 1 commit
  8. 17 Nov, 2010 1 commit
    • Paul E. McKenney's avatar
      rcu: move TINY_RCU from softirq to kthread · b2c0710c
      Paul E. McKenney authored
      If RCU priority boosting is to be meaningful, callback invocation must
      be boosted in addition to preempted RCU readers.  Otherwise, in presence
      of CPU real-time threads, the grace period ends, but the callbacks don't
      get invoked.  If the callbacks don't get invoked, the associated memory
      doesn't get freed, so the system is still subject to OOM.
      
      But it is not reasonable to priority-boost RCU_SOFTIRQ, so this commit
      moves the callback invocations to a kthread, which can be boosted easily.
      Signed-off-by: default avatarPaul E. McKenney <paul.mckenney@linaro.org>
      Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
      b2c0710c
  9. 20 Aug, 2010 4 commits
    • Paul E. McKenney's avatar
      rcu: combine duplicate code, courtesy of CONFIG_PREEMPT_RCU · 7b0b759b
      Paul E. McKenney authored
      The CONFIG_PREEMPT_RCU kernel configuration parameter was recently
      re-introduced, but as an indication of the type of RCU (preemptible
      vs. non-preemptible) instead of as selecting a given implementation.
      This commit uses CONFIG_PREEMPT_RCU to combine duplicate code
      from include/linux/rcutiny.h and include/linux/rcutree.h into
      include/linux/rcupdate.h.  This commit also combines a few other pieces
      of duplicate code that have accumulated.
      Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
      7b0b759b
    • Paul E. McKenney's avatar
      rcu: repair code-duplication FIXMEs · a3dc3fb1
      Paul E. McKenney authored
      Combine the duplicate definitions of ULONG_CMP_GE(), ULONG_CMP_LT(),
      and rcu_preempt_depth() into include/linux/rcupdate.h.
      Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
      a3dc3fb1
    • Paul E. McKenney's avatar
      rcu: permit suppressing current grace period's CPU stall warnings · 53d84e00
      Paul E. McKenney authored
      When using a kernel debugger, a long sojourn in the debugger can get
      you lots of RCU CPU stall warnings once you resume.  This might not be
      helpful, especially if you are using the system console.  This patch
      therefore allows RCU CPU stall warnings to be suppressed, but only for
      the duration of the current set of grace periods.
      
      This differs from Jason's original patch in that it adds support for
      tiny RCU and preemptible RCU, and uses a slightly different method for
      suppressing the RCU CPU stall warning messages.
      Signed-off-by: default avatarJason Wessel <jason.wessel@windriver.com>
      Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
      Tested-by: default avatarJason Wessel <jason.wessel@windriver.com>
      53d84e00
    • Paul E. McKenney's avatar
      rcu: Add a TINY_PREEMPT_RCU · a57eb940
      Paul E. McKenney authored
      Implement a small-memory-footprint uniprocessor-only implementation of
      preemptible RCU.  This implementation uses but a single blocked-tasks
      list rather than the combinatorial number used per leaf rcu_node by
      TREE_PREEMPT_RCU, which reduces memory consumption and greatly simplifies
      processing.  This version also takes advantage of uniprocessor execution
      to accelerate grace periods in the case where there are no readers.
      
      The general design is otherwise broadly similar to that of TREE_PREEMPT_RCU.
      
      This implementation is a step towards having RCU implementation driven
      off of the SMP and PREEMPT kernel configuration variables, which can
      happen once this implementation has accumulated sufficient experience.
      
      Removed ACCESS_ONCE() from __rcu_read_unlock() and added barrier() as
      suggested by Steve Rostedt in order to avoid the compiler-reordering
      issue noted by Mathieu Desnoyers (http://lkml.org/lkml/2010/8/16/183).
      
      As can be seen below, CONFIG_TINY_PREEMPT_RCU represents almost 5Kbyte
      savings compared to CONFIG_TREE_PREEMPT_RCU.  Of course, for non-real-time
      workloads, CONFIG_TINY_RCU is even better.
      
      	CONFIG_TREE_PREEMPT_RCU
      
      	   text	   data	    bss	    dec	   filename
      	     13	      0	      0	     13	   kernel/rcupdate.o
      	   6170	    825	     28	   7023	   kernel/rcutree.o
      				   ----
      				   7026    Total
      
      	CONFIG_TINY_PREEMPT_RCU
      
      	   text	   data	    bss	    dec	   filename
      	     13	      0	      0	     13	   kernel/rcupdate.o
      	   2081	     81	      8	   2170	   kernel/rcutiny.o
      				   ----
      				   2183    Total
      
      	CONFIG_TINY_RCU (non-preemptible)
      
      	   text	   data	    bss	    dec	   filename
      	     13	      0	      0	     13	   kernel/rcupdate.o
      	    719	     25	      0	    744	   kernel/rcutiny.o
      				    ---
      				    757    Total
      Requested-by: default avatarLoïc Minier <loic.minier@canonical.com>
      Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
      a57eb940
  10. 10 May, 2010 3 commits
    • Paul E. McKenney's avatar
      rcu: slim down rcutiny by removing rcu_scheduler_active and friends · bbad9379
      Paul E. McKenney authored
      TINY_RCU does not need rcu_scheduler_active unless CONFIG_DEBUG_LOCK_ALLOC.
      So conditionally compile rcu_scheduler_active in order to slim down
      rcutiny a bit more.  Also gets rid of an EXPORT_SYMBOL_GPL, which is
      responsible for most of the slimming.
      Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
      bbad9379
    • Paul E. McKenney's avatar
      rcu: refactor RCU's context-switch handling · 25502a6c
      Paul E. McKenney authored
      The addition of preemptible RCU to treercu resulted in a bit of
      confusion and inefficiency surrounding the handling of context switches
      for RCU-sched and for RCU-preempt.  For RCU-sched, a context switch
      is a quiescent state, pure and simple, just like it always has been.
      For RCU-preempt, a context switch is in no way a quiescent state, but
      special handling is required when a task blocks in an RCU read-side
      critical section.
      
      However, the callout from the scheduler and the outer loop in ksoftirqd
      still calls something named rcu_sched_qs(), whose name is no longer
      accurate.  Furthermore, when rcu_check_callbacks() notes an RCU-sched
      quiescent state, it ends up unnecessarily (though harmlessly, aside
      from the performance hit) enqueuing the current task if it happens to
      be running in an RCU-preempt read-side critical section.  This not only
      increases the maximum latency of scheduler_tick(), it also needlessly
      increases the overhead of the next outermost rcu_read_unlock() invocation.
      
      This patch addresses this situation by separating the notion of RCU's
      context-switch handling from that of RCU-sched's quiescent states.
      The context-switch handling is covered by rcu_note_context_switch() in
      general and by rcu_preempt_note_context_switch() for preemptible RCU.
      This permits rcu_sched_qs() to handle quiescent states and only quiescent
      states.  It also reduces the maximum latency of scheduler_tick(), though
      probably by much less than a microsecond.  Finally, it means that tasks
      within preemptible-RCU read-side critical sections avoid incurring the
      overhead of queuing unless there really is a context switch.
      Suggested-by: Lai Jiangshan's avatarLai Jiangshan <laijs@cn.fujitsu.com>
      Acked-by: Lai Jiangshan's avatarLai Jiangshan <laijs@cn.fujitsu.com>
      Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
      Cc: Ingo Molnar <mingo@elte.hu>
      Cc: Peter Zijlstra <peterz@infradead.org>
      25502a6c
    • Paul E. McKenney's avatar
      rcu: shrink rcutiny by making synchronize_rcu_bh() be inline · da848c47
      Paul E. McKenney authored
      Because synchronize_rcu_bh() is identical to synchronize_sched(),
      make the former a static inline invoking the latter, saving the
      overhead of an EXPORT_SYMBOL_GPL() and the duplicate code.
      Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
      da848c47
  11. 06 May, 2010 1 commit
    • Tejun Heo's avatar
      sched: replace migration_thread with cpu_stop · 969c7921
      Tejun Heo authored
      Currently migration_thread is serving three purposes - migration
      pusher, context to execute active_load_balance() and forced context
      switcher for expedited RCU synchronize_sched.  All three roles are
      hardcoded into migration_thread() and determining which job is
      scheduled is slightly messy.
      
      This patch kills migration_thread and replaces all three uses with
      cpu_stop.  The three different roles of migration_thread() are
      splitted into three separate cpu_stop callbacks -
      migration_cpu_stop(), active_load_balance_cpu_stop() and
      synchronize_sched_expedited_cpu_stop() - and each use case now simply
      asks cpu_stop to execute the callback as necessary.
      
      synchronize_sched_expedited() was implemented with private
      preallocated resources and custom multi-cpu queueing and waiting
      logic, both of which are provided by cpu_stop.
      synchronize_sched_expedited_count is made atomic and all other shared
      resources along with the mutex are dropped.
      
      synchronize_sched_expedited() also implemented a check to detect cases
      where not all the callback got executed on their assigned cpus and
      fall back to synchronize_sched().  If called with cpu hotplug blocked,
      cpu_stop already guarantees that and the condition cannot happen;
      otherwise, stop_machine() would break.  However, this patch preserves
      the paranoid check using a cpumask to record on which cpus the stopper
      ran so that it can serve as a bisection point if something actually
      goes wrong theree.
      
      Because the internal execution state is no longer visible,
      rcu_expedited_torture_stats() is removed.
      
      This patch also renames cpu_stop threads to from "stopper/%d" to
      "migration/%d".  The names of these threads ultimately don't matter
      and there's no reason to make unnecessary userland visible changes.
      
      With this patch applied, stop_machine() and sched now share the same
      resources.  stop_machine() is faster without wasting any resources and
      sched migration users are much cleaner.
      Signed-off-by: default avatarTejun Heo <tj@kernel.org>
      Acked-by: default avatarPeter Zijlstra <peterz@infradead.org>
      Cc: Ingo Molnar <mingo@elte.hu>
      Cc: Dipankar Sarma <dipankar@in.ibm.com>
      Cc: Josh Triplett <josh@freedesktop.org>
      Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
      Cc: Oleg Nesterov <oleg@redhat.com>
      Cc: Dimitri Sivanich <sivanich@sgi.com>
      969c7921
  12. 26 Feb, 2010 1 commit
    • Paul E. McKenney's avatar
      rcu: Make rcu_read_lock_sched_held() take boot time into account · d9f1bb6a
      Paul E. McKenney authored
      Before the scheduler starts, all tasks are non-preemptible by
      definition. So, during that time, rcu_read_lock_sched_held()
      needs to always return "true".  This patch makes that be so.
      Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
      Cc: laijs@cn.fujitsu.com
      Cc: dipankar@in.ibm.com
      Cc: mathieu.desnoyers@polymtl.ca
      Cc: josh@joshtriplett.org
      Cc: dvhltc@us.ibm.com
      Cc: niv@us.ibm.com
      Cc: peterz@infradead.org
      Cc: rostedt@goodmis.org
      Cc: Valdis.Kletnieks@vt.edu
      Cc: dhowells@redhat.com
      LKML-Reference: <1267135607-7056-2-git-send-email-paulmck@linux.vnet.ibm.com>
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      d9f1bb6a
  13. 13 Jan, 2010 1 commit
  14. 17 Dec, 2009 1 commit
    • Frederic Weisbecker's avatar
      sched: Teach might_sleep() about preemptible RCU · 234da7bc
      Frederic Weisbecker authored
      In practice, it is harmless to voluntarily sleep in a
      rcu_read_lock() section if we are running under preempt rcu, but
      it is illegal if we build a kernel running non-preemptable rcu.
      
      Currently, might_sleep() doesn't notice sleepable operations
      under rcu_read_lock() sections if we are running under
      preemptable rcu because preempt_count() is left untouched after
      rcu_read_lock() in this case. But we want developers who test
      their changes under such config to notice the "sleeping while
      atomic" issues.
      
      So we add rcu_read_lock_nesting to prempt_count() in
      might_sleep() checks.
      
      [ v2: Handle rcu-tiny ]
      Signed-off-by: default avatarFrederic Weisbecker <fweisbec@gmail.com>
      Reviewed-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
      Cc: Peter Zijlstra <peterz@infradead.org>
      LKML-Reference: <1260991265-8451-1-git-send-regression-fweisbec@gmail.com>
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      234da7bc
  15. 22 Nov, 2009 2 commits
    • Paul E. McKenney's avatar
      rcu: Re-arrange code to reduce #ifdef pain · 6ebb237b
      Paul E. McKenney authored
      Remove #ifdefs from kernel/rcupdate.c and
      include/linux/rcupdate.h by moving code to
      include/linux/rcutiny.h, include/linux/rcutree.h, and
      kernel/rcutree.c.
      
      Also remove some definitions that are no longer used.
      Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
      Cc: laijs@cn.fujitsu.com
      Cc: dipankar@in.ibm.com
      Cc: mathieu.desnoyers@polymtl.ca
      Cc: josh@joshtriplett.org
      Cc: dvhltc@us.ibm.com
      Cc: niv@us.ibm.com
      Cc: peterz@infradead.org
      Cc: rostedt@goodmis.org
      Cc: Valdis.Kletnieks@vt.edu
      Cc: dhowells@redhat.com
      LKML-Reference: <1258908830885-git-send-email->
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      6ebb237b
    • Paul E. McKenney's avatar
      rcu: Eliminate unneeded function wrapping · 9f680ab4
      Paul E. McKenney authored
      The functions rcu_init() is a wrapper for __rcu_init(), and also
      sets up the CPU-hotplug notifier for rcu_barrier_cpu_hotplug().
      But TINY_RCU doesn't need CPU-hotplug notification, and the
      rcu_barrier_cpu_hotplug() is a simple wrapper for
      rcu_cpu_notify().
      
      So push rcu_init() out to kernel/rcutree.c and kernel/rcutiny.c
      and get rid of the wrapper function rcu_barrier_cpu_hotplug().
      Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
      Cc: laijs@cn.fujitsu.com
      Cc: dipankar@in.ibm.com
      Cc: mathieu.desnoyers@polymtl.ca
      Cc: josh@joshtriplett.org
      Cc: dvhltc@us.ibm.com
      Cc: niv@us.ibm.com
      Cc: peterz@infradead.org
      Cc: rostedt@goodmis.org
      Cc: Valdis.Kletnieks@vt.edu
      Cc: dhowells@redhat.com
      LKML-Reference: <12589088302320-git-send-email->
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      9f680ab4
  16. 26 Oct, 2009 2 commits
    • Ingo Molnar's avatar
      rcu: Do tiny cleanups in rcutiny · 4ce5b903
      Ingo Molnar authored
      No change in functionality - just straighten out a few small
      stylistic details.
      
      Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
      Cc: David Howells <dhowells@redhat.com>
      Cc: Josh Triplett <josh@joshtriplett.org>
      Cc: laijs@cn.fujitsu.com
      Cc: dipankar@in.ibm.com
      Cc: mathieu.desnoyers@polymtl.ca
      Cc: dvhltc@us.ibm.com
      Cc: niv@us.ibm.com
      Cc: peterz@infradead.org
      Cc: rostedt@goodmis.org
      Cc: Valdis.Kletnieks@vt.edu
      Cc: avi@redhat.com
      Cc: mtosatti@redhat.com
      LKML-Reference: <12565226351355-git-send-email->
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      4ce5b903
    • Paul E. McKenney's avatar
      rcu: "Tiny RCU", The Bloatwatch Edition · 9b1d82fa
      Paul E. McKenney authored
      This patch is a version of RCU designed for !SMP provided for a
      small-footprint RCU implementation.  In particular, the
      implementation of synchronize_rcu() is extremely lightweight and
      high performance. It passes rcutorture testing in each of the
      four relevant configurations (combinations of NO_HZ and PREEMPT)
      on x86.  This saves about 1K bytes compared to old Classic RCU
      (which is no longer in mainline), and more than three kilobytes
      compared to Hierarchical RCU (updated to 2.6.30):
      
      	CONFIG_TREE_RCU:
      
      	   text	   data	    bss	    dec	    filename
      	    183       4       0     187     kernel/rcupdate.o
      	   2783     520      36    3339     kernel/rcutree.o
      				   3526 Total (vs 4565 for v7)
      
      	CONFIG_TREE_PREEMPT_RCU:
      
      	   text	   data	    bss	    dec	    filename
      	    263       4       0     267     kernel/rcupdate.o
      	   4594     776      52    5422     kernel/rcutree.o
      	   			   5689 Total (6155 for v7)
      
      	CONFIG_TINY_RCU:
      
      	   text	   data	    bss	    dec	    filename
      	     96       4       0     100     kernel/rcupdate.o
      	    734      24       0     758     kernel/rcutiny.o
      	    			    858 Total (vs 848 for v7)
      
      The above is for x86.  Your mileage may vary on other platforms.
      Further compression is possible, but is being procrastinated.
      
      Changes from v7 (http://lkml.org/lkml/2009/10/9/388)
      
      o	Apply Lai Jiangshan's review comments (aside from
      might_sleep() 	in synchronize_sched(), which is covered by SMP builds).
      
      o	Fix up expedited primitives.
      
      Changes from v6 (http://lkml.org/lkml/2009/9/23/293).
      
      o	Forward ported to put it into the 2.6.33 stream.
      
      o	Added lockdep support.
      
      o	Make lightweight rcu_barrier.
      
      Changes from v5 (http://lkml.org/lkml/2009/6/23/12).
      
      o	Ported to latest pre-2.6.32 merge window kernel.
      
      	- Renamed rcu_qsctr_inc() to rcu_sched_qs().
      	- Renamed rcu_bh_qsctr_inc() to rcu_bh_qs().
      	- Provided trivial rcu_cpu_notify().
      	- Provided trivial exit_rcu().
      	- Provided trivial rcu_needs_cpu().
      	- Fixed up the rcu_*_enter/exit() functions in linux/hardirq.h.
      
      o	Removed the dependence on EMBEDDED, with a view to making
      	TINY_RCU default for !SMP at some time in the future.
      
      o	Added (trivial) support for expedited grace periods.
      
      Changes from v4 (http://lkml.org/lkml/2009/5/2/91) include:
      
      o	Squeeze the size down a bit further by removing the
      	->completed field from struct rcu_ctrlblk.
      
      o	This permits synchronize_rcu() to become the empty function.
      	Previous concerns about rcutorture were unfounded, as
      	rcutorture correctly handles a constant value from
      	rcu_batches_completed() and rcu_batches_completed_bh().
      
      Changes from v3 (http://lkml.org/lkml/2009/3/29/221) include:
      
      o	Changed rcu_batches_completed(), rcu_batches_completed_bh()
      	rcu_enter_nohz(), rcu_exit_nohz(), rcu_nmi_enter(), and
      	rcu_nmi_exit(), to be static inlines, as suggested by David
      	Howells.  Doing this saves about 100 bytes from rcutiny.o.
      	(The numbers between v3 and this v4 of the patch are not directly
      	comparable, since they are against different versions of Linux.)
      
      Changes from v2 (http://lkml.org/lkml/2009/2/3/333) include:
      
      o	Fix whitespace issues.
      
      o	Change short-circuit "||" operator to instead be "+" in order
      to 	fix performance bug noted by "kraai" on LWN.
      
      		(http://lwn.net/Articles/324348/)
      
      Changes from v1 (http://lkml.org/lkml/2009/1/13/440) include:
      
      o	This version depends on EMBEDDED as well as !SMP, as suggested
      	by Ingo.
      
      o	Updated rcu_needs_cpu() to unconditionally return zero,
      	permitting the CPU to enter dynticks-idle mode at any time.
      	This works because callbacks can be invoked upon entry to
      	dynticks-idle mode.
      
      o	Paul is now OK with this being included, based on a poll at
      the 	Kernel Miniconf at linux.conf.au, where about ten people said
      	that they cared about saving 900 bytes on single-CPU systems.
      
      o	Applies to both mainline and tip/core/rcu.
      Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
      Acked-by: default avatarDavid Howells <dhowells@redhat.com>
      Acked-by: Josh Triplett's avatarJosh Triplett <josh@joshtriplett.org>
      Reviewed-by: Lai Jiangshan's avatarLai Jiangshan <laijs@cn.fujitsu.com>
      Cc: dipankar@in.ibm.com
      Cc: mathieu.desnoyers@polymtl.ca
      Cc: dvhltc@us.ibm.com
      Cc: niv@us.ibm.com
      Cc: peterz@infradead.org
      Cc: rostedt@goodmis.org
      Cc: Valdis.Kletnieks@vt.edu
      Cc: avi@redhat.com
      Cc: mtosatti@redhat.com
      LKML-Reference: <12565226351355-git-send-email->
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      9b1d82fa