user/sven/linux.git/kernel/rcutree_plugin.h, branch v3.3.1

rcu: Apply ACCESS_ONCE() to rcu_boost() return value

2011-12-11T18:33:19Z

Both TINY_RCU's and TREE_RCU's implementations of rcu_boost() access the ->boost_tasks and ->exp_tasks fields without preventing concurrent changes to these fields. This commit therefore applies ACCESS_ONCE in order to prevent compiler mischief. Signed-off-by: Paul E. McKenney Signed-off-by: Paul E. McKenney

Revert "rcu: Permit rt_mutex_unlock() with irqs disabled"

2011-12-11T18:33:18Z

This reverts commit 5342e269b2b58ee0b0b4168a94087faaa60d0567. The approach taken in this patch was deemed too abusive to mutexes, and thus too likely to result in maintenance problems in the future. Instead, we will disallow RCU read-side critical sections that partially overlap with interrupt-disbled code segments. Signed-off-by: Paul E. McKenney

rcu: Adaptive dyntick-idle preparation

2011-12-11T18:32:10Z

If there are other CPUs active at a given point in time, then there is a limit to what a given CPU can do to advance the current RCU grace period. Beyond this limit, attempting to force the RCU grace period forward will do nothing but consume energy burning CPU cycles. Therefore, this commit takes an adaptive approach to RCU_FAST_NO_HZ preparations for idle. It pushes the RCU core state machine for two cycles unconditionally, and then it will push from zero to three additional cycles, but only as long as the RCU core has work for this CPU to do immediately. The rcu_pending() function is used to check whether the RCU core has such work. Signed-off-by: Paul E. McKenney Signed-off-by: Paul E. McKenney

rcu: Keep invoking callbacks if CPU otherwise idle

2011-12-11T18:32:09Z

The rcu_do_batch() function that invokes callbacks for TREE_RCU and TREE_PREEMPT_RCU normally throttles callback invocation to avoid degrading scheduling latency. However, as long as the CPU would otherwise be idle, there is no downside to continuing to invoke any callbacks that have passed through their grace periods. In fact, processing such callbacks in a timely manner has the benefit of increasing the probability that the CPU can enter the power-saving dyntick-idle mode. Therefore, this commit allows callback invocation to continue beyond the preset limit as long as the scheduler does not have some other task to run and as long as context is that of the idle task or the relevant RCU kthread. Signed-off-by: Paul E. McKenney Signed-off-by: Paul E. McKenney

rcu: Permit dyntick-idle with callbacks pending

2011-12-11T18:32:07Z

The current implementation of RCU_FAST_NO_HZ prevents CPUs from entering dyntick-idle state if they have RCU callbacks pending. Unfortunately, this has the side-effect of often preventing them from entering this state, especially if at least one other CPU is not in dyntick-idle state. However, the resulting per-tick wakeup is wasteful in many cases: if the CPU has already fully responded to the current RCU grace period, there will be nothing for it to do until this grace period ends, which will frequently take several jiffies. This commit therefore permits a CPU that has done everything that the current grace period has asked of it (rcu_pending() == 0) even if it still as RCU callbacks pending. However, such a CPU posts a timer to wake it up several jiffies later (6 jiffies, based on experience with grace-period lengths). This wakeup is required to handle situations that can result in all CPUs being in dyntick-idle mode, thus failing to ever complete the current grace period. If a CPU wakes up before the timer goes off, then it cancels that timer, thus avoiding spurious wakeups. Signed-off-by: Paul E. McKenney Signed-off-by: Paul E. McKenney

rcu: Reduce latency of rcu_prepare_for_idle()

2011-12-11T18:32:03Z

Re-enable interrupts across calls to quiescent-state functions and also across force_quiescent_state() to reduce latency. Signed-off-by: Paul E. McKenney Signed-off-by: Paul E. McKenney

rcu: Eliminate RCU_FAST_NO_HZ grace-period hang

2011-12-11T18:32:02Z

With the new implementation of RCU_FAST_NO_HZ, it was possible to hang RCU grace periods as follows: o CPU 0 attempts to go idle, cycles several times through the rcu_prepare_for_idle() loop, then goes dyntick-idle when RCU needs nothing more from it, while still having at least on RCU callback pending. o CPU 1 goes idle with no callbacks. Both CPUs can then stay in dyntick-idle mode indefinitely, preventing the RCU grace period from ever completing, possibly hanging the system. This commit therefore prevents CPUs that have RCU callbacks from entering dyntick-idle mode. This approach also eliminates the need for the end-of-grace-period IPIs used previously. Signed-off-by: Paul E. McKenney Signed-off-by: Paul E. McKenney

rcu: Avoid needlessly IPIing CPUs at GP end

2011-12-11T18:32:01Z

If a CPU enters dyntick-idle mode with callbacks pending, it will need an IPI at the end of the grace period. However, if it exits dyntick-idle mode before the grace period ends, it will be needlessly IPIed at the end of the grace period. Therefore, this commit clears the per-CPU rcu_awake_at_gp_end flag when a CPU determines that it does not need it. This in turn requires disabling interrupts across much of rcu_prepare_for_idle() in order to avoid having nested interrupts clearing this state out from under us. Signed-off-by: Paul E. McKenney Signed-off-by: Paul E. McKenney

rcu: Go dyntick-idle more quickly if CPU has serviced current grace period

2011-12-11T18:32:00Z

The earlier version would attempt to push callbacks through five times before going into dyntick-idle mode if callbacks remained, but the CPU had done all that it needed to do for the current RCU grace periods. This is wasteful: In most cases, once the CPU has done all that it needs to for the current RCU grace periods, it will make no further progress on the callbacks no matter how many times it loops through the RCU core processing and the idle-entry code. This commit therefore goes to dyntick-idle mode whenever the current CPU has done all it can for the current grace period. Signed-off-by: Paul E. McKenney Signed-off-by: Paul E. McKenney

rcu: Add tracing for RCU_FAST_NO_HZ

2011-12-11T18:32:00Z

This commit adds trace_rcu_prep_idle(), which is invoked from rcu_prepare_for_idle() and rcu_wake_cpu() to trace attempts on the part of RCU to force CPUs into dyntick-idle mode. Signed-off-by: Paul E. McKenney Signed-off-by: Paul E. McKenney