user/sven/linux.git/kernel/rcutree.c, branch v3.12.48

rcu: Use rcu_gp_kthread_wake() to wake up grace period kthreads

2014-11-19T17:38:26Z

commit 2aa792e6faf1a00f5accf1f69e87e11a390ba2cd upstream. The rcu_gp_kthread_wake() function checks for three conditions before waking up grace period kthreads: * Is the thread we are trying to wake up the current thread? * Are the gp_flags zero? (all threads wait on non-zero gp_flags condition) * Is there no thread created for this flavour, hence nothing to wake up? If any one of these condition is true, we do not call wake_up(). It was found that there are quite a few avoidable wake ups both during idle time and under stress induced by rcutorture. Idle: Total:66000, unnecessary:66000, case1:61827, case2:66000, case3:0 Total:68000, unnecessary:68000, case1:63696, case2:68000, case3:0 rcutorture: Total:254000, unnecessary:254000, case1:199913, case2:254000, case3:0 Total:256000, unnecessary:256000, case1:201784, case2:256000, case3:0 Here case{1-3} are the cases listed above. We can avoid these wake ups by using rcu_gp_kthread_wake() to conditionally wake up the grace period kthreads. There is a comment about an implied barrier supplied by the wake_up() logic. This barrier is necessary for the awakened thread to see the updated ->gp_flags. This flag is always being updated with the root node lock held. Also, the awakened thread tries to acquire the root node lock before reading ->gp_flags because of which there is proper ordering. Hence this commit tries to avoid calling wake_up() whenever we can by using rcu_gp_kthread_wake() function. Signed-off-by: Pranith Kumar CC: Mathieu Desnoyers Signed-off-by: Paul E. McKenney Cc: Kamal Mostafa Signed-off-by: Jiri Slaby

rcu: Make callers awaken grace-period kthread

2014-11-19T17:38:26Z

commit 48a7639ce80cf279834d0d44865e49ecd714f37d upstream. The rcu_start_gp_advanced() function currently uses irq_work_queue() to defer wakeups of the RCU grace-period kthread. This deferring is necessary to avoid RCU-scheduler deadlocks involving the rcu_node structure's lock, meaning that RCU cannot call any of the scheduler's wake-up functions while holding one of these locks. Unfortunately, the second and subsequent calls to irq_work_queue() are ignored, and the first call will be ignored (aside from queuing the work item) if the scheduler-clock tick is turned off. This is OK for many uses, especially those where irq_work_queue() is called from an interrupt or softirq handler, because in those cases the scheduler-clock-tick state will be re-evaluated, which will turn the scheduler-clock tick back on. On the next tick, any deferred work will then be processed. However, this strategy does not always work for RCU, which can be invoked at process level from idle CPUs. In this case, the tick might never be turned back on, indefinitely defering a grace-period start request. Note that the RCU CPU stall detector cannot see this condition, because there is no RCU grace period in progress. Therefore, we can (and do!) see long tens-of-seconds stalls in grace-period handling. In theory, we could see a full grace-period hang, but rcutorture testing to date has seen only the tens-of-seconds stalls. Event tracing demonstrates that irq_work_queue() is being called repeatedly to no effect during these stalls: The "newreq" event appears repeatedly from a task that is not one of the grace-period kthreads. In theory, irq_work_queue() might be fixed to avoid this sort of issue, but RCU's requirements are unusual and it is quite straightforward to pass wake-up responsibility up through RCU's call chain, so that the wakeup happens when the offending locks are released. This commit therefore makes this change. The rcu_start_gp_advanced(), rcu_start_future_gp(), rcu_accelerate_cbs(), rcu_advance_cbs(), __note_gp_changes(), and rcu_start_gp() functions now return a boolean which indicates when a wake-up is needed. A new rcu_gp_kthread_wake() does the wakeup when it is necessary and safe to do so: No self-wakes, no wake-ups if the ->gp_flags field indicates there is no need (as in someone else did the wake-up before we got around to it), and no wake-ups before the grace-period kthread has been created. Signed-off-by: Paul E. McKenney Cc: Peter Zijlstra Cc: Steven Rostedt Cc: Frederic Weisbecker Reviewed-by: Josh Triplett [ Pranith: backport to 3.13-stable: just rcu_gp_kthread_wake(), prereq for 2aa792e "rcu: Use rcu_gp_kthread_wake() to wake up grace period kthreads" ] Signed-off-by: Pranith Kumar Signed-off-by: Kamal Mostafa Signed-off-by: Jiri Slaby

Merge branches 'doc.2013.08.19a', 'fixes.2013.08.20a', 'sysidle.2013.08.31a' and 'torture.2013.08.20a' into HEAD

2013-08-31T21:44:45Z

doc.2013.08.19a: Documentation updates fixes.2013.08.20a: Miscellaneous fixes sysidle.2013.08.31a: Detect system-wide idle state. torture.2013.08.20a: rcutorture updates.

nohz_full: Force RCU's grace-period kthreads onto timekeeping CPU

2013-08-31T21:44:02Z

Because RCU's quiescent-state-forcing mechanism is used to drive the full-system-idle state machine, and because this mechanism is executed by RCU's grace-period kthreads, this commit forces these kthreads to run on the timekeeping CPU (tick_do_timer_cpu). To do otherwise would mean that the RCU grace-period kthreads would force the system into non-idle state every time they drove the state machine, which would be just a bit on the futile side. Signed-off-by: Paul E. McKenney Cc: Frederic Weisbecker Cc: Steven Rostedt Cc: Lai Jiangshan Reviewed-by: Josh Triplett

nohz_full: Add full-system-idle state machine

2013-08-31T21:43:50Z

This commit adds the state machine that takes the per-CPU idle data as input and produces a full-system-idle indication as output. This state machine is driven out of RCU's quiescent-state-forcing mechanism, which invokes rcu_sysidle_check_cpu() to collect per-CPU idle state and then rcu_sysidle_report() to drive the state machine. The full-system-idle state is sampled using rcu_sys_is_idle(), which also drives the state machine if RCU is idle (and does so by forcing RCU to become non-idle). This function returns true if all but the timekeeping CPU (tick_do_timer_cpu) are idle and have been idle long enough to avoid memory contention on the full_sysidle_state state variable. The rcu_sysidle_force_exit() may be called externally to reset the state machine back into non-idle state. For large systems the state machine is driven out of RCU's force-quiescent-state logic, which provides good scalability at the price of millisecond-scale latencies on the transition to full-system-idle state. This is not so good for battery-powered systems, which are usually small enough that they don't need to care about scalability, but which do care deeply about energy efficiency. Small systems therefore drive the state machine directly out of the idle-entry code. The number of CPUs in a "small" system is defined by a new NO_HZ_FULL_SYSIDLE_SMALL Kconfig parameter, which defaults to 8. Note that this is a build-time definition. Signed-off-by: Paul E. McKenney Cc: Frederic Weisbecker Cc: Steven Rostedt Cc: Lai Jiangshan [ paulmck: Use true and false for boolean constants per Lai Jiangshan. ] Reviewed-by: Josh Triplett [ paulmck: Simplify logic and provide better comments for memory barriers, based on review comments and questions by Lai Jiangshan. ]

rcu: Simplify _rcu_barrier() processing

2013-08-20T18:45:50Z

This commit drops an unneeded ACCESS_ONCE() and simplifies an "our work is done" check in _rcu_barrier(). This applies feedback from Linus (https://lkml.org/lkml/2013/7/26/777) that he gave to similar code in an unrelated patch. Signed-off-by: Paul E. McKenney Reviewed-by: Josh Triplett [ paulmck: Fix comment to match code, reported by Lai Jiangshan. ]

nohz_full: Add full-system-idle arguments to API

2013-08-19T01:59:03Z

This commit adds an isidle and jiffies argument to force_qs_rnp(), dyntick_save_progress_counter(), and rcu_implicit_dynticks_qs() to enable RCU's force-quiescent-state process to check for full-system idle. Signed-off-by: Paul E. McKenney Cc: Frederic Weisbecker Cc: Steven Rostedt Cc: Lai Jiangshan [ paulmck: Use true and false for boolean constants per Lai Jiangshan. ] Reviewed-by: Josh Triplett

nohz_full: Add per-CPU idle-state tracking

2013-08-19T01:58:43Z

This commit adds the code that updates the rcu_dyntick structure's new fields to track the per-CPU idle state based on interrupts and transitions into and out of the idle loop (NMIs are ignored because NMI handlers cannot cleanly read out the time anyway). This code is similar to the code that maintains RCU's idea of per-CPU idleness, but differs in that RCU treats CPUs running in user mode as idle, where this new code does not. Signed-off-by: Paul E. McKenney Acked-by: Frederic Weisbecker Cc: Steven Rostedt Reviewed-by: Josh Triplett

nohz_full: Add rcu_dyntick data for scalable detection of all-idle state

2013-08-19T01:58:31Z

This commit adds fields to the rcu_dyntick structure that are used to detect idle CPUs. These new fields differ from the existing ones in that the existing ones consider a CPU executing in user mode to be idle, where the new ones consider CPUs executing in user mode to be busy. The handling of these new fields is otherwise quite similar to that for the exiting fields. This commit also adds the initialization required for these fields. So, why is usermode execution treated differently, with RCU considering it a quiescent state equivalent to idle, while in contrast the new full-system idle state detection considers usermode execution to be non-idle? It turns out that although one of RCU's quiescent states is usermode execution, it is not a full-system idle state. This is because the purpose of the full-system idle state is not RCU, but rather determining when accurate timekeeping can safely be disabled. Whenever accurate timekeeping is required in a CONFIG_NO_HZ_FULL kernel, at least one CPU must keep the scheduling-clock tick going. If even one CPU is executing in user mode, accurate timekeeping is requires, particularly for architectures where gettimeofday() and friends do not enter the kernel. Only when all CPUs are really and truly idle can accurate timekeeping be disabled, allowing all CPUs to turn off the scheduling clock interrupt, thus greatly improving energy efficiency. This naturally raises the question "Why is this code in RCU rather than in timekeeping?", and the answer is that RCU has the data and infrastructure to efficiently make this determination. Signed-off-by: Paul E. McKenney Acked-by: Frederic Weisbecker Cc: Steven Rostedt Reviewed-by: Josh Triplett

rcu: Eliminate unused APIs intended for adaptive ticks

2013-08-19T01:06:44Z

The rcu_user_enter_after_irq() and rcu_user_exit_after_irq() functions were intended for use by adaptive ticks, but changes in implementation have rendered them unnecessary. This commit therefore removes them. Reported-by: Frederic Weisbecker Signed-off-by: Paul E. McKenney Reviewed-by: Josh Triplett