The nocb callbacks generated before the nocb kthreads are spawned are
enqueued in the nocb queue for later processing. Commit fbce7497ee5af ("rcu:
Parallelize and economize NOCB kthread wakeups") introduced nocb leader kthreads
which checked the nocb_leader_wake flag to see if there were any such pending
callbacks. A case was reported in which newly spawned leader kthreads were not
processing the pending callbacks as this flag was not set, which led to a boot
hang.

The following commit ensures that the newly spawned nocb kthreads process the
pending callbacks by allowing the kthreads to run immediately after spawning
instead of waiting. This is done by inverting the logic of nocb_leader_wake
tests to nocb_leader_sleep which allows us to use the default initialization of
this flag to 0 to let the kthreads run.

Reported-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Link: http://www.spinics.net/lists/kernel/msg1802899.html
[ paulmck: Backported to v3.17-rc2. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Amit Shah <amit.shah@redhat.com>

doc.2014.07.08a: Documentation updates.
fixes.2014.07.09a: Miscellaneous fixes.
maintainers.2014.07.08b: Maintainership updates.
nocbs.2014.07.07a: Callback-offloading fixes.
torture.2014.07.07a: Torture-test updates.

RCU priority boosting currently checks for boosting via a pointer in
task_struct.  However, this is not needed: As Oleg noted, if the
rt_mutex is placed in the rcu_node instead of on the booster's stack,
the boostee can simply check it see if it owns the lock.  This commit
makes this change, shrinking task_struct by one pointer and the kernel
by thirteen lines.

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

The current approach to RCU priority boosting uses an rt_mutex strictly
for its priority-boosting side effects.  The rt_mutex_init_proxy_locked()
function is used by the booster to initialize the lock as held by the
boostee.  The booster then uses rt_mutex_lock() to acquire this rt_mutex,
which priority-boosts the boostee.  When the boostee reaches the end
of its outermost RCU read-side critical section, it checks a field in
its task structure to see whether it has been boosted, and, if so, uses
rt_mutex_unlock() to release the rt_mutex.  The booster can then go on
to boost the next task that is blocking the current RCU grace period.

But reasonable implementations of rt_mutex_unlock() might result in the
boostee referencing the rt_mutex's data after releasing it.  But the
booster might have re-initialized the rt_mutex between the time that the
boostee released it and the time that it later referenced it.  This is
clearly asking for trouble, so this commit introduces a completion that
forces the booster to wait until the boostee has completely finished with
the rt_mutex, thus avoiding the case where the booster is re-initializing
the rt_mutex before the last boostee's last reference to that rt_mutex.

This of course does introduce some overhead, but the priority-boosting
code paths are miles from any possible fastpath, and the overhead of
executing the completion will normally be quite small compared to the
overhead of priority boosting and deboosting, so this should be OK.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

An 80-CPU system with a context-switch-heavy workload can require so
many NOCB kthread wakeups that the RCU grace-period kthreads spend several
tens of percent of a CPU just awakening things.  This clearly will not
scale well: If you add enough CPUs, the RCU grace-period kthreads would
get behind, increasing grace-period latency.

To avoid this problem, this commit divides the NOCB kthreads into leaders
and followers, where the grace-period kthreads awaken the leaders each of
whom in turn awakens its followers.  By default, the number of groups of
kthreads is the square root of the number of CPUs, but this default may
be overridden using the rcutree.rcu_nocb_leader_stride boot parameter.
This reduces the number of wakeups done per grace period by the RCU
grace-period kthread by the square root of the number of CPUs, but of
course by shifting those wakeups to the leaders.  In addition, because
the leaders do grace periods on behalf of their respective followers,
the number of wakeups of the followers decreases by up to a factor of two.
Instead of being awakened once when new callbacks arrive and again
at the end of the grace period, the followers are awakened only at
the end of the grace period.

For a numerical example, in a 4096-CPU system, the grace-period kthread
would awaken 64 leaders, each of which would awaken its 63 followers
at the end of the grace period.  This compares favorably with the 79
wakeups for the grace-period kthread on an 80-CPU system.

Reported-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Commit ac1bea85781e (Make cond_resched() report RCU quiescent states)
fixed a problem where a CPU looping in the kernel with but one runnable
task would give RCU CPU stall warnings, even if the in-kernel loop
contained cond_resched() calls.  Unfortunately, in so doing, it introduced
performance regressions in Anton Blanchard's will-it-scale "open1" test.
The problem appears to be not so much the increased cond_resched() path
length as an increase in the rate at which grace periods complete, which
increased per-update grace-period overhead.

This commit takes a different approach to fixing this bug, mainly by
moving the RCU-visible quiescent state from cond_resched() to
rcu_note_context_switch(), and by further reducing the check to a
simple non-zero test of a single per-CPU variable.  However, this
approach requires that the force-quiescent-state processing send
resched IPIs to the offending CPUs.  These will be sent only once
the grace period has reached an age specified by the boot/sysfs
parameter rcutree.jiffies_till_sched_qs, or once the grace period
reaches an age halfway to the point at which RCU CPU stall warnings
will be emitted, whichever comes first.

Reported-by: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Christoph Lameter <cl@gentwo.org>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
[ paulmck: Made rcu_momentary_dyntick_idle() as suggested by the
  ktest build robot.  Also fixed smp_mb() comment as noted by
  Oleg Nesterov. ]

Merge with e552592e (Reduce overhead of cond_resched() checks for RCU)

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

doc.2014.04.29a:  Documentation updates.
fixes.2014.04.29a:  Miscellaneous fixes.
torture.2014.05.14a:  RCU/Lock torture tests.

This commit allows rcutorture to print additional state for the
RCU grace-period kthreads in cases where RCU seems reluctant to
start a new grace period.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

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 <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

The ->preemptible field in rcu_data is only initialized in the function
rcu_init_percpu_data(), and never used.  This commit therefore removes
this field.

Signed-off-by: Iulia Manda <iulia.manda21@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>