rcu: Reduce cache-miss initialization latencies for large systems

Commit #0209f649 (rcu: limit rcu_node leaf-level fanout) set an upper
limit of 16 on the leaf-level fanout for the rcu_node tree.  This was
needed to reduce lock contention that was induced by the synchronization
of scheduling-clock interrupts, which was in turn needed to improve
energy efficiency for moderate-sized lightly loaded servers.

However, reducing the leaf-level fanout means that there are more
leaf-level rcu_node structures in the tree, which in turn means that
RCU's grace-period initialization incurs more cache misses.  This is
not a problem on moderate-sized servers with only a few tens of CPUs,
but becomes a major source of real-time latency spikes on systems with
many hundreds of CPUs.  In addition, the workloads running on these large
systems tend to be CPU-bound, which eliminates the energy-efficiency
advantages of synchronizing scheduling-clock interrupts.  Therefore,
these systems need maximal values for the rcu_node leaf-level fanout.

This commit addresses this problem by introducing a new kernel parameter
named RCU_FANOUT_LEAF that directly controls the leaf-level fanout.
This parameter defaults to 16 to handle the common case of a moderate
sized lightly loaded servers, but may be set higher on larger systems.

Reported-by: Mike Galbraith <efault@gmx.de>
Reported-by: Dimitri Sivanich <sivanich@sgi.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
rcu: Document why rcu_blocking_is_gp() is safe

The rcu_blocking_is_gp() function tests to see if there is only one
online CPU, and if so, synchronize_sched() and friends become no-ops.
However, for larger systems, num_online_cpus() scans a large vector,
and might be preempted while doing so.  While preempted, any number
of CPUs might come online and go offline, potentially resulting in
num_online_cpus() returning 1 when there never had only been one
CPU online.  This could result in a too-short RCU grace period, which
could in turn result in total failure, except that the only way that
the grace period is too short is if there is an RCU read-side critical
section spanning it.  For RCU-sched and RCU-bh (which are the only
cases using rcu_blocking_is_gp()), RCU read-side critical sections
have either preemption or bh disabled, which prevents CPUs from going
offline.  This in turn prevents actual failures from occurring.

This commit therefore adds a large block comment to rcu_blocking_is_gp()
documenting why it is safe.  This commit also moves rcu_blocking_is_gp()
into kernel/rcutree.c, which should help prevent unwary developers from
mistaking it for a generally useful function.

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