[PATCH tip/core/rcu 0/15] v3 RCU idle/no-CB changes for 3.10

From: Paul E. McKenney
Date: Mon Mar 18 2013 - 17:36:33 EST


This series contains changes to RCU_FAST_NO_HZ idle entry/exit and also
removes restrictions on no-CBs CPUs.

1. Remove restrictions on no-CBs CPUs.
2. Allow some control of no-CBs CPUs at kernel-build time. The option
of most interest is probably the one that makes -all- CPUs be
no-CBs CPUs.
3. Introduce proper blocking to grace-period waits for no-CBs CPUs.
4. Add event tracing for no-CBs CPU callback registration.
5. Add event tracing for no-CBs CPU grace periods.
6. Distinguish the no-CBs kthreads for the various RCU flavors.
Without this patch, CPU 0 would have up to three kthreads all
named "rcuo0", which is less than optimal. These kthreads
are now named "rcuob/0", "rcuop/0", and "rcuos/0".
7. Export RCU_FAST_NO_HZ parameters to sysfs to allow run-time
8. Re-introduce callback acceleration during grace-period cleanup.
Now that the callbacks are associated with specific grace periods,
such acceleration is idempotent, and it is now safe to accelerate
more than needed. (In contrast, in the past, too-frequent callback
acceleration resulted in infrequent RCU failures.)
9. Use the newly numbered callbacks to greatly reduce the CPU overhead
incurred at idle entry by RCU_FAST_NO_HZ. The fact that the
callbacks are now numbered means that instead of repeatedly
cranking the RCU state machine to try to get all callbacks
invoked, we can instead rely on the numbering so that the CPU
can take full advantage of any grace periods that elapse while
it is asleep. CPUs with callbacks still have limited sleep times,
especially if they have at least one non-lazy callback queued.
10-15. Allow CPUs to make known their need for future grace periods,
which is also used to reduce the need for frenetic RCU
state-machine cranking upon RCU_FAST_NO_HZ entry to idle.
10. Move the release of the root rcu_node structure's ->lock
to then end of rcu_start_gp().
11. Repurpose no-CB's grace-period event tracing to that of
future grace periods, which share no-CB's grace-period
12. Move the release of the root rcu_node structure's ->lock
to rcu_start_gp()'s callers.
13. Rename the rcu_node ->n_nocb_gp_requests field to
14. Abstract rcu_start_future_gp() from rcu_nocb_wait_gp()
to that RCU_FAST_NO_HZ can use the no-CB CPUs mechanism
for allowing a CPU to record its need for future grace
15. Make rcu_accelerate_cbs() note the need for future
grace periods, thus avoiding delays in starting grace
periods that currently happen due to the CPUs needing
those grace periods being out of action when the previous
grace period ends.

Changes since v2:

o Broke initial patch into smaller pieces.
o Significant additional testing completed.

Changes since v1:

o Fixed a deadlock in #1 spotted by Xie ChanglongX.
o Updated #2 to bring the abbreviations in line with conventional
per-CPU kthread naming.
o Moved the first two patches into their own group.

Thanx, Paul

b/Documentation/kernel-parameters.txt | 35 -
b/include/linux/rcupdate.h | 1
b/include/trace/events/rcu.h | 71 ++
b/init/Kconfig | 71 ++
b/kernel/rcutree.c | 279 +++++++---
b/kernel/rcutree.h | 43 -
b/kernel/rcutree_plugin.h | 935 +++++++++++++---------------------
b/kernel/rcutree_trace.c | 2
8 files changed, 756 insertions(+), 681 deletions(-)

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