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Subject[PATCH tip/core/rcu 0/15] Improvements to rcu_barrier() and RT response on big systems
Hello!

This patch series contains improvements to the rcu_barrier() family of
primitives and to latency for large systems. These are in a single
series due to conflicts that would otherwise occur. The individual
patches are as follows:

1. Allow the value for RCU_FANOUT_LEAF to be increased (but not
decreased!) via a boot-time parameter, in turn allowing a
default kernel build to be adjusted for low RCU grace-period
initialization latency on a large system.
2. Work around the new default NR_CPUS=4096 by checking the
boot-time-computed nr_cpu_ids, allowing this to override
NR_CPUS. This again reduces RCU grace-period initialization
latency for kernels built with large NR_CPUS running on small
systems.
3. Shrink a macro argument to keep lines under 80 characters.
4. Add a pointer in the rcu_state structure to the corresponding
member of the call_rcu() family of functions in preparation
for increasing rcu_barrier() concurrency.
5. Move _rcu_barrier()'s rcu_head structures to the per-CPU
per-RCU-flavor rcu_data structures so that different flavors
of rcu_barrier() do not need to contend for the rcu_head
structures.
6. Move rcu_barrier()'s rcu_barrier_cpu_count global variable to
a new ->barrier_cpu_count field in the rcu_state structure, so
that different flavors of rcu_barrier() do not need to contend
for this variable.
7. Move rcu_barrier()'s rcu_barrier_completion global variable to
a new ->barrier_completion field in the rcu_state structure, so
that different flavors of rcu_barrier() do not need to contend
for this variable.
8. Move rcu_barrier()'s rcu_barrier_mutex global variable to
a new ->barrier_mutex field in the rcu_state structure, so that
different flavors of rcu_barrier() do not need to contend for
this variable.
9. Introduce counter scheme to allow multiple concurrent executions
of a given flavor of rcu_barrier() to share work.
10. Add event tracing for _rcu_barrier().
11. Add debugfs tracing for _rcu_barrier().
12. Remove unnecessary per-CPU variable argument from
__rcu_process_callbacks().
13. Introduce for_each_rcu_flavor() iterator and use it. This provides
a nicer way to iterate through the RCU flavors to do per-flavor
processing.
14. Apply the for_each_rcu_flavor() iterator to debugfs tracing.
15. Remove dead-code gcc helper from code that is no longer ever dead.
Thanx, Paul

b/Documentation/kernel-parameters.txt | 4
b/include/trace/events/rcu.h | 45 +++++++
b/kernel/rcutree.c | 97 +++++++++++++--
b/kernel/rcutree.h | 23 ++-
b/kernel/rcutree_plugin.h | 4
b/kernel/rcutree_trace.c | 2
kernel/rcutree.c | 213 +++++++++++++++++++++-------------
kernel/rcutree.h | 22 ++-
kernel/rcutree_plugin.h | 126 --------------------
kernel/rcutree_trace.c | 134 ++++++++++++---------
10 files changed, 379 insertions(+), 291 deletions(-)


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