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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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