Commit [d21670] Maximize Restore History

rcu: reduce the number of spurious RCU_SOFTIRQ invocations

Lai Jiangshan noted that up to 10% of the RCU_SOFTIRQ are spurious, and
traced this down to the fact that the current grace-period machinery
will uselessly raise RCU_SOFTIRQ when a given CPU needs to go through
a quiescent state, but has not yet done so. In this situation, there
might well be nothing that RCU_SOFTIRQ can do, and the overhead can be
worth worrying about in the ksoftirqd case. This patch therefore avoids
raising RCU_SOFTIRQ in this situation.

Changes since v1 (http://lkml.org/lkml/2010/3/30/122 from Lai Jiangshan):

o Omit the rcu_qs_pending() prechecks, as they aren't that
much less expensive than the quiescent-state checks.

o Merge with the set_need_resched() patch that reduces IPIs.

o Add the new n_rp_report_qs field to the rcu_pending tracing output.

o Update the tracing documentation accordingly.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Paul E. McKenney Paul E. McKenney 2010-04-15

changed Documentation/RCU/trace.txt
changed kernel/rcutree.c
changed kernel/rcutree.h
changed kernel/rcutree_trace.c
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