If a nohz_full CPU is idle or executing in userspace, it makes good sense
to keep it out of RCU core processing. After all, the RCU grace-period
kthread can see its quiescent states and all of its callbacks are
offloaded, so there is nothing for RCU core processing to do.
However, if a nohz_full CPU is executing in kernel space, the RCU
grace-period kthread cannot do anything for it, so such a CPU must report
its own quiescent states. This commit therefore makes nohz_full CPUs
skip RCU core processing only if the scheduler-clock interrupt caught
them in idle or in userspace.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
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