rcu: Let non-offloaded idle CPUs with callbacks defer tick
When a CPU goes idle, rcu_needs_cpu() is invoked to determine whether or
not RCU needs the scheduler-clock tick to keep interrupting. Right now,
RCU keeps the tick on for a given idle CPU if there are any non-offloaded
callbacks queued on that CPU.
But if all of these callbacks are waiting for a grace period to finish,
there is no point in scheduling a tick before that grace period has any
reasonable chance of completing. This commit therefore delays the tick
in the case where all the callbacks are waiting for a specific grace
period to elapse. In theory, this should result in a 50-70% reduction in
RCU-induced scheduling-clock ticks on mostly-idle CPUs. In practice, TBD.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
4 files changed
