rcu: Employ jiffies-based backstop to callback time limit

rcu: Employ jiffies-based backstop to callback time limit

Currently, if there are more than 100 ready-to-invoke RCU callbacks queued
on a given CPU, the rcu_do_batch() function sets a timeout for invocation
of the series.  This timeout defaulting to three milliseconds, and may
be adjusted using the rcutree.rcu_resched_ns kernel boot parameter.
This timeout is checked using local_clock(), but the overhead of this
function combined with the common-case very small callback-invocation
overhead means that local_clock() is checked every 32nd invocation.

This works well except for longer-than average callbacks.  For example,
a series of 500-microsecond-duration callbacks means that local_clock()
is checked only once every 16 milliseconds, which makes it difficult to
enforce a three-millisecond timeout.

This commit therefore adds a Kconfig option RCU_DOUBLE_CHECK_CB_TIME
that enables backup timeout checking using the coarser grained but
lighter weight jiffies.  If the jiffies counter detects a timeout,
then local_clock() is consulted even if this is not the 32nd callback.
This prevents the aforementioned 16-millisecond latency blow.

Reported-by: Domas Mituzas <dmituzas@meta.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>