rcu/nocb: Use the rcuog CPU's ->nocb_timer

rcu/nocb: Use the rcuog CPU's ->nocb_timer

Currently each CPU has its own ->nocb_timer queued when the nocb_gp
wakeup must be deferred.  This approach has many drawbacks, compared to
a solution based on a single timer per NOCB group:

* There are a lot of timers to maintain.

* The per-rdp ->nocb_lock must be held to queue and cancel the timer
  and this lock can already be heavily contended.

* One timer firing doesn't cancel the other timers in the same group:
  - These other timers can thus cause spurious wakeups
  - Each rdp that queued a timer must lock both ->nocb_lock and then
    ->nocb_gp_lock upon exit from the kernel to idle/user/guest mode.

* We can't cancel all of them if we detect an unflushed bypass in
  nocb_gp_wait(). In fact currently we only ever cancel the ->nocb_timer
  of the leader group.

* The leader group's nocb_timer is cancelled without locking ->nocb_lock
  in nocb_gp_wait().  This currently appears to be safe but is an
  accident waiting to happen.

* Since the timer acquires ->nocb_lock, it requires extra care in the
  NOCB (de-)offloading process, requiring that it be either enabled or
  disabled and then flushed.

This commit instead uses the rcuog kthread's CPU's ->nocb_timer instead.
It is protected by nocb_gp_lock, which is _way_ less contended and
remains so even after this change.  As a matter of fact, the nocb_timer
almost never fires and the deferred wakeup is mostly carried out upon
idle/user/guest entry.  Now the early check performed at this point in
do_nocb_deferred_wakeup() is done on rdp_gp->nocb_defer_wakeup, which
is of course racy.  However, this raciness is harmless because we only
need the guarantee that the timer is queued if we were the last one to
queue it.  Any other situation (another CPU has queued it and we either
see it or not) is fine.

This solves all the issues listed above.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>