How to turn scheduler tick on for current nohz_full CPU?

From: "Paul E. McKenney" <paulmck@linux.ibm.com>
To: fweisbec@gmail.com, tglx@linutronix.de, mingo@kernel.org
Cc: linux-kernel@vger.kernel.org, rcu@vger.kernel.org, peterz@infradead.org
Subject: How to turn scheduler tick on for current nohz_full CPU?
Date: Wed, 24 Jul 2019 04:53:31 -0700	[thread overview]
Message-ID: <20190724115331.GA29059@linux.ibm.com> (raw)

Hello!

One of the callback-invocation forward-progress issues turns out to
be nohz_full CPUs not turning their scheduling-clock interrupt back on
when running in kernel mode.  Given that callback floods can cause RCU's
callback-invocation loop to run for some time, it would be good for this
loop to re-enable this interrupt.  Of course, this problem applies to
pretty much any kernel code that might loop for an extended time period,
not just RCU.

I took a quick look at kernel/time/tick-sched.c and the closest thing
I found was tick_nohz_full_kick_cpu(), except that (1) it isn't clear
that this does much when invoked on the current CPU and (2) it doesn't
help in rcutorture TREE04.  In contrast, disabling NO_HZ_FULL and using
RCU_NOCB_CPU instead works quite well.

So what should I be calling instead of tick_nohz_full_kick_cpu() to
re-enable the current CPU's scheduling-clock interrupt?

							Thanx, Paul