Re: How to turn scheduler tick on for current nohz_full CPU?

From: "Paul E. McKenney" <paulmck@linux.ibm.com>
To: Frederic Weisbecker <frederic@kernel.org>
Cc: fweisbec@gmail.com, tglx@linutronix.de, mingo@kernel.org,
	linux-kernel@vger.kernel.org, rcu@vger.kernel.org,
	peterz@infradead.org
Subject: Re: How to turn scheduler tick on for current nohz_full CPU?
Date: Wed, 24 Jul 2019 06:52:19 -0700	[thread overview]
Message-ID: <20190724135219.GY14271@linux.ibm.com> (raw)
In-Reply-To: <20190724132257.GA1029@lenoir>

On Wed, Jul 24, 2019 at 03:22:59PM +0200, Frederic Weisbecker wrote:
> On Wed, Jul 24, 2019 at 04:53:31AM -0700, Paul E. McKenney wrote:
> > 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?
> 
> Indeed, kernel code is assumed to be quick enough (between two extended grace
> periods) to avoid running the tick for RCU. But some long lasting kernel code
> may require to tick temporarily.
> 
> You can use tick_nohz_dep_set_cpu(cpu, TICK_DEP_MASK_RCU) with the
> following:
> 
> diff --git a/include/linux/tick.h b/include/linux/tick.h
> index f92a10b5e112..3f476e2a4bf7 100644
> --- a/include/linux/tick.h
> +++ b/include/linux/tick.h
> @@ -108,7 +108,8 @@ enum tick_dep_bits {
>  	TICK_DEP_BIT_POSIX_TIMER	= 0,
>  	TICK_DEP_BIT_PERF_EVENTS	= 1,
>  	TICK_DEP_BIT_SCHED		= 2,
> -	TICK_DEP_BIT_CLOCK_UNSTABLE	= 3
> +	TICK_DEP_BIT_CLOCK_UNSTABLE	= 3,
> +	TICK_DEP_BIT_RCU		= 4
>  };
>  
>  #define TICK_DEP_MASK_NONE		0
> @@ -116,6 +117,7 @@ enum tick_dep_bits {
>  #define TICK_DEP_MASK_PERF_EVENTS	(1 << TICK_DEP_BIT_PERF_EVENTS)
>  #define TICK_DEP_MASK_SCHED		(1 << TICK_DEP_BIT_SCHED)
>  #define TICK_DEP_MASK_CLOCK_UNSTABLE	(1 << TICK_DEP_BIT_CLOCK_UNSTABLE)
> +#define TICK_DEP_MASK_RCU		(1 << TICK_DEP_BIT_RCU)
>  
>  #ifdef CONFIG_NO_HZ_COMMON
>  extern bool tick_nohz_enabled;

I will give this a try, thank you!  (Testing will take a few days.)

							Thanx, Paul