Re: [PATCH RFC v2 tip/core/rcu 03/22] rcutorture: Add flag to produce non-busy-wait task stalls

From: "Paul E. McKenney" <paulmck@kernel.org>
To: Hillf Danton <hdanton@sina.com>
Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
	kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com,
	dipankar@in.ibm.com, akpm@linux-foundation.org,
	mathieu.desnoyers@efficios.com, josh@joshtriplett.org,
	tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org,
	dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com,
	oleg@redhat.com, joel@joelfernandes.org
Subject: Re: [PATCH RFC v2 tip/core/rcu 03/22] rcutorture: Add flag to produce non-busy-wait task stalls
Date: Thu, 19 Mar 2020 05:38:12 -0700
Message-ID: <20200319123812.GA3199@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20200319104614.11444-1-hdanton@sina.com>

On Thu, Mar 19, 2020 at 06:46:14PM +0800, Hillf Danton wrote:
> 
> On Wed, 18 Mar 2020 17:10:41 -0700
> > From: "Paul E. McKenney" <paulmck@kernel.org>
> > 
> > This commit aids testing of RCU task stall warning messages by adding
> > an rcutorture.stall_cpu_block module parameter that results in the
> > induced stall sleeping within the RCU read-side critical section.
> > Spinning with interrupts disabled is still available via the
> > rcutorture.stall_cpu_irqsoff module parameter, and specifying neither
> > of these two module parameters will spin with preemption disabled.
> > 
> > Note that sleeping (as opposed to preemption) results in additional
> > complaints from RCU at context-switch time, so yet more testing.
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > ---
> >  Documentation/admin-guide/kernel-parameters.txt |  5 +++++
> >  kernel/rcu/rcutorture.c                         | 15 +++++++++------
> >  2 files changed, 14 insertions(+), 6 deletions(-)
> > 
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index 6d16b78..17eff15 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -4161,6 +4161,11 @@
> >  			Duration of CPU stall (s) to test RCU CPU stall
> >  			warnings, zero to disable.
> >  
> > +	rcutorture.stall_cpu_block= [KNL]
> > +			Sleep while stalling if set.  This will result
> > +			in warnings from preemptible RCU in addition
> > +			to any other stall-related activity.
> > +
> >  	rcutorture.stall_cpu_holdoff= [KNL]
> >  			Time to wait (s) after boot before inducing stall.
> >  
> > diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> > index b3301f3..ada5b91 100644
> > --- a/kernel/rcu/rcutorture.c
> > +++ b/kernel/rcu/rcutorture.c
> > @@ -102,6 +102,7 @@ torture_param(int, stall_cpu, 0, "Stall duration (s), zero to disable.");
> >  torture_param(int, stall_cpu_holdoff, 10,
> >  	     "Time to wait before starting stall (s).");
> >  torture_param(int, stall_cpu_irqsoff, 0, "Disable interrupts while stalling.");
> > +torture_param(int, stall_cpu_block, 0, "Sleep while stalling.");
> >  torture_param(int, stat_interval, 60,
> >  	     "Number of seconds between stats printk()s");
> >  torture_param(int, stutter, 5, "Number of seconds to run/halt test");
> > @@ -1599,6 +1600,7 @@ static int rcutorture_booster_init(unsigned int cpu)
> >   */
> >  static int rcu_torture_stall(void *args)
> >  {
> > +	int idx;
> >  	unsigned long stop_at;
> >  
> >  	VERBOSE_TOROUT_STRING("rcu_torture_stall task started");
> > @@ -1610,21 +1612,22 @@ static int rcu_torture_stall(void *args)
> >  	if (!kthread_should_stop()) {
> >  		stop_at = ktime_get_seconds() + stall_cpu;
> >  		/* RCU CPU stall is expected behavior in following code. */
> > -		rcu_read_lock();
> > +		idx = cur_ops->readlock();
> >  		if (stall_cpu_irqsoff)
> >  			local_irq_disable();
> > -		else
> > +		else if (!stall_cpu_block)
> >  			preempt_disable();
> >  		pr_alert("rcu_torture_stall start on CPU %d.\n",
> > -			 smp_processor_id());
> > +			 raw_smp_processor_id());
> >  		while (ULONG_CMP_LT((unsigned long)ktime_get_seconds(),
> >  				    stop_at))
> > -			continue;  /* Induce RCU CPU stall warning. */
> > +			if (stall_cpu_block)
> > +				schedule_timeout_uninterruptible(HZ);
> 
> Why is the scheduled-in task so special that it will be running on
> the current CPU with irq disabled?

You lost me on this one.  IRQs are not at all disabled.

Oh, you mean the _uninterruptible suffix?

That only affects accounting during the sleep.  Since this is rcutorture,
the exact suffix is not all that relevant.  So the current state is that
rcutorture tends to use _uninterruptible.

Or am I missing your point?

							Thanx, Paul

> >  		if (stall_cpu_irqsoff)
> >  			local_irq_enable();
> > -		else
> > +		else if (!stall_cpu_block)
> >  			preempt_enable();
> > -		rcu_read_unlock();
> > +		cur_ops->readunlock(idx);
> >  		pr_alert("rcu_torture_stall end.\n");
> >  	}
> >  	torture_shutdown_absorb("rcu_torture_stall");
> > -- 
> > 2.9.5
> 