Message-ID: <20111116224447.GO2355@linux.vnet.ibm.com>
Date:	Wed, 16 Nov 2011 14:44:47 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	patches@...aro.org, "Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH tip/core/rcu 2/9] rcu: Add rcutorture system-shutdown
 capability

On Wed, Nov 16, 2011 at 02:15:45PM -0800, Josh Triplett wrote:
> On Wed, Nov 16, 2011 at 12:32:26PM -0800, Paul E. McKenney wrote:
> > On Tue, Nov 15, 2011 at 01:46:15PM -0800, Josh Triplett wrote:
> > > On Tue, Nov 15, 2011 at 12:27:58PM -0800, Paul E. McKenney wrote:
> > > > From: Paul E. McKenney <paul.mckenney@...aro.org>
> > > > 
> > > > Although it is easy to run rcutorture tests under KVM, there is currently
> > > > no nice way to run such a test for a fixed time period, collect all of
> > > > the rcutorture data, and then shut the system down cleanly.  This commit
> > > > therefore adds an rcutorture module parameter named "shutdown_secs" that
> > > > specified the run duration in seconds, after which rcutorture terminates
> > > > the test and powers the system down.  The default value for "shutdown_secs"
> > > > is zero, which disables shutdown.
> > > > 
> > > > Signed-off-by: Paul E. McKenney <paul.mckenney@...aro.org>
> > > > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > > 
> > > >From your recent post on this, I thought you found a solution through
> > > the init= parameter, which seems preferable.
> > 
> > For some things, the init= parameter does work great.  I do intend to
> > use it when collecting event-tracing and debugfs data, for example.
> > 
> > However, there is still a need for RCU torture testing that will operate
> > correctly regardless of how userspace is set up.  That, and there are
> > quite a few different kernel test setup, each with their own peculiar
> > capabilities and limitations.  So what happened was that before people
> > suggested the init= approach, I implemented enough of the in-kernel
> > approach to appreciate how much it simplifies life for the common case of
> > "just torture-test RCU".  As in I should have done this long ago.
> 
> Seems like it would work just as easily to point init at a statically
> linked C program which just sleeps for a fixed time and then shuts down.
> However, given the special-purpose nature of rcutorture, I won't
> complain that strongly.

I did consider a statically linked C program, but that can introduce the
need for cross-compilation into situations that do not otherwise need it.

> > > > --- a/kernel/rcutorture.c
> > > > +++ b/kernel/rcutorture.c
> > > > @@ -61,9 +61,10 @@ static int test_no_idle_hz;	/* Test RCU's support for tickless idle CPUs. */
> > > >  static int shuffle_interval = 3; /* Interval between shuffles (in sec)*/
> > > >  static int stutter = 5;		/* Start/stop testing interval (in sec) */
> > > >  static int irqreader = 1;	/* RCU readers from irq (timers). */
> > > > -static int fqs_duration = 0;	/* Duration of bursts (us), 0 to disable. */
> > > > -static int fqs_holdoff = 0;	/* Hold time within burst (us). */
> > > > +static int fqs_duration;	/* Duration of bursts (us), 0 to disable. */
> > > > +static int fqs_holdoff;		/* Hold time within burst (us). */
> > > 
> > > Looks like these lines picked up unrelated whitespace changes in this
> > > commit.
> > 
> > Turns out that my initial patch added another variable that I explicitly
> > initialized to zero.  Of course, checkpatch yelled at me about this, so
> > I figured I should fix the other nearby occurrences of this while I was
> > at it.  Doesn't really seem to me to be worth a separate patch, though.
> 
> Ah, I missed the removal of the initializer.  However, I don't see the
> harm in splitting out the trivial two-line patch, rather than folding it
> into an unrelated change which just happens to change lines nearby.

Ummm...  Laziness on my part?  ;-)

> > > > +static int
> > > > +rcu_torture_shutdown(void *arg)
> > > > +{
> > > > +	VERBOSE_PRINTK_STRING("rcu_torture_shutdown task started");
> > > > +	while (ULONG_CMP_LT(jiffies, shutdown_time) &&
> > > > +	       !kthread_should_stop()) {
> > > > +		if (verbose)
> > > > +			printk(KERN_ALERT "%s" TORTURE_FLAG
> > > > +			       "rcu_torture_shutdown task: %lu "
> > > > +			       "jiffies remaining\n",
> > > > +			       torture_type, shutdown_time - jiffies);
> > > > +		schedule_timeout_interruptible(HZ);
> > > > +	}
> > > 
> > > Any particular reason to wake up once a second here?  If !verbose, this could just
> > > sleep until shutdown time.  (And does the verbose output really help
> > > here, given printk timestamps?)
> > 
> > It actually did help me find a bug where it was failing to shut down.
> > I could use different code paths, but that would defeat the debugging.
> > 
> > So I increased the sleep time to 30 seconds.  Fair enough?
> 
> Well, now that you've debugged rcutorture's shutdown routine, would it
> suffice to have a printk when you actually go to shut down, without
> waking up for previous printks when not shutting down yet?
> 
> (The poll time doesn't really matter, and sleeping for 30 seconds before
> checking the time means you might overshoot by up to 30 seconds.  I'd
> like to avoid polling to begin with when you know exactly how long you
> need to sleep.)

Indeed, good points!  But please see below for what this function turns
into when taking that approach.

> > > > +	if (ULONG_CMP_LT(jiffies, shutdown_time)) {
> > > > +		VERBOSE_PRINTK_STRING("rcu_torture_shutdown task stopping");
> > > > +		return 0;
> > > > +	}
> > > > +
> > > > +	/* OK, shut down the system. */
> > > > +
> > > > +	VERBOSE_PRINTK_STRING("rcu_torture_shutdown task shutting down system");
> > > > +	shutdown_task = NULL;	/* Avoid self-kill deadlock. */
> > > 
> > > Not that it matters much here, but won't this cause a leak?
> > 
> > Only if we are shutting down.  And the alternative is a deadlock
> > where this task invokes kthread_stop() on itself.  ;-)
> 
> Hence why I said it didn't matter much. :)

;-)

								Thanx, Paul

------------------------------------------------------------------------

rcu_torture_shutdown(void *arg)
{
	long delta;
	unsigned long jiffies_snap;

	VERBOSE_PRINTK_STRING("rcu_torture_shutdown task started");
	jiffies_snap = ACCESS_ONCE(jiffies);
	while (ULONG_CMP_LT(jiffies_snap, shutdown_time) &&
	       !kthread_should_stop()) {
		delta = shutdown_time - jiffies_snap;
		if (verbose)
			printk(KERN_ALERT "%s" TORTURE_FLAG
			       "rcu_torture_shutdown task: %lu "
			       "jiffies remaining\n",
			       torture_type, delta);
		schedule_timeout_interruptible(delta);
		jiffies_snap = ACCESS_ONCE(jiffies);
	}
	if (ULONG_CMP_LT(jiffies_snap, shutdown_time)) {
		VERBOSE_PRINTK_STRING("rcu_torture_shutdown task stopping");
		return 0;
	}

	/* OK, shut down the system. */

	VERBOSE_PRINTK_STRING("rcu_torture_shutdown task shutting down system");
	shutdown_task = NULL;	/* Avoid self-kill deadlock. */
	rcu_torture_cleanup();	/* Get the success/failure message. */
	kernel_power_off();	/* Shut down the system. */
	return 0;
}

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