Re: [PATCH tip/core/rcu 2/9] rcu: Add rcutorture system-shutdowncapability

[Josh Triplett]

On Wed, Nov 16, 2011 at 02:44:47PM -0800, Paul E. McKenney wrote:
> 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@xxxxxxxxxx>
> > > > > 
> > > > > 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@xxxxxxxxxx>
> > > > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > > > 
> > > > >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.

Wouldn't you need to cross-compile the kernel anyway in such situations?

> > > > > +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.

See below for responses; that version seems like an improvement, though
it could still improve further.

> rcu_torture_shutdown(void *arg)
> {
> 	long delta;
> 	unsigned long jiffies_snap;
> 
> 	VERBOSE_PRINTK_STRING("rcu_torture_shutdown task started");
> 	jiffies_snap = ACCESS_ONCE(jiffies);

Why do you need to snapshot jiffies in this version but not in the
version you originally posted?

> 	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);

I suggested dropping this print entirely; under normal circumstances it
should never print.  It will only print if
schedule_timeout_interruptible wakes up spuriously.

> 		schedule_timeout_interruptible(delta);
> 		jiffies_snap = ACCESS_ONCE(jiffies);
> 	}

Any reason this entire loop body couldn't just become
msleep_interruptible()?

> 	if (ULONG_CMP_LT(jiffies_snap, shutdown_time)) {
> 		VERBOSE_PRINTK_STRING("rcu_torture_shutdown task stopping");
> 		return 0;
> 	}

Writing this as "if (kthread_should_stop())" seems clearer.

- Josh Triplett
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