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Message-ID: <20111116225856.GD2325@leaf>
Date: Wed, 16 Nov 2011 14:58:56 -0800
From: Josh Triplett <josh@...htriplett.org>
To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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 02:44:47PM -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@...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.
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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