Date:	Wed, 18 Jan 2012 14:43:04 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Simon Glass <sjg@...omium.org>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	LKML <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	linux-serial@...r.kernel.org, "Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: [PATCH 3/3] serial: 8250: Add a wakeup_capable module param

On Wed, Jan 18, 2012 at 02:15:59PM -0800, Simon Glass wrote:
> Hi Paul,
> 
> On Wed, Jan 18, 2012 at 1:42 PM, Paul E. McKenney
> <paulmck@...ux.vnet.ibm.com> wrote:
> > On Wed, Jan 18, 2012 at 01:08:13PM -0800, Simon Glass wrote:
> >> [+cc Rafael J. Wysocki <rjw@...k.pl> who I think wrote the wakeup.c code]
> >>
> >> Hi Alan, Paul,
> >>
> >> On Tue, Jan 17, 2012 at 8:17 PM, Paul E. McKenney
> >> <paulmck@...ux.vnet.ibm.com> wrote:
> >> > On Tue, Jan 17, 2012 at 08:10:36PM +0000, Alan Cox wrote:
> >> >> On Tue, 17 Jan 2012 10:56:03 -0800
> >> >> Simon Glass <sjg@...omium.org> wrote:
> >> >>
> >> >> > Since serial_core now does not make serial ports wake-up capable by
> >> >> > default, add a parameter to support this feature in the 8250 UART.
> >> >> > This is the only UART where I think this feature is useful.
> >> >>
> >> >> NAK
> >> >>
> >> >> Things should just work for users. Magic parameters is not an
> >> >> improvement. If its a performance problem someone needs to fix the rcu
> >> >> sync overhead or stop using rcu on that path.
> >>
> >> OK fair enough, I agree. Every level I move down the source tree
> >> affects more people though.
> >>
> >> >
> >> > I must say that I lack context here, even after looking at the patch,
> >> > but the synchronize_rcu_expedited() primitives can be used if the latency
> >> > of synchronize_rcu() is too large.
> >> >
> >>
> >> Let me provide a bit of context. The serial_core code seems to be the
> >> only place in the kernel that does this:
> >>
> >>               device_init_wakeup(tty_dev, 1);
> >>               device_set_wakeup_enable(tty_dev, 0);
> >>
> >> The first call makes the device wakeup capable and enables wakeup, The
> >> second call disabled wakeup.
> >>
> >> The code that removes the wakeup source looks like this:
> >>
> >> void wakeup_source_remove(struct wakeup_source *ws)
> >> {
> >>       if (WARN_ON(!ws))
> >>               return;
> >>
> >>       spin_lock_irq(&events_lock);
> >>       list_del_rcu(&ws->entry);
> >>       spin_unlock_irq(&events_lock);
> >>       synchronize_rcu();
> >> }
> >>
> >> The sync is there because we are about to destroy the actual ws
> >> structure (in wakeup_source_destroy()). I wonder if it should be in
> >> wakeup_source_destroy() but that wouldn't help me anyway.
> >>
> >> synchronize_rcu_expedited() is a bit faster but not really fast
> >> enough. Anyway surely people will complain if I put this in the wakeup
> >> code - it will affect all wakeup users. It seems to me that the right
> >> solution is to avoid enabling and then immediately disabling wakeup.
> >
> > Hmmm...  What hardware are you running this one?  Normally,
> > synchronize_rcu_expedited() will be a couple of orders of magnitude
> > faster than synchronize_rcu().
> >
> >> I assume we can't and shouldn't change device_init_wakeup() . We could
> >> add a call like device_init_wakeup_disabled() which makes the device
> >> wakeup capable but does not actually enable it. Does that work?
> >
> > If the only reason for the synchronize_rcu() is to defer the pair of
> > kfree()s in wakeup_source_destroy(), then another possible approach
> > would be to remove the synchronize_rcu() from wakeup_source_remove()
> > and then use call_rcu() to defer the two kfree()s.
> >
> > If this is a reasonable change to make, the approach is as follows:
> >
> > 1.      Add a struct rcu_head to wakeup_source, call it "rcu".
> >        Or adjust the following to suit your choice of name.
> >
> > 2.      Replace the pair of kfree()s with:
> >
> >                call_rcu(&ws->rcu, wakeup_source_destroy_rcu);
> >
> > 3.      Create the wakeup_source_destroy_rcu() as follows:
> >
> >        static void wakeup_source_destroy_rcu(struct rcu_head *head)
> >        {
> >                struct wakeup_source *ws =
> >                        container_of(head, struct wakeup_source, rcu);
> >
> >                kfree(ws->name);
> >                kfree(ws);
> >        }
> >
> > Of course, this assumes that it is OK for wakeup_source_unregister()
> > to return before the memory is freed up.  This often is OK, but there
> > are some cases where the caller requires that there be no further
> > RCU readers with access to the old data.  In these cases, you really
> > do need the wait.
> 
> Thanks very much for that. I'm not sure if it is a reasonable change,
> but it does bug me that we add it to a data structure knowing that we
> will immediately remove it!
> 
> >From what I can see, making a device wakeup-enabled mostly happens on
> init or in response to a request to the driver (presumably from user
> space). In the latter case I suspect the synchronise_rcu() is fine. In
> the former it feels like we should make up our minds which of the
> three options is required (incapable, capable but not enabled, capable
> and enabled).
> 
> I will try a patch first based on splitting the two options (capable
> and enable) and see if that get a NAK.
> 
> Then I will come back to your solution - it seems fine to me and not a
> lot of code. Do we have to worry about someone enabling, disabled,
> enabling and then disabling wakeup quickly? Will this method break in
> that case if the second call to call_rcu() uses the same wc->rcu?

There are a couple of questions here, let me take them one at a time:

1.	If you just disabled, can you immediately re-enable?

	The answer is "yes".  The reason that this works is that you
	allocate a new structure for the re-enabling, and that new
	structure has its own rcu_head field.

2.	If you repeatedly disable and re-enable in a tight loop,
	can this cause problems?

	The answer to this is also "yes" -- you can run the system
	out of memory doing that.  However, there are a number of
	simple ways to avoid this problem:

	a.	Do a synchronize_rcu() on every (say) thousandth
		disable operation.

	b.	As above, but only do the synchronize_rcu() if
		all 1,000 disable operations occurred within
		(say) a second of each other.

	c.	As above, but actually count the number of
		pending call_rcu() callbacks.

	Both (a) and (b) can be carried out on a per-CPU basis if there
	is no convenient locked structure in which to track the state.
	You cannot carry (c) out on a per-CPU basis because RCU callbacks
	can sometimes be invoked on a different CPU from the one that
	call_rcu()ed them.  Rare, but it can happen.

	I would expect that option (a) would work in almost all cases.

If this can be exercised freely from user space, then you probably
really do need #2 above.

						Thanx, Paul

> Regards,
> Simon
> >
> >                                                        Thanx, Paul
> >
> 

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