Message-ID: <CAPnjgZ3Rg80YE6qFNB+VxqX9aPd4oQYsDa2pGAsEsJHiNFYAbw@mail.gmail.com>
Date:	Wed, 18 Jan 2012 14:51:20 -0800
From:	Simon Glass <sjg@...omium.org>
To:	paulmck@...ux.vnet.ibm.com
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

Hi Paul,

On Wed, Jan 18, 2012 at 2:43 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> 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.

OK I see, thank you. It does sound a bit complicated although the
chances of anyone actually doing this are probably remote.

I will send my patch to avoid getting into this situation and see what
you think.

Regards,
Simon
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