Re: [PATCH 3/3] serial: 8250: Add a wakeup_capable module param
From: Simon Glass
Date: Wed Jan 18 2012 - 21:36:02 EST
Hi Paul,
On Wed, Jan 18, 2012 at 5:37 PM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> On Thu, Jan 19, 2012 at 01:02:58AM +0100, Rafael J. Wysocki wrote:
>> On Wednesday, January 18, 2012, Paul E. McKenney 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@xxxxxxxxxxxxxxxxxx> wrote:
>> > > > On Wed, Jan 18, 2012 at 01:08:13PM -0800, Simon Glass wrote:
>> > > >> [+cc Rafael J. Wysocki <rjw@xxxxxxx> who I think wrote the wakeup.c code]
>> > > >>
>> > > >> Hi Alan, Paul,
>> > > >>
>> > > >> On Tue, Jan 17, 2012 at 8:17 PM, Paul E. McKenney
>> > > >> <paulmck@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxx> 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.
>>
>> Yes, you can, but then I'd say it's not necessary for user space to
>> be able to carry that out in a tight loop. So, it seems, alternatively,
>> we could make that loop a bit less tight, e.g. by adding an arbitrary
>> sleep to the user space interface for the "disable" case.
>
> Good point, that would work just as well and be simpler.
OK, well I am expecting that this will now be a very small patch to
change just serial_core.
Thanks for your help with this.
Regards,
Simon
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