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    SubjectRe: [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@linux.vnet.ibm.com> wrote:
    > > On Wed, Jan 18, 2012 at 01:08:13PM -0800, Simon Glass wrote:
    > >> [+cc Rafael J. Wysocki <rjw@sisk.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@linux.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@chromium.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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