From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Rafael J. Wysocki" Subject: Re: [PATCH 3/3] serial: 8250: Add a wakeup_capable module param Date: Thu, 19 Jan 2012 01:02:58 +0100 Message-ID: <201201190102.58788.rjw@sisk.pl> References: <1326826563-32215-1-git-send-email-sjg@chromium.org> <20120118224304.GJ2431@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Return-path: Received: from ogre.sisk.pl ([217.79.144.158]:40930 "EHLO ogre.sisk.pl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753740Ab2ARX7a (ORCPT ); Wed, 18 Jan 2012 18:59:30 -0500 In-Reply-To: <20120118224304.GJ2431@linux.vnet.ibm.com> Sender: linux-serial-owner@vger.kernel.org List-Id: linux-serial@vger.kernel.org To: paulmck@linux.vnet.ibm.com Cc: Simon Glass , Alan Cox , LKML , Greg Kroah-Hartman , linux-serial@vger.kernel.org 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 > > wrote: > > > On Wed, Jan 18, 2012 at 01:08:13PM -0800, Simon Glass wrote: > > >> [+cc Rafael J. Wysocki who I think wrote the wakeup.c code] > > >> > > >> Hi Alan, Paul, > > >> > > >> On Tue, Jan 17, 2012 at 8:17 PM, Paul E. McKenney > > >> 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 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. Thanks, Rafael