From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755237Ab2ARWoP (ORCPT ); Wed, 18 Jan 2012 17:44:15 -0500 Received: from e32.co.us.ibm.com ([32.97.110.150]:39476 "EHLO e32.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754894Ab2ARWoN (ORCPT ); Wed, 18 Jan 2012 17:44:13 -0500 Date: Wed, 18 Jan 2012 14:43:04 -0800 From: "Paul E. McKenney" To: Simon Glass Cc: Alan Cox , LKML , Greg Kroah-Hartman , linux-serial@vger.kernel.org, "Rafael J. Wysocki" Subject: Re: [PATCH 3/3] serial: 8250: Add a wakeup_capable module param Message-ID: <20120118224304.GJ2431@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1326826563-32215-1-git-send-email-sjg@chromium.org> <1326826563-32215-3-git-send-email-sjg@chromium.org> <20120117201036.6d99f98f@pyramind.ukuu.org.uk> <20120118041720.GA2431@linux.vnet.ibm.com> <20120118214215.GH2431@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-Content-Scanned: Fidelis XPS MAILER x-cbid: 12011822-3270-0000-0000-00000350AE86 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. Thanx, Paul > Regards, > Simon > > > >                                                        Thanx, Paul > > >