From mboxrd@z Thu Jan 1 00:00:00 1970 From: mark gross <640e9920@gmail.com> Subject: Re: [PATCH 1/8] PM: Opportunistic suspend support. Date: Sun, 30 May 2010 13:32:17 -0700 Message-ID: <20100530203217.GD25545__38607.5595430613$1275251663$gmane$org@gvim.org> References: <1274863342.5882.4850.camel@twins> <1274863987.5882.4892.camel@twins> <20100526124929.GA32580@srcf.ucam.org> <1274878665.27810.354.camel@twins> <20100526132051.GA1834@srcf.ucam.org> <20100527172354.43e46cef@notabene.brown> <20100529025215.GB11600@gvim.org> <20100530180846.408e50be@notabene.brown> Reply-To: markgross@thegnar.org Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Return-path: Content-Disposition: inline In-Reply-To: <20100530180846.408e50be@notabene.brown> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: linux-pm-bounces@lists.linux-foundation.org Errors-To: linux-pm-bounces@lists.linux-foundation.org To: Neil Brown Cc: Wu Fengguang , Andi Kleen , markgross@thegnar.org, Len Brown , linux-doc@vger.kernel.org, Peter Zijlstra , Greg KH , Kernel development list , Jesse Barnes , James Bottomley , Tejun Heo , tytso@mit.edu, Linux-pm mailing list , Dmitry Torokhov , Andrew Morton List-Id: linux-pm@vger.kernel.org On Sun, May 30, 2010 at 06:08:46PM +1000, Neil Brown wrote: > On Fri, 28 May 2010 21:04:53 -0700 > Arve Hj=F8nnev=E5g wrote: > = > > On Fri, May 28, 2010 at 7:52 PM, mark gross <640e9920@gmail.com> wrote: > > > On Thu, May 27, 2010 at 05:23:54PM +1000, Neil Brown wrote: > > >> On Wed, 26 May 2010 14:20:51 +0100 > > >> Matthew Garrett wrote: > > >> > > >> > On Wed, May 26, 2010 at 02:57:45PM +0200, Peter Zijlstra wrote: > > >> > > > >> > > I fail to see why. In both cases the woken userspace will contac= t a > > >> > > central governing task, either the kernel or the userspace suspe= nd > > >> > > manager, and inform it there is work to be done, and please don't > > >> > > suspend now. > > >> > > > >> > Thinking about this, you're right - we don't have to wait, but tha= t does > > >> > result in another problem. Imagine we get two wakeup events > > >> > approximately simultaneously. In the kernel-level universe the ker= nel > > >> > knows when both have been handled. In the user-level universe, we = may > > >> > have one task schedule, bump the count, handle the event, drop the= count > > >> > and then we attempt a suspend again because the second event handl= er > > >> > hasn't had an opportunity to run yet. We'll then attempt a suspend= and > > >> > immediately bounce back up. That's kind of wasteful, although it'd= be > > >> > somewhat mitigated by checking that right at the top of suspend en= try > > >> > and returning -EAGAIN or similar. > > >> > > > >> > > >> (I'm coming a little late to this party, so excuse me if I say somet= hing that > > >> has already been covered however...) > > >> > > >> The above triggers a sequence of thoughts which (When they settled d= own) look > > >> a bit like this. > > >> > > >> At the hardware level, there is a thing that we could call a "suspend > > >> blocker". =A0It is an interrupt (presumably level-triggered) that ca= uses the > > >> processor to come out of suspend, or not to go into it. > > >> > > >> Maybe it makes sense to export a similar thing from the kernel to us= er-space. > > >> When any event happens that would wake the device (and drivers need = to know > > >> about these already), it would present something to user-space to sa= y that > > >> the event happened. > > >> > > >> When user-space processes the event, it clears the event indicator. > > > > > > we did I proposed making the suspend enabling a oneshot type of thing > > > and all sorts of weak arguments came spewing forth. =A0I honestly cou= ldn't > > > tell if I was reading valid input or fanboy BS. > > > > > = > > Can you be more specific? If you are talking about only letting > > drivers abort suspend, not block it, then the main argument against > > that is that you are forcing user-space to poll until the driver stops > > aborting suspend (which according to people arguing against us using > > suspend would make the power-manager a "bad" process). Or are you > > talking about blocking the request from user-space until all other > > suspend-blockers have been released and then doing a single suspend > > cycle before returning. This would not be as bad, but it would force > > the user-space power manager to be multi-threaded since it now would > > have way to cancel the request. Either way, what problem are you > > trying to solve by making it a one-shot request? > > = Sorry about missing Avr's email, I've been fighting with getting my email forwarding working right. The problems I want to solve with the one-shot styled interface are: 1) of having to sprinkle suspend blocking sections from isr up to usermode and get them right. 2) provide a platform / architecture independent framework supporting other low power modes. I've just posted a patch that expresses what I was trying to express. Its hard to take design over email without tossing code back and forth. It's my turn to toss some code. > = > Simply aborting the suspend cannot work as you rightly say - the suspend > daemon would then spin until other user-space processes get into action. Not if you have sensible event messaging to the user-space processes. I've posted a patch that attempts to add some messaging. > Simply blocking while there are any unhandled 'wakeup events' - then abor= ting > if there were any - is how I think it should work. However as it > doesn't work that way now I don't think it is safe to make it work that w= ay > unconditionally. If we did we could find that existing configurations al= ways > block suspend indefinitely with would clearly be a regression. > = > I think we still need some sort of "suspend_prepare". This would have two > particular effects. > 1/ it sets the start time for interpreting the word "were" above. i.e. t= he > suspend would abort of there were any unhandled wakeup events since the > "suspend_prepare" was issued. > 2/ It would allow unhandled wakeup events to abort the suspend. If no > suspend_prepare had been issued, then only "new" wakeup events would > be allowed to abort the suspend (i.e. the old racy version of suspend). > = > So the suspend daemon does: > = > wait for there to be no user-space suspend blocks > issue suspend_prepare > check there are still no suspend blocks > if there are, loop (possibly issue suspend_abort if needed) > issue suspend request > loop > = > processes that handle wakeup events would > = > poll for event to be available > request suspend-block > consume event > release suspend-block > loop In my mind I'm thinking of something like: attempt low power entry (suspend) if blocked, get -EBUSY back select on block_file until block is released retry suspend else // entered low power and exited read wake_event do something with event; typically wait a bit and loop. --mgross > = > (where consuming the event would quite possibly cause some other > suspend-block to become active - e.g. it might request that the display > be unlocked which would block suspends for a time). > = > NeilBrown