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Date:   Fri, 21 Feb 2020 00:49:18 +0000
From:   Anchal Agarwal <anchalag@amazon.com>
To:     "Durrant, Paul" <pdurrant@amazon.co.uk>,
        Roger Pau =?iso-8859-1?Q?Monn=E9?= <roger.pau@citrix.com>
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        "Woodhouse, David" <dwmw@amazon.co.uk>, <anchalag@amazon.com>
Subject: Re: [Xen-devel] [RFC PATCH v3 06/12] xen-blkfront: add callbacks for
 PM suspend and hibernation
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On Thu, Feb 20, 2020 at 10:01:52AM -0700, Durrant, Paul wrote:
> > -----Original Message-----
> > From: Roger Pau Monné <roger.pau@citrix.com>
> > Sent: 20 February 2020 16:49
> > To: Durrant, Paul <pdurrant@amazon.co.uk>
> > Cc: Agarwal, Anchal <anchalag@amazon.com>; Valentin, Eduardo
> > <eduval@amazon.com>; len.brown@intel.com; peterz@infradead.org;
> > benh@kernel.crashing.org; x86@kernel.org; linux-mm@kvack.org;
> > pavel@ucw.cz; hpa@zytor.com; tglx@linutronix.de; sstabellini@kernel.org;
> > fllinden@amaozn.com; Kamata, Munehisa <kamatam@amazon.com>;
> > mingo@redhat.com; xen-devel@lists.xenproject.org; Singh, Balbir
> > <sblbir@amazon.com>; axboe@kernel.dk; konrad.wilk@oracle.com;
> > bp@alien8.de; boris.ostrovsky@oracle.com; jgross@suse.com;
> > netdev@vger.kernel.org; linux-pm@vger.kernel.org; rjw@rjwysocki.net;
> > linux-kernel@vger.kernel.org; vkuznets@redhat.com; davem@davemloft.net;
> > Woodhouse, David <dwmw@amazon.co.uk>
> > Subject: Re: [Xen-devel] [RFC PATCH v3 06/12] xen-blkfront: add callbacks
> > for PM suspend and hibernation
> > 
> > On Thu, Feb 20, 2020 at 04:23:13PM +0000, Durrant, Paul wrote:
> > > > -----Original Message-----
> > > > From: Roger Pau Monné <roger.pau@citrix.com>
> > > > Sent: 20 February 2020 15:45
> > > > To: Durrant, Paul <pdurrant@amazon.co.uk>
> > > > Cc: Agarwal, Anchal <anchalag@amazon.com>; Valentin, Eduardo
> > > > <eduval@amazon.com>; len.brown@intel.com; peterz@infradead.org;
> > > > benh@kernel.crashing.org; x86@kernel.org; linux-mm@kvack.org;
> > > > pavel@ucw.cz; hpa@zytor.com; tglx@linutronix.de;
> > sstabellini@kernel.org;
> > > > fllinden@amaozn.com; Kamata, Munehisa <kamatam@amazon.com>;
> > > > mingo@redhat.com; xen-devel@lists.xenproject.org; Singh, Balbir
> > > > <sblbir@amazon.com>; axboe@kernel.dk; konrad.wilk@oracle.com;
> > > > bp@alien8.de; boris.ostrovsky@oracle.com; jgross@suse.com;
> > > > netdev@vger.kernel.org; linux-pm@vger.kernel.org; rjw@rjwysocki.net;
> > > > linux-kernel@vger.kernel.org; vkuznets@redhat.com;
> > davem@davemloft.net;
> > > > Woodhouse, David <dwmw@amazon.co.uk>
> > > > Subject: Re: [Xen-devel] [RFC PATCH v3 06/12] xen-blkfront: add
> > callbacks
> > > > for PM suspend and hibernation
> > > >
> > > > On Thu, Feb 20, 2020 at 08:54:36AM +0000, Durrant, Paul wrote:
> > > > > > -----Original Message-----
> > > > > > From: Xen-devel <xen-devel-bounces@lists.xenproject.org> On Behalf
> > Of
> > > > > > Roger Pau Monné
> > > > > > Sent: 20 February 2020 08:39
> > > > > > To: Agarwal, Anchal <anchalag@amazon.com>
> > > > > > Cc: Valentin, Eduardo <eduval@amazon.com>; len.brown@intel.com;
> > > > > > peterz@infradead.org; benh@kernel.crashing.org; x86@kernel.org;
> > linux-
> > > > > > mm@kvack.org; pavel@ucw.cz; hpa@zytor.com; tglx@linutronix.de;
> > > > > > sstabellini@kernel.org; fllinden@amaozn.com; Kamata, Munehisa
> > > > > > <kamatam@amazon.com>; mingo@redhat.com; xen-
> > > > devel@lists.xenproject.org;
> > > > > > Singh, Balbir <sblbir@amazon.com>; axboe@kernel.dk;
> > > > > > konrad.wilk@oracle.com; bp@alien8.de; boris.ostrovsky@oracle.com;
> > > > > > jgross@suse.com; netdev@vger.kernel.org; linux-pm@vger.kernel.org;
> > > > > > rjw@rjwysocki.net; linux-kernel@vger.kernel.org;
> > vkuznets@redhat.com;
> > > > > > davem@davemloft.net; Woodhouse, David <dwmw@amazon.co.uk>
> > > > > > Subject: Re: [Xen-devel] [RFC PATCH v3 06/12] xen-blkfront: add
> > > > callbacks
> > > > > > for PM suspend and hibernation
> > > > > >
> > > > > > Thanks for this work, please see below.
> > > > > >
> > > > > > On Wed, Feb 19, 2020 at 06:04:24PM +0000, Anchal Agarwal wrote:
> > > > > > > On Tue, Feb 18, 2020 at 10:16:11AM +0100, Roger Pau Monné wrote:
> > > > > > > > On Mon, Feb 17, 2020 at 11:05:53PM +0000, Anchal Agarwal
> > wrote:
> > > > > > > > > On Mon, Feb 17, 2020 at 11:05:09AM +0100, Roger Pau Monné
> > wrote:
> > > > > > > > > > On Fri, Feb 14, 2020 at 11:25:34PM +0000, Anchal Agarwal
> > > > wrote:
> > > > > > > > > Quiescing the queue seemed a better option here as we want
> > to
> > > > make
> > > > > > sure ongoing
> > > > > > > > > requests dispatches are totally drained.
> > > > > > > > > I should accept that some of these notion is borrowed from
> > how
> > > > nvme
> > > > > > freeze/unfreeze
> > > > > > > > > is done although its not apple to apple comparison.
> > > > > > > >
> > > > > > > > That's fine, but I would still like to requests that you use
> > the
> > > > same
> > > > > > > > logic (as much as possible) for both the Xen and the PM
> > initiated
> > > > > > > > suspension.
> > > > > > > >
> > > > > > > > So you either apply this freeze/unfreeze to the Xen suspension
> > > > (and
> > > > > > > > drop the re-issuing of requests on resume) or adapt the same
> > > > approach
> > > > > > > > as the Xen initiated suspension. Keeping two completely
> > different
> > > > > > > > approaches to suspension / resume on blkfront is not suitable
> > long
> > > > > > > > term.
> > > > > > > >
> > > > > > > I agree with you on overhaul of xen suspend/resume wrt blkfront
> > is a
> > > > > > good
> > > > > > > idea however, IMO that is a work for future and this patch
> > series
> > > > should
> > > > > > > not be blocked for it. What do you think?
> > > > > >
> > > > > > It's not so much that I think an overhaul of suspend/resume in
> > > > > > blkfront is needed, it's just that I don't want to have two
> > completely
> > > > > > different suspend/resume paths inside blkfront.
> > > > > >
> > > > > > So from my PoV I think the right solution is to either use the
> > same
> > > > > > code (as much as possible) as it's currently used by Xen initiated
> > > > > > suspend/resume, or to also switch Xen initiated suspension to use
> > the
> > > > > > newly introduced code.
> > > > > >
> > > > > > Having two different approaches to suspend/resume in the same
> > driver
> > > > > > is a recipe for disaster IMO: it adds complexity by forcing
> > developers
> > > > > > to take into account two different suspend/resume approaches when
> > > > > > there's no need for it.
> > > > >
> > > > > I disagree. S3 or S4 suspend/resume (or perhaps we should call them
> > > > power state transitions to avoid confusion) are quite different from
> > Xen
> > > > suspend/resume.
> > > > > Power state transitions ought to be, and indeed are, visible to the
> > > > software running inside the guest. Applications, as well as drivers,
> > can
> > > > receive notification and take whatever action they deem appropriate.
> > > > > Xen suspend/resume OTOH is used when a guest is migrated and the
> > code
> > > > should go to all lengths possible to make any software running inside
> > the
> > > > guest (other than Xen specific enlightened code, such as PV drivers)
> > > > completely unaware that anything has actually happened.
> > > >
> > > > So from what you say above PM state transitions are notified to all
> > > > drivers, and Xen suspend/resume is only notified to PV drivers, and
> > > > here we are speaking about blkfront which is a PV driver, and should
> > > > get notified in both cases. So I'm unsure why the same (or at least
> > > > very similar) approach can't be used in both cases.
> > > >
> > > > The suspend/resume approach proposed by this patch is completely
> > > > different than the one used by a xenbus initiated suspend/resume, and
> > > > I don't see a technical reason that warrants this difference.
> > > >
> > >
> > > Within an individual PV driver it may well be ok to use common
> > mechanisms for connecting to the backend but issues will arise if any
> > subsequent action is visible to the guest. E.g. a network frontend needs
> > to issue gratuitous ARPs without anything else in the network stack (or
> > monitoring the network stack) knowing that it has happened.
> > >
> > > > I'm not saying that the approach used here is wrong, it's just that I
> > > > don't see the point in having two different ways to do suspend/resume
> > > > in the same driver, unless there's a technical reason for it, which I
> > > > don't think has been provided.
> > >
> > > The technical justification is that the driver needs to know what kind
> > of suspend or resume it is doing, so that it doesn't do the wrong thing.
> > There may also be differences in the state of the system e.g. in Windows,
> > at least some of the resume-from-xen-suspend code runs with interrupts
> > disabled (which is necessary to make sure enough state is restored before
> > things become visible to other kernel code).
> > >
> > > >
> > > > I would be fine with switching xenbus initiated suspend/resume to also
> > > > use the approach proposed here: freeze the queues and drain the shared
> > > > rings before suspending.
> > > >
> > >
> > > I think abstracting away at the xenbus level to some degree is probably
> > feasible, but some sort of flag should be passed to the individual drivers
> > so they know what circumstances they are operating under.
> > >
> > > > > So, whilst it may be possible to use common routines to, for
> > example,
> > > > re-establish PV frontend/backend communication, PV frontend code
> > should be
> > > > acutely aware of the circumstances they are operating in. I can cite
> > > > example code in the Windows PV driver, which have supported guest
> > S3/S4
> > > > power state transitions since day 1.
> > > >
> > > > Hm, please bear with me, as I'm not sure I fully understand. Why isn't
> > > > the current suspend/resume logic suitable for PM transitions?
> > > >
> > >
> > > I don’t know the details for Linux but it may well be to do with
> > assumptions made about the system e.g. the ability to block waiting for
> > something to happen on another CPU (which may have already been quiesced
> > in a PM context).
> > >
> > > > As said above, I'm happy to switch xenbus initiated suspend/resume to
> > > > use the logic in this patch, but unless there's a technical reason for
> > > > it I don't see why blkfront should have two completely different
> > > > approaches to suspend/resume depending on whether it's a PM or a
> > > > xenbus state change.
> > > >
> > >
> > > Hopefully what I said above illustrates why it may not be 100% common.
> > 
> > Yes, that's fine. I don't expect it to be 100% common (as I guess
> > that the hooks will have different prototypes), but I expect
> > that routines can be shared, and that the approach taken can be the
> > same.
> > 
> > For example one necessary difference will be that xenbus initiated
> > suspend won't close the PV connection, in case suspension fails. On PM
> > suspend you seem to always close the connection beforehand, so you
> > will always have to re-negotiate on resume even if suspension failed.
> >
I don't get what you mean, 'suspension failure' during disconnecting frontend from 
backend? [as in this case we mark frontend closed and then wait for completion]
Or do you mean suspension fail in general post bkacend is disconnected from
frontend for blkfront? 

In case of later, if anything fails after the dpm_suspend(),
things need to be thawed or set back up so it should ok to always 
re-negotitate just to avoid errors. 

> > What I'm mostly worried about is the different approach to ring
> > draining. Ie: either xenbus is changed to freeze the queues and drain
> > the shared rings, or PM uses the already existing logic of not
> > flushing the rings an re-issuing in-flight requests on resume.
> > 
> 
> Yes, that's needs consideration. I don’t think the same semantic can be suitable for both. E.g. in a xen-suspend we need to freeze with as little processing as possible to avoid dirtying RAM late in the migration cycle, and we know that in-flight data can wait. But in a transition to S4 we need to make sure that at least all the in-flight blkif requests get completed, since they probably contain bits of the guest's memory image and that's not going to get saved any other way.
> 
>   Paul
I agree with Paul here. Just so as you know, I did try a hacky way in the past 
to re-queue requests in the past and failed miserably.
I doubt[just from my experimentation]re-queuing the requests will work for PM 
Hibernation for the same reason Paul mentioned above unless you give me pressing
reason why it should work.
Also, won't it effect the migration time if we start waiting for all the
inflight requests to complete[last min page faults] ?


Thanks,
Anchal