On Fri, Feb 21, 2020 at 10:33:42AM +0000, Durrant, Paul wrote: > > -----Original Message----- > > From: Roger Pau Monné <roger.pau@xxxxxxxxxx> > > Sent: 21 February 2020 10:22 > > To: Durrant, Paul <pdurrant@xxxxxxxxxxxx> > > Cc: Agarwal, Anchal <anchalag@xxxxxxxxxx>; Valentin, Eduardo > > <eduval@xxxxxxxxxx>; len.brown@xxxxxxxxx; peterz@xxxxxxxxxxxxx; > > benh@xxxxxxxxxxxxxxxxxxx; x86@xxxxxxxxxx; linux-mm@xxxxxxxxx; > > pavel@xxxxxx; hpa@xxxxxxxxx; tglx@xxxxxxxxxxxxx; sstabellini@xxxxxxxxxx; > > fllinden@xxxxxxxxxx; Kamata, Munehisa <kamatam@xxxxxxxxxx>; > > mingo@xxxxxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxxx; Singh, Balbir > > <sblbir@xxxxxxxxxx>; axboe@xxxxxxxxx; konrad.wilk@xxxxxxxxxx; > > bp@xxxxxxxxx; boris.ostrovsky@xxxxxxxxxx; jgross@xxxxxxxx; > > netdev@xxxxxxxxxxxxxxx; linux-pm@xxxxxxxxxxxxxxx; rjw@xxxxxxxxxxxxx; > > linux-kernel@xxxxxxxxxxxxxxx; vkuznets@xxxxxxxxxx; davem@xxxxxxxxxxxxx; > > Woodhouse, David <dwmw@xxxxxxxxxxxx> > > Subject: Re: [Xen-devel] [RFC PATCH v3 06/12] xen-blkfront: add callbacks > > for PM suspend and hibernation > > > > On Fri, Feb 21, 2020 at 09:56:54AM +0000, Durrant, Paul wrote: > > > > -----Original Message----- > > > > From: Roger Pau Monné <roger.pau@xxxxxxxxxx> > > > > Sent: 21 February 2020 09:22 > > > > To: Durrant, Paul <pdurrant@xxxxxxxxxxxx> > > > > Cc: Agarwal, Anchal <anchalag@xxxxxxxxxx>; Valentin, Eduardo > > > > <eduval@xxxxxxxxxx>; len.brown@xxxxxxxxx; peterz@xxxxxxxxxxxxx; > > > > benh@xxxxxxxxxxxxxxxxxxx; x86@xxxxxxxxxx; linux-mm@xxxxxxxxx; > > > > pavel@xxxxxx; hpa@xxxxxxxxx; tglx@xxxxxxxxxxxxx; > > sstabellini@xxxxxxxxxx; > > > > fllinden@xxxxxxxxxx; Kamata, Munehisa <kamatam@xxxxxxxxxx>; > > > > mingo@xxxxxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxxx; Singh, Balbir > > > > <sblbir@xxxxxxxxxx>; axboe@xxxxxxxxx; konrad.wilk@xxxxxxxxxx; > > > > bp@xxxxxxxxx; boris.ostrovsky@xxxxxxxxxx; jgross@xxxxxxxx; > > > > netdev@xxxxxxxxxxxxxxx; linux-pm@xxxxxxxxxxxxxxx; rjw@xxxxxxxxxxxxx; > > > > linux-kernel@xxxxxxxxxxxxxxx; vkuznets@xxxxxxxxxx; > > davem@xxxxxxxxxxxxx; > > > > Woodhouse, David <dwmw@xxxxxxxxxxxx> > > > > Subject: Re: [Xen-devel] [RFC PATCH v3 06/12] xen-blkfront: add > > callbacks > > > > for PM suspend and hibernation > > > > > > > > On Thu, Feb 20, 2020 at 05:01:52PM +0000, Durrant, Paul wrote: > > > > > > > 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. > > > > > > > > > > > > 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. > > > > > > > > Thanks, that makes sense and something along this lines should be > > > > added to the commit message IMO. > > > > > > > > Wondering about S4, shouldn't we expect the queues to already be > > > > empty? As any subsystem that wanted to store something to disk should > > > > make sure requests have been successfully completed before > > > > suspending. > > > > > > What about writing the suspend image itself? Normal filesystem I/O > > > will have been flushed of course, but whatever vestigial kernel > > > actually writes out the hibernation file may well expect a final > > > D0->D3 on the storage device to cause a flush. > > > > Hm, I have no idea really. I think whatever writes to the disk before > > suspend should actually make sure requests have completed, but what > > you suggest might also be a possibility. > > > > Can you figure out whether there are requests on the ring or in the > > queue before suspending? > > Well there's clearly pending stuff in the ring if rsp_prod != req_prod :-) Right, I assume there's no document that states what's the expected state for queues &c when switching PM states, so we have to assume that there might be in-flight requests on the ring and in the driver queues. > As for internal queues, I don't know how blkfront manages that (or whether it has any pending work queue at all). There are no internal queues, just the generic ones from blk_mq which every block device has IIRC. Thanks, Roger.
