On Thu, Sep 21, 2017 at 09:42:26AM +0200, Igor Mammedov wrote: > On Thu, 21 Sep 2017 08:04:55 +0200 > Cédric Le Goater <c...@kaod.org> wrote: > > > On 09/21/2017 05:54 AM, Nikunj A Dadhania wrote: > > > David Gibson <da...@gibson.dropbear.id.au> writes: > > > > > >> On Wed, Sep 20, 2017 at 12:48:55PM +0530, Nikunj A Dadhania wrote: > > >>> David Gibson <da...@gibson.dropbear.id.au> writes: > > >>> > > >>>> On Wed, Sep 20, 2017 at 12:10:48PM +0530, Nikunj A Dadhania wrote: > > >>>>> David Gibson <da...@gibson.dropbear.id.au> writes: > > >>>>> > > >>>>>> On Wed, Sep 20, 2017 at 10:43:19AM +0530, Nikunj A Dadhania wrote: > > >>>>>>> David Gibson <da...@gibson.dropbear.id.au> writes: > > >>>>>>> > > >>>>>>>> On Wed, Sep 20, 2017 at 09:50:24AM +0530, Nikunj A Dadhania wrote: > > >>>>>>>> > > >>>>>>>>> David Gibson <da...@gibson.dropbear.id.au> writes: > > >>>>>>>>> > > >>>>>>>>>> On Fri, Sep 15, 2017 at 02:39:16PM +0530, Nikunj A Dadhania > > >>>>>>>>>> wrote: > > >>>>>>>>>>> David Gibson <da...@gibson.dropbear.id.au> writes: > > >>>>>>>>>>> > > >>>>>>>>>>>> On Fri, Sep 15, 2017 at 01:53:15PM +0530, Nikunj A Dadhania > > >>>>>>>>>>>> wrote: > > >>>>>>>>>>>>> David Gibson <da...@gibson.dropbear.id.au> writes: > > >>>>>>>>>>>>> > > >>>>>>>>>>>>>>> > > >>>>>>>>>>>>>>> I thought, I am doing the same here for PowerNV, number of > > >>>>>>>>>>>>>>> online cores > > >>>>>>>>>>>>>>> is equal to initial online vcpus / threads per core > > >>>>>>>>>>>>>>> > > >>>>>>>>>>>>>>> int boot_cores_nr = smp_cpus / smp_threads; > > >>>>>>>>>>>>>>> > > >>>>>>>>>>>>>>> Only difference that I see in PowerNV is that we have > > >>>>>>>>>>>>>>> multiple chips > > >>>>>>>>>>>>>>> (max 2, at the moment) > > >>>>>>>>>>>>>>> > > >>>>>>>>>>>>>>> cores_per_chip = smp_cpus / (smp_threads * > > >>>>>>>>>>>>>>> pnv->num_chips); > > >>>>>>>>>>>>>> > > >>>>>>>>>>>>>> This doesn't make sense to me. Cores per chip should > > >>>>>>>>>>>>>> *always* equal > > >>>>>>>>>>>>>> smp_cores, you shouldn't need another calculation for it. > > >>>>>>>>>>>>>> > > >>>>>>>>>>>>>>> And in case user has provided sane smp_cores, we use it. > > >>>>>>>>>>>>>> > > >>>>>>>>>>>>>> If smp_cores isn't sane, you should simply reject it, not > > >>>>>>>>>>>>>> try to fix > > >>>>>>>>>>>>>> it. That's just asking for confusion. > > >>>>>>>>>>>>> > > >>>>>>>>>>>>> This is the case where the user does not provide a > > >>>>>>>>>>>>> topology(which is a > > >>>>>>>>>>>>> valid scenario), not sure we should reject it. So qemu > > >>>>>>>>>>>>> defaults > > >>>>>>>>>>>>> smp_cores/smt_threads to 1. I think it makes sense to > > >>>>>>>>>>>>> over-ride. > > >>>>>>>>>>>> > > >>>>>>>>>>>> If you can find a way to override it by altering smp_cores > > >>>>>>>>>>>> when it's > > >>>>>>>>>>>> not explicitly specified, then ok. > > >>>>>>>>>>> > > >>>>>>>>>>> Should I change the global smp_cores here as well ? > > >>>>>>>>>> > > >>>>>>>>>> I'm pretty uneasy with that option. > > >>>>>>>>> > > >>>>>>>>> Me too. > > >>>>>>>>> > > >>>>>>>>>> It would take a fair bit of checking to ensure that changing > > >>>>>>>>>> smp_cores > > >>>>>>>>>> is safe here. An easier to verify option would be to make the > > >>>>>>>>>> generic > > >>>>>>>>>> logic which splits up an unspecified -smp N into cores and > > >>>>>>>>>> sockets > > >>>>>>>>>> more flexible, possibly based on machine options for max values. > > >>>>>>>>>> > > >>>>>>>>>> That might still be more trouble than its worth. > > >>>>>>>>> > > >>>>>>>>> I think the current approach is the simplest and less intrusive, > > >>>>>>>>> as we > > >>>>>>>>> are handling a case where user has not bothered to provide a > > >>>>>>>>> detailed > > >>>>>>>>> topology, the best we can do is create single threaded cores > > >>>>>>>>> equal to > > >>>>>>>>> number of cores. > > >>>>>>>> > > >>>>>>>> No, sorry. Having smp_cores not correspond to the number of cores > > >>>>>>>> per > > >>>>>>>> chip in all cases is just not ok. Add an error message if the > > >>>>>>>> topology isn't workable for powernv by all means. But users > > >>>>>>>> having to > > >>>>>>>> use a longer command line is better than breaking basic assumptions > > >>>>>>>> about what numbers reflect what topology. > > >>>>>>> > > >>>>>>> Sorry to ask again, as I am still not convinced, we do similar > > >>>>>>> adjustment in spapr where the user did not provide the number of > > >>>>>>> cores, > > >>>>>>> but qemu assumes them as single threaded cores and created > > >>>>>>> cores(boot_cores_nr) that were not same as smp_cores ? > > >>>>>> > > >>>>>> What? boot_cores_nr has absolutely nothing to do with adjusting the > > >>>>>> topology, and it certainly doesn't assume they're single threaded. > > >>>>> > > >>>>> When we start a TCG guest and user provides following commandline, > > >>>>> e.g. > > >>>>> "-smp 4", smt_threads is set to 1 by default in vl.c. So the guest > > >>>>> boots > > >>>>> with 4 cores, each having 1 thread. > > >>>> > > >>>> Ok.. and what's the problem with that behaviour on powernv? > > >>> > > >>> As smp_thread defaults to 1 in vl.c, similarly smp_cores also has the > > >>> default value of 1 in vl.c. In powernv, we were setting nr-cores like > > >>> this: > > >>> > > >>> object_property_set_int(chip, smp_cores, "nr-cores", > > >>> &error_fatal); > > >>> > > >>> Even when there were multiple cpus (-smp 4), when the guest boots up, we > > >>> just get one core (i.e. smp_cores was 1) with single thread(smp_threads > > >>> was 1), which is wrong as per the command-line that was provided. > > >> > > >> Right, so, -smp 4 defaults to 4 sockets, each with 1 core of 1 > > >> thread. If you can't supply 4 sockets you should error, but you > > >> shouldn't go and change the number of cores per socket. > > > > > > OK, that makes sense now. And I do see that smp_cpus is 4 in the above > > > case. Now looking more into it, i see that powernv has something called > > > "num_chips", isnt this same as sockets ? Do we need num_chips separately? > > > > > > > yes that would do for cpus, but how do we retrieve the number of > > sockets ? I don't see a smp_sockets. > I'd suggest to rewrite QEMU again :) > > more exactly, -smp parsing is global and sometimes doesn't suite > target device model/machine. > Idea was to make it's options machine properties to get rid of globals > and then let leaf machine redefine parsing behaviour. > here is Drew's take on it: > > [Qemu-devel] [PATCH RFC 00/16] Rework SMP parameters > https://www.mail-archive.com/qemu-devel@nongnu.org/msg376961.html > > considering there weren't pressing need, the series has been pushed > to the end of TODO list. Maybe you can revive it and make work for > pnv and other machines.
Right, making the core smp parsing more flexible might be good.
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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