From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:41617) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1duenI-0007jr-ME for qemu-devel@nongnu.org; Wed, 20 Sep 2017 09:12:33 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1duemf-0006tt-Rl for qemu-devel@nongnu.org; Wed, 20 Sep 2017 09:12:00 -0400 Date: Wed, 20 Sep 2017 21:53:42 +1000 From: David Gibson Message-ID: <20170920115342.GQ5520@umbus.fritz.box> References: <20170915085115.GN5250@umbus.fritz.box> <87y3pgl45f.fsf@abhimanyu.i-did-not-set--mail-host-address--so-tickle-me> <20170919082421.GU27153@umbus> <871sn2hugn.fsf@abhimanyu.i-did-not-set--mail-host-address--so-tickle-me> <20170920045524.GH5520@umbus.fritz.box> <87y3pagdg0.fsf@abhimanyu.i-did-not-set--mail-host-address--so-tickle-me> <20170920061756.GJ5520@umbus.fritz.box> <87vakdhnyn.fsf@abhimanyu.i-did-not-set--mail-host-address--so-tickle-me> <20170920065700.GO5520@umbus.fritz.box> <87poalhm74.fsf@abhimanyu.i-did-not-set--mail-host-address--so-tickle-me> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="i/CQJCAqWP/GQJtX" Content-Disposition: inline In-Reply-To: <87poalhm74.fsf@abhimanyu.i-did-not-set--mail-host-address--so-tickle-me> Subject: Re: [Qemu-devel] [PATCH] ppc/pnv: fix cores per chip for multiple cpus List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Nikunj A Dadhania Cc: qemu-ppc@nongnu.org, qemu-devel@nongnu.org, clg@kaod.org, bharata@linux.vnet.ibm.com, benh@kernel.crashing.org --i/CQJCAqWP/GQJtX Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Wed, Sep 20, 2017 at 12:48:55PM +0530, Nikunj A Dadhania wrote: > David Gibson writes: >=20 > > On Wed, Sep 20, 2017 at 12:10:48PM +0530, Nikunj A Dadhania wrote: > >> David Gibson writes: > >>=20 > >> > On Wed, Sep 20, 2017 at 10:43:19AM +0530, Nikunj A Dadhania wrote: > >> >> David Gibson writes: > >> >>=20 > >> >> > On Wed, Sep 20, 2017 at 09:50:24AM +0530, Nikunj A Dadhania wrote: > >> >> >> David Gibson writes: > >> >> >>=20 > >> >> >> > On Fri, Sep 15, 2017 at 02:39:16PM +0530, Nikunj A Dadhania wr= ote: > >> >> >> >> David Gibson writes: > >> >> >> >>=20 > >> >> >> >> > On Fri, Sep 15, 2017 at 01:53:15PM +0530, Nikunj A Dadhania= wrote: > >> >> >> >> >> David Gibson writes: > >> >> >> >> >>=20 > >> >> >> >> >> >>=20 > >> >> >> >> >> >> I thought, I am doing the same here for PowerNV, number= of online cores > >> >> >> >> >> >> is equal to initial online vcpus / threads per core > >> >> >> >> >> >>=20 > >> >> >> >> >> >> int boot_cores_nr =3D smp_cpus / smp_threads; > >> >> >> >> >> >>=20 > >> >> >> >> >> >> Only difference that I see in PowerNV is that we have m= ultiple chips > >> >> >> >> >> >> (max 2, at the moment) > >> >> >> >> >> >>=20 > >> >> >> >> >> >> cores_per_chip =3D smp_cpus / (smp_threads * pn= v->num_chips); > >> >> >> >> >> > > >> >> >> >> >> > This doesn't make sense to me. Cores per chip should *a= lways* 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, no= t try to fix > >> >> >> >> >> > it. That's just asking for confusion. > >> >> >> >> >>=20 > >> >> >> >> >> This is the case where the user does not provide a topolog= y(which is a > >> >> >> >> >> valid scenario), not sure we should reject it. So qemu def= aults > >> >> >> >> >> 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. > >> >> >> >>=20 > >> >> >> >> Should I change the global smp_cores here as well ? > >> >> >> > > >> >> >> > I'm pretty uneasy with that option. > >> >> >>=20 > >> >> >> Me too. > >> >> >>=20 > >> >> >> > It would take a fair bit of checking to ensure that changing s= mp_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 soc= kets > >> >> >> > more flexible, possibly based on machine options for max value= s. > >> >> >> > > >> >> >> > That might still be more trouble than its worth. > >> >> >>=20 > >> >> >> 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 det= ailed > >> >> >> topology, the best we can do is create single threaded cores equ= al to > >> >> >> number of cores. > >> >> > > >> >> > No, sorry. Having smp_cores not correspond to the number of core= s 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 havi= ng to > >> >> > use a longer command line is better than breaking basic assumptio= ns > >> >> > about what numbers reflect what topology. > >> >>=20 > >> >> 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 co= res, > >> >> 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. > >>=20 > >> 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 boo= ts > >> with 4 cores, each having 1 thread. > > > > Ok.. and what's the problem with that behaviour on powernv? >=20 > 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: >=20 > object_property_set_int(chip, smp_cores, "nr-cores", &error_fatal= ); >=20 > 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. --=20 David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. 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