Re: [Xen-devel] dom0less + sched=null => broken in staging

From: Julien Grall <julien.grall@gmail.com>
To: Dario Faggioli <dfaggioli@suse.com>
Cc: "George.Dunlap@eu.citrix.com" <George.Dunlap@eu.citrix.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	Julien Grall <julien.grall@arm.com>,
	Stefano Stabellini <sstabellini@kernel.org>
Subject: Re: [Xen-devel] dom0less + sched=null => broken in staging
Date: Wed, 14 Aug 2019 00:07:57 +0100	[thread overview]
Message-ID: <CAF3u54CpObhGqbWvgMHno3QUEVWtugKyD19L3N=PmQS45_SSUQ@mail.gmail.com> (raw)
In-Reply-To: <e82af45e0841f9cf8f27ac93c698e4c4a2d4c44e.camel@suse.com>

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On Tue, 13 Aug 2019, 23:39 Dario Faggioli, <dfaggioli@suse.com> wrote:

> On Tue, 2019-08-13 at 19:43 +0100, Julien Grall wrote:
> > On 8/13/19 6:34 PM, Dario Faggioli wrote:
> > > On Tue, 2019-08-13 at 17:52 +0100, Julien Grall wrote:
> > > >
> > > So, unless the flag gets cleared again, or something else happens
> > > that
> > > makes the vCPU(s) fail the vcpu_runnable() check in
> > > domain_unpause()->vcpu_wake(), I don't see why the wakeup that let
> > > the
> > > null scheduler start scheduling the vCPU doesn't happen... as it
> > > instead does on x86 or !dom0less ARM (because, as far as I've
> > > understood, it's only dom0less that doesn't work, it this correct?)
> >
> > Yes, I quickly tried to use NULL scheduler with just dom0 and it
> > boots.
> >
> Ok.
>
> > Interestingly, I can't see the log:
> >
> > (XEN) Freed 328kB init memory.
> >
> > This is called as part of init_done before CPU0 goes into the idle
> > loop.
> >
> > Adding more debug, it is getting stuck when calling
> > domain_unpause_by_controller for dom0. Specifically vcpu_wake on
> > dom0v0.
> >
> Wait... Is this also with just dom0, or when trying dom0less with some
> domUs?
>

Dom0 is unpaused after all the domUs. In other words, the scheduler will
see domUs first.

> > The loop to assign a pCPU in null_vcpu_wake() is turning into an
> > infinite loop. Indeed the loop is trying to pick CPU0 for dom0v0 that
> > is
> > already used by dom1v0. So the problem is in pick_cpu() or the data
> > used
> > by it.
> >
> Ah, interesting...
>
> > It feels to me this is an affinity problem. Note that I didn't
> > request
> > to pin dom0 vCPUs.
> >
> Yep, looking better, I think I've seen something suspicious now. I'll
> send another debug patch.
>

You may want to see my last e-mail first just in case it rings a bell. :) I
did more debugging during the evening.

Cheers,

> Regards
> --
> Dario Faggioli, Ph.D
> http://about.me/dario.faggioli
> Virtualization Software Engineer
> SUSE Labs, SUSE https://www.suse.com/
> -------------------------------------------------------------------
> <<This happens because _I_ choose it to happen!>> (Raistlin Majere)
>
> _______________________________________________
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> Xen-devel@lists.xenproject.org
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