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

From: Dario Faggioli <dfaggioli@suse.com>
To: "sstabellini@kernel.org" <sstabellini@kernel.org>,
	"julien.grall@arm.com" <julien.grall@arm.com>
Cc: "George.Dunlap@eu.citrix.com" <George.Dunlap@eu.citrix.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Subject: Re: [Xen-devel] dom0less + sched=null => broken in staging
Date: Tue, 13 Aug 2019 22:34:10 +0000	[thread overview]
Message-ID: <e82af45e0841f9cf8f27ac93c698e4c4a2d4c44e.camel@suse.com> (raw)
In-Reply-To: <fdbf9a17-acbf-3402-6b69-d1af0193df53@arm.com>


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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?

> 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.

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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