From mboxrd@z Thu Jan 1 00:00:00 1970 From: Juergen Gross Subject: Re: Shutdown panic in disable_nonboot_cpus after cpupool-numa-split Date: Mon, 07 Jul 2014 16:28:18 +0200 Message-ID: <53BAAE82.2090200@suse.com> References: <53BA857A.8070608@canonical.com> <53BA8BD1.4020506@citrix.com> <53BA94D0.80201@suse.com> <53BA9773.6090004@canonical.com> <53BA9AB7.70105@suse.com> <53BAA9DD.4040403@canonical.com> Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1"; Format="flowed" Content-Transfer-Encoding: quoted-printable Return-path: In-Reply-To: <53BAA9DD.4040403@canonical.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Stefan Bader , Andrew Cooper , "xen-devel@lists.xensource.com" List-Id: xen-devel@lists.xenproject.org On 07/07/2014 04:08 PM, Stefan Bader wrote: > On 07.07.2014 15:03, J=FCrgen Gro=DF wrote: >> On 07/07/2014 02:49 PM, Stefan Bader wrote: >>> On 07.07.2014 14:38, J=FCrgen Gro=DF wrote: >>>> On 07/07/2014 02:00 PM, Andrew Cooper wrote: >>>>> On 07/07/14 12:33, Stefan Bader wrote: >>>>>> I recently noticed that I get a panic (rebooting the system) on shu= tdown in >>>>>> some >>>>> > cases. This happened only on my AMD system and also not all the = time. >>>>> Finally >>>>> > realized that it is related to the use of using cpupool-numa-spl= it >>>>> > (libxl with xen-4.4 maybe, but not 100% sure 4.3 as well). >>>>> > >>>>> > What happens is that on shutdown the hypervisor runs >>>>> disable_nonboot_cpus which >>>>> > call cpu_down for each online cpu. There is a BUG_ON in the code= for >>>>> the case of >>>>> > cpu_down returning -EBUSY. This happens in my case as soon as the >>>>> first cpu that >>>>> > has been moved to pool-1 by cpupool-numa-split is attempted. The= error is >>>>> > returned by running the notifier_call_chain and I suspect that e= nds >>>>> up calling >>>>> > cpupool_cpu_remove which always returns EBUSY for cpus not in po= ol0. >>>>> > >>>>> > I am not sure which end needs to be fixed but looping over all o= nline >>>>> cpus in >>>>> > disable_nonboot_cpus sounds plausible. So maybe the check for po= ol-0 in >>>>> > cpupool_cpu_remove is wrong...? >>>>> > >>>>> > -Stefan >>>>> >>>>> Hmm yes - this looks completely broken. >>>>> >>>>> cpupool_cpu_remove() only has a single caller which is from cpu_down(= ), >>>>> and will unconditionally fail for cpus outside of the default pool. >>>>> >>>>> It is not obvious at all how this is supposed to work, and the comment >>>>> beside cpupool_cpu_remove() doesn't help. >>>>> >>>>> Can you try the following (only compile tested) patch, which looks >>>>> plausibly like it might DTRT. The for_each_cpupool() is a little nas= tly >>>>> but there appears to be no cpu_to_cpupool mapping available. >>>> >>>> Your patch has the disadvantage to support hot-unplug of the last cpu = in >>>> a cpupool. The following should work, however: >>> >>> Disadvantage and support sounded a bit confusing. But I think it means >>> hot-unplugging the last cpu of a pool is bad and should not be working. >> >> Correct. >> >>> >>>> >>>> diff --git a/xen/common/cpupool.c b/xen/common/cpupool.c >>>> index 4a0e569..73249d3 100644 >>>> --- a/xen/common/cpupool.c >>>> +++ b/xen/common/cpupool.c >>>> @@ -471,12 +471,24 @@ static void cpupool_cpu_add(unsigned int cpu) >>>> */ >>>> static int cpupool_cpu_remove(unsigned int cpu) >>>> { >>>> - int ret =3D 0; >>>> + int ret =3D -EBUSY; >>>> + struct cpupool **c; >>>> >>>> spin_lock(&cpupool_lock); >>>> - if ( !cpumask_test_cpu(cpu, cpupool0->cpu_valid)) >>>> - ret =3D -EBUSY; >>>> + if ( cpumask_test_cpu(cpu, cpupool0->cpu_valid) ) >>>> + ret =3D 0; >>>> else >>>> + { >>>> + for_each_cpupool(c) >>>> + { >>>> + if ( cpumask_test_cpu(cpu, (*c)->cpu_suspended ) ) >>> >>> The rest seems to keep the semantics the same as before (though does th= at mean >>> unplugging the last cpu of pool-0 is ok?) But why testing for suspended= here to >>> succeed (and not valid)? >> >> Testing valid would again enable to remove the last cpu of a cpupool in >> case of hotplugging. cpu_suspended is set if all cpus are to be removed >> due to shutdown, suspend to ram/disk, ... > > Ah, ok. Thanks for the detail explanation. So I was trying this change in > parallel and can confirm that it gets rid of the panic on shutdown. But w= hen I > try to offline any cpu in pool1 (if echoing 0 into /sys/devices/xen_cpu/x= en_cpu? > is the correct method) I always get EBUSY. IOW I cannot hot-unplug any cp= u that > is in a pool other than 0. It does only work after removing it from pool1= , then > add it to pool0 and then echo 0 into online. That's how it was designed some years ago. I don't want to change the behavior in the hypervisor. Adding some tool support could make sense, however. Juergen