Re: [Xen-devel] POD: soft lockups in dom0 kernel

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 06/12/13 13:52, Dietmar Hahn wrote:
> Am Freitag 06 Dezember 2013, 12:00:02 schrieb David Vrabel:
>> On 06/12/13 11:30, Jan Beulich wrote:
>>>>>> On 06.12.13 at 12:07, David Vrabel <david.vrabel@xxxxxxxxxx> wrote:
>>>> We do not want to disable the soft lockup detection here as it has found
>>>> a bug.  We can't have tasks that are unschedulable for minutes as it
>>>> would only take a handful of such tasks to hose the system.
>>>
>>> My understanding is that the soft lockup detection is what its name
>>> says - a mechanism to find cases where the kernel software locked
>>> up. Yet that's not the case with long running hypercalls.
>>
>> Well ok, it's not a lockup in the kernel but it's still a task that
>> cannot be descheduled for minutes of wallclock time.  This is still a
>> bug that needs to be fixed.
>>
>>>> We should put an explicit preemption point in.  This will fix it for the
>>>> CONFIG_PREEMPT_VOLUNTARY case which I think is the most common
>>>> configuration.  Or perhaps this should even be a cond_reched() call to
>>>> fix it for fully non-preemptible as well.
>>>
>>> How do you imagine to do this? When the hypervisor preempts a
>>> hypercall, all the kernel gets to see is that it drops back into the
>>> hypercall page, such that the next thing to happen would be
>>> re-execution of the hypercall. You can't call anything at that point,
>>> all that can get run here are interrupts (i.e. event upcalls). Or do
>>> you suggest to call cond_resched() from within
>>> __xen_evtchn_do_upcall()?
>>
>> I've not looked at how.
>>
>>> And even if you do - how certain is it that what gets its continuation
>>> deferred won't interfere with other things the kernel wants to do
>>> (since if you'd be doing it that way, you'd cover all hypercalls at
>>> once, not just those coming through privcmd, and hence you could
>>> end up with partially completed multicalls or other forms of batching,
>>> plus you'd need to deal with possibly active lazy modes).
>>
>> I would only do this for hypercalls issued by the privcmd driver.
> 
> But I also got soft lockups when unmapping a bigger chunk of guest memory
> (our BS2000 OS) in the dom0 kernel via vunmap(). This calls in the end
> HYPERVISOR_update_va_mapping() and may take a very long time.
> From a kernel module I found no solution to split the virtual address area to
> be able to call schedule(). Because all needed kernel functions are not
> exported to be usable in modules. The only possible solution was to turn of
> the soft lockup detection.

vunmap() does a hypercall per-page since it calls ptep_get_and_clear()
so there are no long running hypercalls here.

zap_pmd_range() (which is used for munmap()) already has appropriate
cond_resched() calls after every zap_pte_range() so I think there needs
to be a cond_resched() call added into vunmap_pmd_range() as well.

David

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