Linux Virtualization - Jul 2016 - [PATCH v2 2/4] powerpc/spinlock: support vcpu preempted check

Cc Paolo, kvm ml
2016-07-06 12:58 GMT+08:00 xinhui <xinhui.pan at
linux.vnet.ibm.com>:
> Hi, wanpeng
>
> On 2016?07?05? 17:57, Wanpeng Li wrote:
>>
>> Hi Xinhui,
>> 2016-06-28 22:43 GMT+08:00 Pan Xinhui <xinhui.pan at
linux.vnet.ibm.com>:
>>>
>>> This is to fix some lock holder preemption issues. Some other locks
>>> implementation do a spin loop before acquiring the lock itself.
Currently
>>> kernel has an interface of bool vcpu_is_preempted(int cpu). It take
the
>>> cpu
>>> as parameter and return true if the cpu is preempted. Then kernel
can
>>> break
>>> the spin loops upon on the retval of vcpu_is_preempted.
>>>
>>> As kernel has used this interface, So lets support it.
>>>
>>> Only pSeries need supoort it. And the fact is powerNV are built
into same
>>> kernel image with pSeries. So we need return false if we are runnig
as
>>> powerNV. The another fact is that lppaca->yiled_count keeps zero
on
>>> powerNV. So we can just skip the machine type.
>>
>>
>> Lock holder vCPU preemption can be detected by hardware pSeries or
>> paravirt method?
>>
> There is one shard struct between kernel and powerVM/KVM. And we read the
> yield_count of this struct to detect if one vcpu is running or not.
> SO it's easy for ppc to implement such interface. Note that yield_count
is
> set by powerVM/KVM.
> and only pSeries can run a guest for now. :)
>
> I also review x86 related code, looks like we need add one hyer-call to get
> such vcpu preemption info?
There is no such stuff to record lock holder in x86 kvm, maybe we
don't need to depend on PLE handler algorithm to guess it if we can
know lock holder vCPU directly.

Regards,
Wanpeng Li

On Wed, Jul 06, 2016 at 02:46:34PM +0800, Wanpeng Li
wrote:
> > SO it's easy for ppc to implement such interface. Note that
yield_count is
> > set by powerVM/KVM.
> > and only pSeries can run a guest for now. :)
> >
> > I also review x86 related code, looks like we need add one hyer-call
to get
> > such vcpu preemption info?
> 
> There is no such stuff to record lock holder in x86 kvm, maybe we
> don't need to depend on PLE handler algorithm to guess it if we can
> know lock holder vCPU directly.
x86/kvm has vcpu->preempted to indicate if a vcpu is currently preempted
or not. I'm just not sure if that is visible to the guest or how to make
it so.

2016-07-06 15:58 GMT+08:00 Peter Zijlstra <peterz at
infradead.org>:
> On Wed, Jul 06, 2016 at 02:46:34PM +0800, Wanpeng Li wrote:
>> > SO it's easy for ppc to implement such interface. Note that
yield_count is
>> > set by powerVM/KVM.
>> > and only pSeries can run a guest for now. :)
>> >
>> > I also review x86 related code, looks like we need add one
hyer-call to get
>> > such vcpu preemption info?
>>
>> There is no such stuff to record lock holder in x86 kvm, maybe we
>> don't need to depend on PLE handler algorithm to guess it if we can
>> know lock holder vCPU directly.
>
> x86/kvm has vcpu->preempted to indicate if a vcpu is currently preempted
> or not. I'm just not sure if that is visible to the guest or how to
make
> it so.
Yeah, I miss it. I can be the volunteer to do it if there is any idea,
ping Paolo. :)

Regards,
Wanpeng Li