Re: [PATCH v2 0/4] implement vcpu preempted check

[Waiman Long]

On 07/12/2016 12:16 AM, Juergen Gross wrote:
> On 11/07/16 17:10, Waiman Long wrote:
> > On 07/06/2016 02:52 AM, Peter Zijlstra wrote:
> > > On Tue, Jun 28, 2016 at 10:43:07AM -0400, Pan Xinhui wrote:
> > > > change fomr v1:
> > > >      a simplier definition of default vcpu_is_preempted
> > > >      skip mahcine type check on ppc, and add config. remove dedicated
> > > > macro.
> > > >      add one patch to drop overload of rwsem_spin_on_owner and
> > > > mutex_spin_on_owner.
> > > >      add more comments
> > > >      thanks boqun and Peter's suggestion.
> > > >
> > > > This patch set aims to fix lock holder preemption issues.
> > > >
> > > > test-case:
> > > > perf record -a perf bench sched messaging -g 400 -p&&   perf report
> > > >
> > > > 18.09%  sched-messaging  [kernel.vmlinux]  [k] osq_lock
> > > > 12.28%  sched-messaging  [kernel.vmlinux]  [k] rwsem_spin_on_owner
> > > >    5.27%  sched-messaging  [kernel.vmlinux]  [k] mutex_unlock
> > > >    3.89%  sched-messaging  [kernel.vmlinux]  [k] wait_consider_task
> > > >    3.64%  sched-messaging  [kernel.vmlinux]  [k] _raw_write_lock_irq
> > > >    3.41%  sched-messaging  [kernel.vmlinux]  [k] mutex_spin_on_owner.is
> > > >    2.49%  sched-messaging  [kernel.vmlinux]  [k] system_call
> > > >
> > > > We introduce interface bool vcpu_is_preempted(int cpu) and use it in
> > > > some spin
> > > > loops of osq_lock, rwsem_spin_on_owner and mutex_spin_on_owner.
> > > > These spin_on_onwer variant also cause rcu stall before we apply this
> > > > patch set
> > > Paolo, could you help out with an (x86) KVM interface for this?
> > >
> > > Waiman, could you see if you can utilize this to get rid of the
> > > SPIN_THRESHOLD in qspinlock_paravirt?
> > That API is certainly useful to make the paravirt spinlock perform
> > better. However, I am not sure if we can completely get rid of the
> > SPIN_THRESHOLD at this point. It is not just the kvm, the xen code need
> > to be modified as well.
> This should be rather easy. The relevant information is included in the
> runstate data mapped into kernel memory. I can provide a patch for Xen
> if needed.
>
>
> Juergen

Thanks for the offering. We will wait until Xinhui's patch comes through 
before working on the next step.

As for the elimination of SPIN_THRESHOLD, the queue head may not always 
have the right CPU number of the lock holder. So I don't think we can 
eliminate that for the queue head spinning. I think we can eliminates 
the SPIN_THRESHOLD spinning for the other queue node vCPUs.

Cheers,
Longman