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

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Message-ID: <CANRm+Cyu0T=gWbZQT5WzdknYQWspRmC2wTt6_KLS4tP9QVuWUA@mail.gmail.com>
Date:	Wed, 6 Jul 2016 21:03:01 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>,
	linux-s390 <linux-s390@...r.kernel.org>,
	Davidlohr Bueso <dave@...olabs.net>, mpe@...erman.id.au,
	boqun.feng@...il.com, will.deacon@....com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Waiman Long <waiman.long@....com>,
	virtualization@...ts.linux-foundation.org,
	Ingo Molnar <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>, benh@...nel.crashing.org,
	schwidefsky@...ibm.com, Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	linuxppc-dev@...ts.ozlabs.org, kvm <kvm@...r.kernel.org>
Subject: Re: [PATCH v2 0/4] implement vcpu preempted check

2016-07-06 20:28 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
>
>
> On 06/07/2016 14:08, Wanpeng Li wrote:
>> 2016-07-06 18:44 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
>>>
>>>
>>> On 06/07/2016 08:52, 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?
>>>
>>> If it's just for spin loops, you can check if the version field in the
>>> steal time structure has changed.
>>
>> Steal time will not be updated until ahead of next vmentry except
>> wrmsr MSR_KVM_STEAL_TIME. So it can't represent it is preempted
>> currently, right?
>
> Hmm, you're right.  We can use bit 0 of struct kvm_steal_time's flags to
> indicate that pad[0] is a "VCPU preempted" field; if pad[0] is 1, the
> VCPU has been scheduled out since the last time the guest reset the bit.
>  The guest can use an xchg to test-and-clear it.  The bit can be
> accessed at any time, independent of the version field.

I will try to implement it tomorrow, thanks for your proposal. :)

Regards,
Wanpeng Li