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

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Date:	Mon, 11 Jul 2016 11:10:15 -0400
From:	Waiman Long <waiman.long@....com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>,
	<linux-kernel@...r.kernel.org>, <linuxppc-dev@...ts.ozlabs.org>,
	<virtualization@...ts.linux-foundation.org>,
	<linux-s390@...r.kernel.org>, <mingo@...hat.com>,
	<mpe@...erman.id.au>, <paulus@...ba.org>,
	<benh@...nel.crashing.org>, <paulmck@...ux.vnet.ibm.com>,
	<will.deacon@....com>, <boqun.feng@...il.com>, <dave@...olabs.net>,
	<schwidefsky@...ibm.com>, <pbonzini@...hat.com>
Subject: Re: [PATCH v2 0/4] implement vcpu preempted check

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.

Cheers,
Longman