linux-kernel - Re: [PATCH v2 2/4] powerpc/spinlock: support vcpu preempted check

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Message-Id: <577C8FE3.3010208@linux.vnet.ibm.com>
Date:	Wed, 06 Jul 2016 12:58:11 +0800
From:	xinhui <xinhui.pan@...ux.vnet.ibm.com>
To:	Wanpeng Li <kernellwp@...il.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linuxppc-dev@...ts.ozlabs.org,
	virtualization@...ts.linux-foundation.org,
	linux-s390 <linux-s390@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, mpe@...erman.id.au,
	Paul Mackerras <paulus@...ba.org>, benh@...nel.crashing.org,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Waiman Long <waiman.long@....com>, will.deacon@....com,
	boqun.feng@...il.com, Davidlohr Bueso <dave@...olabs.net>,
	schwidefsky@...ibm.com
Subject: Re: [PATCH v2 2/4] powerpc/spinlock: support vcpu preempted check

Hi, wanpeng

On 2016年07月05日 17:57, Wanpeng Li wrote:
> Hi Xinhui,
> 2016-06-28 22:43 GMT+08:00 Pan Xinhui <xinhui.pan@...ux.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?

thanks
xinui
> Regards,
> Wanpeng Li
>