linux-kernel - Re: [PATCH 2/3] powerpc/spinlock: support vcpu preempted check

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Message-Id: <57720D4B.8090406@linux.vnet.ibm.com>
Date:	Tue, 28 Jun 2016 13:38:19 +0800
From:	xinhui <xinhui.pan@...ux.vnet.ibm.com>
To:	Boqun Feng <boqun.feng@...il.com>
CC:	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	paulmck@...ux.vnet.ibm.com, peterz@...radead.org, mingo@...hat.com,
	mpe@...erman.id.au, paulus@...ba.org, benh@...nel.crashing.org,
	Waiman.Long@....com, will.deacon@....com, dave@...olabs.net
Subject: Re: [PATCH 2/3] powerpc/spinlock: support vcpu preempted check



On 2016年06月28日 13:03, Boqun Feng wrote:
> On Tue, Jun 28, 2016 at 11:39:18AM +0800, xinhui wrote:
> [snip]
>>>> +{
>>>> +	struct lppaca *lp = &lppaca_of(cpu);
>>>> +
>>>> +	if (unlikely(!(lppaca_shared_proc(lp) ||
>>>> +			lppaca_dedicated_proc(lp))))
>>>
>>> Do you want to detect whether we are running in a guest(ie. pseries
>>> kernel) here? Then I wonder whether "machine_is(pseries)" works here.
>>>
>> I tried as you said yesterday. but .h file has dependencies.
>> As you said, if we add #ifdef PPC_PSERIES, this is not a big problem. only powernv will be affected as they are built into same kernel img.
>>
>
> I never said this it not a big problem ;-)
>
> The problem here is that we only need to detect the vcpu preemption in
> a guest, and there could be several ways we can detect whether the
> kernel is running in a guest. It's worthwhile to try find the best one
> for this. Besides, it's really better that you can make sure we are
> runing out of options before you introduce something like
> lppaca_dedicated_proc().
>
> I have a feeling that yield_count is non-zero only if we are running in
> a guest, if so, we can use this and save several loads. But surely we
> need the confirmation from ppc maintainers.
>
yes, on powernv, print the lppaca.yield_count and it is always zero. looks like only hypervisor and os can touch/modify it.


> Regards,
> Boqun
>