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

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Date:	Tue, 28 Jun 2016 13:03:56 +0800
From:	Boqun Feng <boqun.feng@...il.com>
To:	xinhui <xinhui.pan@...ux.vnet.ibm.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 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.

Regards,
Boqun

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