linux-kernel - Re: [Xen-devel] [PATCH RFC v2 4/4] xen/arm: account for stolen ticks

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Date:	Tue, 7 May 2013 14:49:46 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Ian Campbell <Ian.Campbell@...rix.com>
CC:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"marc.zyngier@....com" <marc.zyngier@....com>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
	"will.deacon@....com" <will.deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [Xen-devel] [PATCH RFC v2 4/4] xen/arm: account for stolen
 ticks

On Tue, 7 May 2013, Ian Campbell wrote:
> > > > @@ -301,6 +320,10 @@ static int __init xen_init_events(void)
> > > >  
> > > >  	on_each_cpu(xen_percpu_init, NULL, 0);
> > > >  
> > > > +	pv_time_ops.steal_clock = xen_stolen_accounting;
> > > > +	static_key_slow_inc(&paravirt_steal_enabled);
> > > > +	static_key_slow_inc(&paravirt_steal_rq_enabled);
> > > 
> > > We don't seem to do this on x86 -- is that a bug on x86 on Xen?
> > 
> > On x86 we do all the accounting in do_stolen_accounting, called from our
> > own interrupt handler (xen_timer_interrupt).
> > I don't think we would gain anything by using the common infrastructure,
> > we would actually loose the idle ticks accounting we do there.
> > 
> > Speaking of which, I don't think that pv_time_ops.steal_clock would
> > properly increase CPUTIME_IDLE the way we do in do_stolen_accounting.
> > 
> > How much of an issue is that?
> 
> Doesn't the generic account_idle_time handle this?

AFAICT only if the rq is idle, while do_stolen_accounting would account
for ticks in RUNSTATE_blocked
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