linux-kernel - Re: [thisops uV3 15/18] Xen: Use this_cpu

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Date:	Tue, 30 Nov 2010 15:03:20 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
cc:	akpm@...ux-foundation.org, Pekka Enberg <penberg@...helsinki.fi>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	linux-kernel@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Tejun Heo <tj@...nel.org>, linux-mm@...ck.org
Subject: Re: [thisops uV3 15/18] Xen: Use this_cpu_ops

On Tue, 30 Nov 2010, Jeremy Fitzhardinge wrote:

> On 11/30/2010 11:07 AM, Christoph Lameter wrote:
> >  static irqreturn_t xen_timer_interrupt(int irq, void *dev_id)
> >  {
> > -	struct clock_event_device *evt = &__get_cpu_var(xen_clock_events);
> >  	irqreturn_t ret;
> >
> >  	ret = IRQ_NONE;
> > -	if (evt->event_handler) {
> > -		evt->event_handler(evt);
> > +	if (__this_cpu_read(xen_clock_events.event_handler)) {
> > +		__this_cpu_read(xen_clock_events.event_handler)(evt);
>
> Really?  What code does this generate?  If this is generating two
> segment-prefixed reads rather than getting the address and doing normal
> reads on it, then I don't think it is an improvement.

Lets drop that hunk. No point to do optimizations at that location then.

evt is also not defined then. Without the evt address determination via
__get_cpu_var we have at least 2 prefixed load and one address
calculation to figure out the parameter to pass. No win.

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