linux-kernel - Re: [PATCH v8] mm: make lru_add_drain

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Date:	Wed, 14 Aug 2013 14:07:00 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Chris Metcalf <cmetcalf@...era.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Cody P Schafer <cody@...ux.vnet.ibm.com>
Subject: Re: [PATCH v8] mm: make lru_add_drain_all() selective

On Wed, 14 Aug 2013 16:50:29 -0400 Tejun Heo <tj@...nel.org> wrote:

> On Wed, Aug 14, 2013 at 01:44:30PM -0700, Andrew Morton wrote:
> > > +static bool need_activate_page_drain(int cpu)
> > > +{
> > > +	return pagevec_count(&per_cpu(activate_page_pvecs, cpu)) != 0;
> > > +}
> > 
> > static int need_activate_page_drain(int cpu)
> > {
> > 	return pagevec_count(&per_cpu(activate_page_pvecs, cpu));
> > }
> > 
> > would be shorter and faster.  bool rather sucks that way.  It's a
> > performance-vs-niceness thing.  I guess one has to look at the call
> > frequency when deciding.
> 
> "!= 0" can be dropped but I'm fairly sure the compiler would be able
> to figure out that the type conversion can be skipped.  It's a trivial
> optimization.

The != 0 can surely be removed and that shouldn't make any difference
to generated code.

The compiler will always need to do the int-to-bool conversion and
that's overhead which is added by using bool.

It's possible that the compiler will optmise away the int-to-bool
conversion when inlining this function into a callsite.  I don't know
whether the compiler _does_ do this and it will be version dependent.


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