Re: [PATCH 6/7] memcg: speed up by percpu

[Andy Whitcroft <apw@shadowen.org>]

On Wed, Mar 19, 2008 at 10:41:56PM +0100, Peter Zijlstra wrote:
> On Wed, 2008-03-19 at 22:19 +0100, Peter Zijlstra wrote:
> 
> > > +static void save_result(struct page_cgroup  *base, unsigned long idx)
> > > +{
> > > +	int hash = idx & (PAGE_CGROUP_NR_CACHE - 1);
> > > +	struct page_cgroup_cache *pcp;
> > > +	/* look up is done under preempt_disable(). then, don't call
> > > +	   this under interrupt(). */
> > > +	preempt_disable();
> > > +	pcp = &__get_cpu_var(pcpu_page_cgroup_cache);
> > > +	pcp->ents[hash].idx = idx;
> > > +	pcp->ents[hash].base = base;
> > > +	preempt_enable();
> > > +}
> 
> Another instance where get_cpu_var(), put_cpu_var() would be preferable.
> 
> Raw preempt_{disable,enable)() calls are discouraged, because just as
> the BKL they are opaque, they don't tell us what data is protected from
> what, or if we rely upon interaction with the RCU grace period or things
> like that.
> 
> These things are a pain to sort out if you try to change the preemption
> model after a few years.
> 
> Ingo, does it make sense to make checkpatch.pl warn about raw
> preempt_{disable,enable} calls?

That signature would likely be detectable even if all preempt_disable()s
were not reportable.  I do wonder looking at the shape of that whether
there is a ganged version for where you want more than one variable; I
can't see one.

-apw

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