Re: [PATCH 04/21] Generic percpu refcounting

[Kent Overstreet]

On Tue, May 14, 2013 at 02:59:45PM -0700, Tejun Heo wrote:
> A couple more things.
> 
> On Mon, May 13, 2013 at 06:18:41PM -0700, Kent Overstreet wrote:
> ...
> > +/**
> > + * percpu_ref_put - decrement a dynamic percpu refcount
> > + *
> > + * Returns true if the result is 0, otherwise false; only checks for the ref
> > + * hitting 0 after percpu_ref_kill() has been called. Analagous to
> > + * atomic_dec_and_test().
> > + */
> > +static inline int percpu_ref_put(struct percpu_ref *ref)
> 
> bool?

Was int to match atomic_dec_and_test(), but switching to bool.

> 
> > +{
> > +	unsigned __percpu *pcpu_count;
> > +	int ret = 0;
> > +
> > +	preempt_disable();
> > +
> > +	pcpu_count = ACCESS_ONCE(ref->pcpu_count);
> > +
> > +	if (pcpu_count)
> 
> We probably want likely() here.

Yeah, I suppose so.

> 
> > +		__this_cpu_dec(*pcpu_count);
> > +	else
> > +		ret = atomic_dec_and_test(&ref->count);
> > +
> > +	preempt_enable();
> > +
> > +	return ret;
> 
> With likely() added, I think the compiler should be able to recognize
> that the branch on pcpu_count should exclude later branch in the
> caller to test for the final put in most cases but I'm a bit worried
> whether that would always be the case and wonder whether ->release
> based interface would be better.  Another concern is that the above
> interface is likely to encourage its users to put the release
> implementation in the same function.  e.g.

I... don't follow what you mean hear at all - what exactly would the
compiler do differently? and how would passing a release function
matter?

> 	void my_put(my_obj)
> 	{
> 		if (!percpu_ref_put(&my_obj->ref))
> 			return;
> 		destroy my_obj;
> 		free my_obj;
> 	}
> 
> Which in turn is likely to nudge the developer or compiler towards not
> inlining the fast path.

I'm kind of skeptical partial inlining would be worth it for just an
atomic_dec_and_test()...

> So, while I do like the simplicity of put() returning %true on the
> final put, I suspect it's more likely to slowing down fast paths due
> to its interface compared to having separate ->release function
> combined with void put().  Any ideas?

Oh, you mean having one branch instead of two when we're in percpu mode.
Yeah, that is a good point.

I bet with the likely() added the compiler is going to generate the same
code either way, but I suppose I can have a look at what gcc actually
does...
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