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...
--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
