Usage of unlikely in RCU code

[Julie Sullivan]

On Wed, Aug 3, 2011 at 7:37 AM, Mulyadi Santosa
<mulyadi.santosa at gmail.com> wrote:
> Dear Julie....
>
> On Tue, Aug 2, 2011 at 04:28, Julie Sullivan <kernelmail.jms at gmail.com> wrote:
>> Specifically, the conditional
>>
>>  if (t->rcu_read_lock_nesting == 0 &&
>>            unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
>>                rcu_read_unlock_special(t);
>>
>> I've googled for 'likely/unlikely' and i've seen
>>
>>  if (unlikely(...
>>
>> before in kernel code, and I might understand if the logical operator
>> here were || not &&, but why is the 'unlikely' attribute only applied
>> to the second operand here?
>
> i guess that's because first condition "t->rcu_read_lock_nesting == 0"
> has about 50%-50% chance.... so putting likely() or unlikely() has no
> use here...
>
> i read somewhere (or maybe someone's opinion) that you only use
> likely() or unlikely() if you are about above 90% (or maybe...if
> possible 99%) sure that condition will be true or false....otherwise,
> avoid likely()/unlikely() and let the compiler+processor do the usual
> ordering/branch prediction.
>
> just my 2 cents...
>
> --
> regards,
>
> Mulyadi Santosa
> Freelance Linux trainer and consultant
>
> blog: the-hydra.blogspot.com
> training: mulyaditraining.blogspot.com
>


Thanks Mulyadi, maybe someday when I get round to performance testing
I'll try this out for myself...


Cheers
Julie