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SubjectRe: Kernel broken on processors without performance counters
On Tue, Jul 21, 2015 at 8:49 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Tue, Jul 21, 2015 at 05:43:27PM +0200, Thomas Gleixner wrote:
>> On Tue, 21 Jul 2015, Peter Zijlstra wrote:
>>
>> > > -#endif /* _LINUX_JUMP_LABEL_H */
>> > > +static inline void static_key_enable(struct static_key *key)
>> > > +{
>> > > + int count = static_key_count(key);
>> > > +
>> > > + WARN_ON_ONCE(count < 0 || count > 1);
>> > > +
>> > > + if (!count)
>> > > + static_key_slow_inc(key);
>> > > +}
>> > > +
>> > > +static inline void static_key_disable(struct static_key *key)
>> > > +{
>> > > + int count = static_key_count(key);
>> > > +
>> > > + WARN_ON_ONCE(count < 0 || count > 1);
>> > > +
>> > > + if (count)
>> > > + static_key_slow_dec(key);
>> > > +}
>>
>> The functions above are not part of the interface which should be used
>> in code, right?
>
> They are in fact. They make it easier to deal with the refcount thing
> when all you want is boolean states.
>
>> > > +/* -------------------------------------------------------------------------- */
>> > > +
>> > > +/*
>> > > + * likely -- default enabled, puts the branch body in-line
>> > > + */
>> > > +
>> > > +struct static_likely_key {
>> > > + struct static_key key;
>> > > +};
>> > > +
>> > > +#define STATIC_LIKELY_KEY_INIT (struct static_likely_key){ .key = STATIC_KEY_INIT_TRUE, }
>> > > +
>> > > +static inline bool static_likely_branch(struct static_likely_key *key)
>> > > +{
>> > > + return static_key_true(&key->key);
>> > > +}
>> > > +
>> > > +/*
>> > > + * unlikely -- default disabled, puts the branch body out-of-line
>> > > + */
>> > > +
>> > > +struct static_unlikely_key {
>> > > + struct static_key key;
>> > > +};
>> > > +
>> > > +#define STATIC_UNLIKELY_KEY_INIT (struct static_unlikely_key){ .key = STATIC_KEY_INIT_FALSE, }
>> > > +
>> > > +static inline bool static_unlikely_branch(struct static_unlikely_key *key)
>> > > +{
>> > > + return static_key_false(&key->key);
>> > > +}
>> > > +
>> > > +/*
>> > > + * Advanced usage; refcount, branch is enabled when: count != 0
>> > > + */
>> > > +
>> > > +#define static_branch_inc(_k) static_key_slow_inc(&(_k)->key)
>> > > +#define static_branch_dec(_k) static_key_slow_dec(&(_k)->key)
>>
>> Inlines please
>>
>> > > +/*
>> > > + * Normal usage; boolean enable/disable.
>> > > + */
>> > > +
>> > > +#define static_branch_enable(_k) static_key_enable(&(_k)->key)
>> > > +#define static_branch_disable(_k) static_key_disable(&(_k)->key)
>>
>> Ditto
>>
>> All in all much more understandable than the existing horror.
>
> They cannot in fact be inlines because we play type tricks. Note how _k
> can be either struct static_likely_key or struct static_unlikely_key.
>
>

To clarify my (mis-)understanding:

There are two degrees of freedom in a static_key. They can start out
true or false, and they can be unlikely or likely. Are those two
degrees of freedom in fact tied together?

--Andy


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