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    SubjectRe: [PATCH 00/10] jump label: introduce very_[un]likely + cleanups + docs

    * Jason Baron <jbaron@redhat.com> wrote:

    > On Thu, Feb 23, 2012 at 06:18:42PM -0500, Mathieu Desnoyers wrote:
    > > * Ingo Molnar (mingo@elte.hu) wrote:
    > > >
    > > > * Ingo Molnar <mingo@elte.hu> wrote:
    > > >
    > > > > So, a modified scheme would be:
    > > > >
    > > > > #include <linux/static_key.h>
    > > > >
    > > > > struct static_key key = STATIC_KEY_INIT_TRUE;
    > > > >
    > > > > if (static_key_false(&key))
    > > > > do unlikely code
    > > > > else
    > > > > do likely code
    > > > >
    > > > > Or:
    > > > >
    > > > > if (static_key_true(&key))
    > > > > do likely code
    > > > > else
    > > > > do unlikely code
    > > > >
    > > > > The static key is modified via:
    > > > >
    > > > > static_key_slow_inc(&key);
    > > > > ...
    > > > > static_key_slow_dec(&key);
    > > > >
    > > > > Is that API fine? I'll rework the series to such an effect if
    > > > > everyone agrees.
    > > >
    > > > I.e. something like the patch below on top of
    > > > tip:perf/jump-labels.
    > > >
    > > > Untested - will test it and will refactor the series if
    > > > everyone's happy.
    > >
    > > Hi Ingo,
    > >
    > > Reading your documentation updates makes me realise that adding the
    > > "inline" keyword in there would make the whole thing even clearer:
    > >
    > > struct static_key key = STATIC_KEY_INLINE_TRUE_INIT;
    > > struct static_key key = STATIC_KEY_INLINE_FALSE_INIT;
    > >
    > > static_key_inline_true() / static_key_inline_false()
    > >
    > > to show that the "true/false" in there does not mean that the key will
    > > always be true or false (the key value can indeed by changed by calling
    > > static_key_slow_inc/dec), but that the inlined path is either the true
    > > of false branch.
    > >
    >
    > Also, as part of the API, there is a test to check the branch
    > direction - which was 'jump_label_true(key)', but is now also
    > 'static_key_true(key)', [...]

    Yeah, there is such an overlap - I've renamed it to
    static_key_enabled(), which makes sense anyway as the original
    was jump_label_enabled()..

    Btw., shouldnt it be an inline function? Currently it's:

    bool static_key_enabled(struct static_key *key)
    {
    return (atomic_read(&key->enabled) > 0);
    }

    which is the perfect candidate for inlining. The difference to
    static_key_true() is the lack of the jump label patching and the
    lack of an unlikely() hint.

    > [...] so we are going to have to change either the branch site
    > or the test for true/false name. The above
    > 'static_key_inline_true/false' solves that.

    It's generally good practice to make the mostly commonly used
    method names the simplest/shortest names - i.e. I don't think we
    should make it longer via adding an _inline to every use.

    In that sense static_key_true() has pretty optimal length - we'd
    like these tests to also be visually unintrusive.

    So in the latest patch (still under testing, will push it out
    soon) we have:

    static_key_true()
    static_key_false()
    static_key_enabled()

    > Also, I do like having a consistent 'static_key_*' prefix for
    > the entire API - definitely an improvement for me.

    Yeah.

    Thanks,

    Ingo


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