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    SubjectRe: [PATCH][RFC] trace: profile likely and unlikely annotations
    On Tue, Oct 28, 2008 at 12:12:48AM -0400, Steven Rostedt wrote:
    >
    > Andrew Morton recently suggested having an in-kernel way to profile
    > likely and unlikely macros. This patch achieves that goal.

    Maybe I'm confused, but when I read through the patch, it looks like
    that 'hit' is incremented whenever the condition is true, and 'missed'
    is incremented whenever the condition is false, correct?

    Is that what you intended? So for profile_unlikely, "missed" is good,
    and "hit" is bad, and for profile_likely, "hit" is good, and "missed"
    is bad. That seems horribly confusing.

    If that wasn't what you intended, the meaning of "hit" and "missed"
    seems to be highly confusing, either way. Can we perhaps use some
    other terminology? Simply using "True" and "False" would be better,
    since there's no possible confusion what the labels mean.

    > +#define unlikely(x) ({ \
    > + int ______r; \
    > + static struct ftrace_likely_data ______f \
    > + __attribute__((__aligned__(4))) \
    > + __attribute__((section("_ftrace_unlikely"))); \
    > + if (unlikely_notrace(!______f.ip)) \
    > + ______f.ip = __THIS_IP__; \
    > + ______r = unlikely_notrace(x); \
    > + ftrace_likely_update(&______f, ______r); \
    > + ______r; \
    > + })

    Note that unlikely(x) calls ftrace_likely_update(), which does this:

    > +void ftrace_likely_update(struct ftrace_likely_data *f, int val)
    > +{
    > + /* FIXME: Make this atomic! */
    > + if (val)
    > + f->hit++;
    > + else
    > + f->missed++;
    > +}
    > +EXPORT_SYMBOL(ftrace_likely_update);


    So that seems to mean that if unlikely(x) is false, then _____r is 0,
    which means we increment f->missed. Or am I missing something?

    I would have thought that if unlikely(x) is false, that's *good*,
    since it means the unlikely label was correct. And normally, when
    people think about cache hits vs cache misses, hits are good and
    misses are bad. Which is why I think the terminology is highly
    confusing...

    - Ted


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