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    SubjectRe: [RFC PATCH 0/6] jump label v3
    On Wed, Nov 18, 2009 at 07:54:24PM -0800, Roland McGrath wrote:
    > 2. optimal compiled hot path code
    > You and Richard have been working on this in gcc and we know the state
    > of it now. When we get the cold labels feature done, it will be ideal
    > for -O(2?). But people mostly use -Os and there no block reordering
    > gets done now (I think perhaps this even means likely/unlikely don't
    > really change which path is the straight line, just the source order
    > of the blocks still determines it). So we hope for more incremental
    > improvements here, and maybe even really optimal code for -O2 soon.
    > But at least for -Os it may not be better than "unconditional jump
    > around" as the "straight line" path in the foreseeable future. As
    > noted, that alone is still a nice savings over the status quo for the
    > disabled case. (You gave an "average cycles saved" for this vs a load
    > and test, but do you have any comparisons of how those two compare to
    > no tracepoint at all?)

    i've run that in the past, and for the nop + jump sequence its between
    2 - 4 cycles on average vs. no tracepoint.

    > 3. bookkeeping magic to find all the jumps to enable for a given tracepoint
    > Here you have a working first draft, but it looks pretty clunky.
    > That strcmp just makes me gag. For a first version that's still
    > pretty simple, I think it should be trivial to use a pointer
    > comparison there. For tracepoints, it can be the address of the
    > struct tracepoint. For the general case, it can be the address of
    > the global that would be flag variable in case of no asm goto support.
    > For more incremental improvements, we could cut down on running
    > through the entire table for every switch. If there are many
    > different switches (as there are already for many different
    > tracepoints), then you really just want to run through the
    > insn-patch list for the particular switch when you toggle it.
    > It's possible to group this all statically at link time, but all
    > the linker magic hacking required to get that to go is probably
    > more trouble than it's worth.
    > A simple hack is to run through the big unsorted table at boot time
    > and turn it into a contiguous table for each switch. Then
    > e.g. hang each table off the per-switch global variable by the same
    > name that in a no-asm-goto build would be the simple global flag.

    that probably makes the most sense. Do a sort of the jump table and then
    store an offset,length pair with each switch. I was thinking of this as follow
    on optimization (the tracepoint code is already O(N) per switch toggle, where
    is N = total number of all tracepoint site locations, and not O(n), where
    n = number of sites per tracepoint). Certainly, if this is a gating issue for
    this patchset, I can fix it now.

    > Finally, for using this for general purposes unrelated to tracepoints,
    > I envision something like:
    > DECLARE_MOSTLY_NOT(foobar);
    > foo(int x, int y)
    > {
    > if (x > y && mostly_not(foobar))
    > do_foobar(x - y);
    > }
    > ... set_mostly_not(foobar, onoff);
    > where it's:
    > #define DECLARE_MOSTLY_NOT(name) ... __something_##name
    > #define mostly_not(name) ({ int _doit = 0; __label__ _yes; \
    > JUMP_LABEL(name, _yes, __something_##name); \
    > if (0) _yes: __cold _doit = 1; \
    > unlikely (_doit); })
    > I don't think we've tried to figure out how well this compiles yet.
    > But it shows the sort of thing that we can do to expose this feature
    > in a way that's simple and unrestrictive for kernel code to use casually.

    cool. the assembly output would be interesting here...



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