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    Subject[patch] x86: atomic64_t: Improve atomic64_add_return()

    * Linus Torvalds <torvalds@linux-foundation.org> wrote:

    > On Thu, 2 Jul 2009, Eric Dumazet wrote:
    > >
    > > Using a fixed initial value (instead of __atomic64_read()) is even faster,
    > > it apparently permits cpu to use an appropriate bus transaction.
    >
    > Yeah, I guess it does a "read-for-write-ownership" and allows the
    > thing to be done as a single cache transaction.
    >
    > If we read it first, it will first get the cacheline for
    > shared-read, and then the cmpxchg8b will need to turn it from
    > shared to exclusive.
    >
    > Of course, the _optimal_ situation would be if the cmpxchg8b
    > didn't actually do the write at all when the value matches (and
    > all cores could just keep it shared), but I guess that's not going
    > to happen.
    >
    > Too bad there is no pure 8-byte read op. Using MMX has too many
    > downsides.
    >
    > Btw, your numbers imply that for the atomic64_add_return(), we
    > really would be much better off not reading the original value at
    > all. Again, in that case, we really do want the
    > "read-for-write-ownership" cache transaction, not a read.

    Something like the patch below?

    Please review it carefully, as the perfcounter exposure to the
    conditional-arithmetics atomic64_t APIs is very low:

    earth4:~/tip> for N in $(git grep atomic64_ | grep perf_ |
    sed 's/(/ /g'); do echo $N; done | grep ^atomic64_ | sort | uniq -c | sort -n

    1 atomic64_add_negative
    1 atomic64_inc_return
    2 atomic64_xchg
    3 atomic64_cmpxchg
    3 atomic64_sub
    7 atomic64_t
    11 atomic64_add
    21 atomic64_set
    22 atomic64_read

    So while i have tested it on a 32-bit box, it's only lightly tested
    (and possibly broken) due to the low exposure of the API.

    Thanks,

    Ingo

    ----------------------->
    Subject: x86: atomic64_t: Improve atomic64_add_return()
    From: Ingo Molnar <mingo@elte.hu>
    Date: Fri Jul 03 12:39:07 CEST 2009

    Linus noted (based on Eric Dumazet's numbers) that we would
    probably be better off not trying an atomic_read() in
    atomic64_add_return() but intead intentionally let the first
    cmpxchg8b fail - to get a cache-friendly 'give me ownership
    of this cacheline' transaction. That can then be followed
    by the real cmpxchg8b which sets the value local to the CPU.

    Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Eric Dumazet <eric.dumazet@gmail.com>
    Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: Mike Galbraith <efault@gmx.de>
    Cc: Paul Mackerras <paulus@samba.org>
    Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
    Cc: Frederic Weisbecker <fweisbec@gmail.com>
    Cc: David Howells <dhowells@redhat.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Arnd Bergmann <arnd@arndb.de>
    LKML-Reference: <alpine.LFD.2.01.0907021653030.3210@localhost.localdomain>
    Signed-off-by: Ingo Molnar <mingo@elte.hu>
    ---
    arch/x86/lib/atomic64_32.c | 15 ++++++++++++---
    1 file changed, 12 insertions(+), 3 deletions(-)

    Index: linux/arch/x86/lib/atomic64_32.c
    ===================================================================
    --- linux.orig/arch/x86/lib/atomic64_32.c
    +++ linux/arch/x86/lib/atomic64_32.c
    @@ -76,13 +76,22 @@ u64 atomic64_read(atomic64_t *ptr)
    */
    u64 atomic64_add_return(u64 delta, atomic64_t *ptr)
    {
    - u64 old_val, new_val;
    + /*
    + * Try first with a (probably incorrect) assumption about
    + * what we have there. We'll do two loops most likely,
    + * but we'll get an ownership MESI transaction straight away
    + * instead of a read transaction followed by a
    + * flush-for-ownership transaction:
    + */
    + u64 old_val, new_val, real_val = 1ULL << 32;

    do {
    - old_val = atomic_read(ptr);
    + old_val = real_val;
    new_val = old_val + delta;

    - } while (atomic64_cmpxchg(ptr, old_val, new_val) != old_val);
    + real_val = atomic64_cmpxchg(ptr, old_val, new_val);
    +
    + } while (real_val != old_val);

    return new_val;
    }

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