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    SubjectRe: [tip:core/locking] x86/smp: Move waiting on contended ticket lock out of line
    On Thu, Feb 28, 2013 at 1:14 PM, Rik van Riel <> wrote:
    > I have modified one of the semop tests to use multiple semaphores.

    Ooh yeah. This shows contention quite nicely. And it's all from
    ipc_lock, and looking at the top-10 loffenders of the profile:

    43.01% semop-multi [kernel.kallsyms] [k] _raw_spin_lock
    4.73% semop-multi [kernel.kallsyms] [k] avc_has_perm_flags
    4.52% semop-multi [kernel.kallsyms] [k] ipc_has_perm.isra.21
    2.43% semop-multi [kernel.kallsyms] [k] ipcperms

    The 43% isn't actually all that interesting, it just shows that there
    is contention and we're waiting for other user. Yes, we waste almost
    half the CPU time on locking, but ignore that for a moment.

    The "more than 10% of the total time is spent in ipc permission code"
    *is* the interesting part. Because that 10%+ is actually more like 20%
    if you ignore the "wait for lock" part. And it's all done *inside* the

    In other words, I can pretty much guarantee that the contention will
    go down a lot if we just move the security check outside the spinlock.
    According to the above numbers, we're currently spending basically
    1/5th of our remaining CPU resources serialized for absolutely no good
    reason. THAT is the kind of thing we shouldn't do.

    The rest of the big offenders seem to be mostly done outside the
    spinlock, although it's hard to tell how much of the 10% of
    sys_semtimedop() iis also under the lock. There's probably other
    things there than just the permission checking.

    I'm not seeing any real reason the permission checking couldn't be
    done just under the RCU lock, before we get the spinlock. Except for
    the fact that the "helper" routines in ipc/util.c are written the way
    they are, so it's a layering violation. But I really think that would
    be a *reasonably* low-hanging fruit thing to do.

    Changing the locking itself to be more fine-grained, and doing it
    across many different ipc semaphores would be a major pain. So I do
    suspect that the work Michel Lespinasse did is probably worth doing
    anyway in addition to at least trying to fix the horrible lack of
    scalability of the code a bit.


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