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    SubjectRe: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods


    On Mon, 15 Jun 2009, Ingo Molnar wrote:
    >
    > [ If on the other hand it's a speedup of a few cycles then we have
    > the problem of me suddenly liking this patch a whole lot more ;-) ]

    I missed the patch.

    It's quite possible that replacing "iret" with a regular "ret" (for the
    kernel->kernel transition) is a real speedup. That said, there's a few
    things to think about:

    - CPU return stack caches/predictors. I suspect that "iret" and
    exceptions don't generally touch them (but who knows - maybe they do),
    while a regular "ret" definitely does. I dunno about "retf".

    This can cause very subtle performance slowdowns, where the slowdown
    happens somewhere else. And it could be _very_ uarch-dependent (ie only
    happen on some architectures, while having no performance downside on
    others)

    - kernel->kernel exceptions _should_ be rare, with the exception of
    actual real external interrupts. So the path to optimize should always
    be the user-space exception path. That one will need 'iret', but I'd
    also not want to see more testing in that hot-path. I suspect we
    already always test for user-mode anyway (due to signal handling etc
    work), but if it adds new tests to that path, any kernel->kernel
    speedup is likely totally pointless.

    That said, it would be nice to avoid 'iret' if only because of its subtle
    interactions with the while NMI flag.

    Linus


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