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    SubjectRe: [LKP] [mm] 9bc8039e71: will-it-scale.per_thread_ops -64.1% regression
    On Mon, Nov 5, 2018 at 10:28 AM Yang Shi <yang.shi@linux.alibaba.com> wrote:
    >
    > Actually, the commit is mainly for optimizing the long stall time caused
    > by holding mmap_sem by write when unmapping or shrinking large mapping.
    > It downgrades write mmap_sem to read when zapping pages. So, it looks
    > the downgrade incurs more context switches. This is kind of expected.
    >
    > However, the test looks just shrink the mapping with one normal 4K page
    > size. It sounds the overhead of context switches outpace the gain in
    > this case at the first glance.

    I'm not seeing why there should be a context switch in the first place.

    Even if you have lots of concurrent brk() users, they should all block
    exactly the same way as before (a write lock blocks against a write
    lock, but it *also* blocks against a downgraded read lock).

    So no, I don't want just some limit to hide this problem for that
    particular test. There's something else going on.

    Linus

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