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    SubjectRe: [PATCH 1/2] x86/mm: maintain a percpu "in get_user_pages_fast" flag
    Jeremy Fitzhardinge a écrit :
    > Avi Kivity wrote:
    >> Jeremy Fitzhardinge wrote:
    >>> get_user_pages_fast() relies on cross-cpu tlb flushes being a barrier
    >>> between clearing and setting a pte, and before freeing a pagetable page.
    >>> It usually does this by disabling interrupts to hold off IPIs, but
    >>> some tlb flush implementations don't use IPIs for tlb flushes, and
    >>> must use another mechanism.
    >>>
    >>> In this change, add in_gup_cpumask, which is a cpumask of cpus currently
    >>> performing a get_user_pages_fast traversal of a pagetable. A cross-cpu
    >>> tlb flush function can use this to determine whether it should hold-off
    >>> on the flush until the gup_fast has finished.
    >>>
    >>> @@ -255,6 +260,10 @@ int get_user_pages_fast(unsigned long start, int
    >>> nr_pages, int write,
    >>> * address down to the the page and take a ref on it.
    >>> */
    >>> local_irq_disable();
    >>> +
    >>> + cpu = smp_processor_id();
    >>> + cpumask_set_cpu(cpu, in_gup_cpumask);
    >>> +
    >>
    >> This will bounce a cacheline, every time. Please wrap in CONFIG_XEN
    >> and skip at runtime if Xen is not enabled.
    >
    > Every time? Only when running successive gup_fasts on different cpus,
    > and only twice per gup_fast. (What's the typical page count? I see that
    > kvm and lguest are page-at-a-time users, but presumably direct IO has
    > larger batches.)

    If I am not mistaken, shared futexes where hitting hard mm semaphore.
    Then gup_fast was introduced in kernel/futex.c to remove this contention point.
    Yet, this contention point was process specific, not a global one :)

    And now, you want to add a global hot point, that would slow
    down unrelated processes, only because they use shared futexes, thousand
    times per second...

    >
    > Alternatively, it could have per-cpu flags and the other side could
    > construct the mask (I originally had that, but this was simpler).

    Simpler but would be a regression for legacy applications still using shared
    futexes (because statically linked with old libc)


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