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    SubjectRe: [RFC PATCH 3/3] sched/fair: Use different cachelines for readers and writers of load_avg
    On 11/30/2015 05:29 PM, Peter Zijlstra wrote:
    > On Mon, Nov 30, 2015 at 02:13:32PM -0500, Waiman Long wrote:
    >>> This would only work if the structure itself is allocated with cacheline
    >>> alignment, and looking at sched_create_group(), we use a plain kzalloc()
    >>> for this, which doesn't guarantee any sort of alignment beyond machine
    >>> word size IIRC.
    >> With a RHEL 6 derived .config file, the size of the task_group structure was
    >> 460 bytes on a 32-bit x86 kernel. Adding a ____cacheline_aligned tag
    >> increase the size to 512 bytes. So it did make the structure a multiple of
    >> the cacheline size. With both slub and slab, the allocated task group
    >> pointers from kzalloc() in sched_create_group() were all multiples of 0x200.
    >> So they were properly aligned for the ____cacheline_aligned tag to work.
    > Not sure we should rely on sl*b doing the right thing here.
    > KMALLOC_MIN_ALIGN is explicitly set to sizeof(long long). If you want
    > explicit alignment, one should use KMEM_CACHE().

    I think the current kernel use power-of-2 kmemcaches to satisfy kalloc()
    requests except when the size is less than or equal to 192 where there
    are some non-power-of-2 kmemcaches available. Given that the task_group
    structure is large enough with FAIR_GROUP_SCHED enabled, we shouldn't
    hit the case that the allocated buffer is not cacheline aligned.

    Cheers,
    Longman


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