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    Subject[PATCH] mm/vmalloc: reduce the number of lazy_max_pages to reduce latency
    Date
    On Marvell berlin arm64 platforms, I see the preemptoff tracer report
    a max 26543 us latency at __purge_vmap_area_lazy, this latency is an
    awfully bad for STB. And the ftrace log also shows __free_vmap_area
    contributes most latency now. I noticed that Joel mentioned the same
    issue[1] on x86 platform and gave two solutions, but it seems no patch
    is sent out for this purpose.

    This patch adopts Joel's first solution, but I use 16MB per core
    rather than 8MB per core for the number of lazy_max_pages. After this
    patch, the preemptoff tracer reports a max 6455us latency, reduced to
    1/4 of original result.

    [1] http://lkml.iu.edu/hypermail/linux/kernel/1603.2/04803.html

    Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
    ---
    mm/vmalloc.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/mm/vmalloc.c b/mm/vmalloc.c
    index 91f44e7..66f377a 100644
    --- a/mm/vmalloc.c
    +++ b/mm/vmalloc.c
    @@ -596,7 +596,7 @@ static unsigned long lazy_max_pages(void)

    log = fls(num_online_cpus());

    - return log * (32UL * 1024 * 1024 / PAGE_SIZE);
    + return log * (16UL * 1024 * 1024 / PAGE_SIZE);
    }

    static atomic_t vmap_lazy_nr = ATOMIC_INIT(0);
    --
    2.9.3
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