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    Subject[RFC 4/4]x86: avoid tlbstate lock if no enough cpus
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    This one isn't related to previous patch. If online cpus are below
    NUM_INVALIDATE_TLB_VECTORS, we don't need the lock. The comments
    in the code declares we don't need the check, but a hot lock still
    needs an atomic operation and expensive, so add the check here.

    Signed-off-by: Shaohua Li <shaohua.li@intel.com>
    ---
    arch/x86/mm/tlb.c | 14 +++++++-------
    1 file changed, 7 insertions(+), 7 deletions(-)

    Index: linux/arch/x86/mm/tlb.c
    ===================================================================
    --- linux.orig/arch/x86/mm/tlb.c 2010-11-02 10:31:51.000000000 +0800
    +++ linux/arch/x86/mm/tlb.c 2010-11-02 14:53:27.000000000 +0800
    @@ -174,17 +174,16 @@ static void flush_tlb_others_ipi(const s
    {
    unsigned int sender;
    union smp_flush_state *f;
    + bool do_lock = false;

    /* Caller has disabled preemption */
    sender = this_cpu_read(tlb_vector_offset);
    f = &flush_state[sender];

    - /*
    - * Could avoid this lock when
    - * num_online_cpus() <= NUM_INVALIDATE_TLB_VECTORS, but it is
    - * probably not worth checking this for a cache-hot lock.
    - */
    - raw_spin_lock(&f->tlbstate_lock);
    + if (num_online_cpus() > NUM_INVALIDATE_TLB_VECTORS) {
    + do_lock = true;
    + raw_spin_lock(&f->tlbstate_lock);
    + }

    f->flush_mm = mm;
    f->flush_va = va;
    @@ -202,7 +201,8 @@ static void flush_tlb_others_ipi(const s

    f->flush_mm = NULL;
    f->flush_va = 0;
    - raw_spin_unlock(&f->tlbstate_lock);
    + if (do_lock)
    + raw_spin_unlock(&f->tlbstate_lock);
    }

    void native_flush_tlb_others(const struct cpumask *cpumask,



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