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    SubjectRe: [RFC 4/4]x86: avoid tlbstate lock if no enough cpus
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    Le mercredi 03 novembre 2010 à 14:44 +0800, Shaohua Li a écrit :
    > 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) {

    Ouch, you remove a comment that pretty well explained the problem.

    Last time I checked, num_online_cpus() was pretty expensive on a 4096
    cpus machine, since 4096 bits array is 512 bytes long.

    Are you sure you didnt want to use nr_cpu_ids here ?

    > + 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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