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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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