[RFC 4/4]x86: avoid tlbstate lock if no enough cpus

[Shaohua Li]

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@xxxxxxxxx>
---
 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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