linux-kernel - Re: [RFC 4/4]x86: avoid tlbstate lock if no enough cpus

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Date:	Thu, 04 Nov 2010 13:21:19 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	lkml <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Andi Kleen <andi@...stfloor.org>,
	"hpa@...or.com" <hpa@...or.com>
Subject: Re: [RFC 4/4]x86: avoid tlbstate lock if no enough cpus

On Wed, 2010-11-03 at 17:08 +0800, Eric Dumazet wrote:
> Le mercredi 03 novembre 2010 à 16:41 +0800, Shaohua Li a écrit :
> 
> > yes, this is ok. we might need avoid some cpu hotplug race too. I'll
> > post a new patch later.
> > 
> > 
> 
> Hmm, maybe only set the variable "must take the lock", never unset it.
I followed your suggestions to use nr_cpu_ids, it should be good enough.

Thanks,
Shaohua


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.

Uses nr_cpu_ids here as suggested by Eric Dumazet.

Signed-off-by: Shaohua Li <shaohua.li@...el.com>
---
 arch/x86/mm/tlb.c |   11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

Index: linux/arch/x86/mm/tlb.c
===================================================================
--- linux.orig/arch/x86/mm/tlb.c	2010-11-04 10:59:09.000000000 +0800
+++ linux/arch/x86/mm/tlb.c	2010-11-04 13:17:51.000000000 +0800
@@ -179,12 +179,8 @@ static void flush_tlb_others_ipi(const s
 	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 (nr_cpu_ids > NUM_INVALIDATE_TLB_VECTORS)
+		raw_spin_lock(&f->tlbstate_lock);
 
 	f->flush_mm = mm;
 	f->flush_va = va;
@@ -202,7 +198,8 @@ static void flush_tlb_others_ipi(const s
 
 	f->flush_mm = NULL;
 	f->flush_va = 0;
-	raw_spin_unlock(&f->tlbstate_lock);
+	if (nr_cpu_ids > NUM_INVALIDATE_TLB_VECTORS)
+		raw_spin_unlock(&f->tlbstate_lock);
 }
 
 void native_flush_tlb_others(const struct cpumask *cpumask,


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