linux-kernel - Re: [thiscpuops upgrade 10/10] Lockless (and preemptless) fastpaths for slub

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Date:	Tue, 23 Nov 2010 22:37:37 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	akpm@...ux-foundation.org, Pekka Enberg <penberg@...helsinki.fi>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [thiscpuops upgrade 10/10] Lockless (and preemptless) fastpaths
 for slub

Use power of two increment in all cases.

Signed-off-by: Christoph Lameter <cl@...ux.com>

Index: linux-2.6/mm/slub.c
===================================================================
--- linux-2.6.orig/mm/slub.c	2010-11-23 22:22:03.596998064 -0600
+++ linux-2.6/mm/slub.c	2010-11-23 21:53:09.422591631 -0600
@@ -1490,19 +1490,21 @@
  * during cmpxchg. The transactions start with the cpu number and are then
  * incremented by CONFIG_NR_CPUS.
  */
+#define TID_STEP  roundup_pow_of_two(CONFIG_NR_CPUS)
+
 static inline unsigned long next_tid(unsigned long tid)
 {
-	return tid + CONFIG_NR_CPUS;
+	return tid + TID_STEP;
 }

 static inline unsigned int tid_to_cpu(unsigned long tid)
 {
-	return tid % CONFIG_NR_CPUS;
+	return tid % TID_STEP;
 }

 static inline unsigned long tid_to_event(unsigned long tid)
 {
-	return tid / CONFIG_NR_CPUS;
+	return tid / TID_STEP;
 }

 static inline unsigned int init_tid(int cpu)

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