sched, cpuacct: refactoring cpuusage_read / cpuusage_write

Impact: micro-optimize the code on 64-bit architectures

In the thread regarding to 'export percpu cpuacct cgroup stats'
http://lkml.org/lkml/2008/12/7/13

akpm pointed out that current cpuacct code is inefficient.  This patch
refactoring the following:

* make cpu_rq locking only on 32-bit
* change iterator to each_present_cpu instead of each_possible_cpu to
  make it hotplug friendly.

It's a bit of code churn, but I was rewarded with 160 byte code size saving
on x86-64 arch and zero code size change on i386.

Signed-off-by: Ken Chen <kenchen@google.com>
Cc: Paul Menage <menage@google.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 39 insertions, 17 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 88215066efae..41b7e2d524d6 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -9275,6 +9275,41 @@ cpuacct_destroy(struct cgroup_subsys *ss, struct cgroup *cgrp)
         kfree(ca);
 }
 
+static u64 cpuacct_cpuusage_read(struct cpuacct *ca, int cpu)
+{
+        u64 *cpuusage = percpu_ptr(ca->cpuusage, cpu);
+        u64 data;
+
+#ifndef CONFIG_64BIT
+        /*
+         * Take rq->lock to make 64-bit read safe on 32-bit platforms.
+         */
+        spin_lock_irq(&cpu_rq(cpu)->lock);
+        data = *cpuusage;
+        spin_unlock_irq(&cpu_rq(cpu)->lock);
+#else
+        data = *cpuusage;
+#endif
+
+        return data;
+}
+
+static void cpuacct_cpuusage_write(struct cpuacct *ca, int cpu, u64 val)
+{
+        u64 *cpuusage = percpu_ptr(ca->cpuusage, cpu);
+
+#ifndef CONFIG_64BIT
+        /*
+         * Take rq->lock to make 64-bit write safe on 32-bit platforms.
+         */
+        spin_lock_irq(&cpu_rq(cpu)->lock);
+        *cpuusage = val;
+        spin_unlock_irq(&cpu_rq(cpu)->lock);
+#else
+        *cpuusage = val;
+#endif
+}
+
 /* return total cpu usage (in nanoseconds) of a group */
 static u64 cpuusage_read(struct cgroup *cgrp, struct cftype *cft)
 {
@@ -9282,17 +9317,8 @@ static u64 cpuusage_read(struct cgroup *cgrp, struct cftype *cft)
         u64 totalcpuusage = 0;
         int i;
 
-        for_each_possible_cpu(i) {
-                u64 *cpuusage = percpu_ptr(ca->cpuusage, i);
-
-                /*
-                 * Take rq->lock to make 64-bit addition safe on 32-bit
-                 * platforms.
-                 */
-                spin_lock_irq(&cpu_rq(i)->lock);
-                totalcpuusage += *cpuusage;
-                spin_unlock_irq(&cpu_rq(i)->lock);
-        }
+        for_each_present_cpu(i)
+                totalcpuusage += cpuacct_cpuusage_read(ca, i);
 
         return totalcpuusage;
 }
@@ -9309,13 +9335,9 @@ static int cpuusage_write(struct cgroup *cgrp, struct cftype *cftype,
                 goto out;
         }
 
-        for_each_possible_cpu(i) {
-                u64 *cpuusage = percpu_ptr(ca->cpuusage, i);
+        for_each_present_cpu(i)
+                cpuacct_cpuusage_write(ca, i, 0);
 
-                spin_lock_irq(&cpu_rq(i)->lock);
-                *cpuusage = 0;
-                spin_unlock_irq(&cpu_rq(i)->lock);
-        }
 out:
         return err;
 }