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Subject[111/123] sched: cpuacct: Use bigger percpu counter batch values for stats counters
From: Anton Blanchard <>

commit fa535a77bd3fa32b9215ba375d6a202fe73e1dd6 upstream

enabled we can call cpuacct_update_stats with values much larger
than percpu_counter_batch. This means the call to
percpu_counter_add will always add to the global count which is
protected by a spinlock and we end up with a global spinlock in
the scheduler.

Based on an idea by KOSAKI Motohiro, this patch scales the batch
value by cputime_one_jiffy such that we have the same batch
limit as we would if CONFIG_VIRT_CPU_ACCOUNTING was disabled.
His patch did this once at boot but that initialisation happened
too early on PowerPC (before time_init) and it was never updated
at runtime as a result of a hotplug cpu add/remove.

This patch instead scales percpu_counter_batch by
cputime_one_jiffy at runtime, which keeps the batch correct even
after cpu hotplug operations. We cap it at INT_MAX in case of

For architectures that do not support
CONFIG_VIRT_CPU_ACCOUNTING, cputime_one_jiffy is the constant 1
and gcc is smart enough to optimise min(s32
percpu_counter_batch, INT_MAX) to just percpu_counter_batch at
least on x86 and PowerPC. So there is no need to add an #ifdef.

On a 64 thread PowerPC box with CONFIG_VIRT_CPU_ACCOUNTING and
CONFIG_CGROUP_CPUACCT enabled, a context switch microbenchmark
is 234x faster and almost matches a CONFIG_CGROUP_CPUACCT
disabled kernel:

CONFIG_CGROUP_CPUACCT disabled: 16906698 ctx switches/sec
CONFIG_CGROUP_CPUACCT enabled: 61720 ctx switches/sec
CONFIG_CGROUP_CPUACCT + patch: 16663217 ctx switches/sec

Tested with:

make context_switch
for i in `seq 0 63`; do taskset -c $i ./context_switch & done
vmstat 1

Signed-off-by: Anton Blanchard <>
Reviewed-by: KOSAKI Motohiro <>
Acked-by: Balbir Singh <>
Tested-by: Balbir Singh <>
Cc: Peter Zijlstra <>
Cc: Martin Schwidefsky <>
Cc: Tony Luck <>
Signed-off-by: Andrew Morton <>
Signed-off-by: Ingo Molnar <>
Signed-off-by: Mike Galbraith <>
Signed-off-by: Greg Kroah-Hartman <>
kernel/sched.c | 20 +++++++++++++++++++-
1 file changed, 19 insertions(+), 1 deletion(-)

--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -10934,12 +10934,30 @@ static void cpuacct_charge(struct task_s

+ * When CONFIG_VIRT_CPU_ACCOUNTING is enabled one jiffy can be very large
+ * in cputime_t units. As a result, cpuacct_update_stats calls
+ * percpu_counter_add with values large enough to always overflow the
+ * per cpu batch limit causing bad SMP scalability.
+ *
+ * To fix this we scale percpu_counter_batch by cputime_one_jiffy so we
+ * batch the same amount of time with CONFIG_VIRT_CPU_ACCOUNTING disabled
+ * and enabled. We cap it at INT_MAX which is the largest allowed batch value.
+ */
+#ifdef CONFIG_SMP
+#define CPUACCT_BATCH \
+ min_t(long, percpu_counter_batch * cputime_one_jiffy, INT_MAX)
+#define CPUACCT_BATCH 0
* Charge the system/user time to the task's accounting group.
static void cpuacct_update_stats(struct task_struct *tsk,
enum cpuacct_stat_index idx, cputime_t val)
struct cpuacct *ca;
+ int batch = CPUACCT_BATCH;

if (unlikely(!
@@ -10948,7 +10966,7 @@ static void cpuacct_update_stats(struct
ca = task_ca(tsk);

do {
- percpu_counter_add(&ca->cpustat[idx], val);
+ __percpu_counter_add(&ca->cpustat[idx], val, batch);
ca = ca->parent;
} while (ca);

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