[PATCH] sched: cpuacct: Use bigger percpu counter batch values for stats counters

[Anton Blanchard <anton@samba.org>]

Hi,

Another try at this percpu_counter batch issue with CONFIG_VIRT_CPU_ACCOUNTING
and CONFIG_CGROUP_CPUACCT enabled. Thoughts?

--

When CONFIG_VIRT_CPU_ACCOUNTING and CONFIG_CGROUP_CPUACCT are 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 overflow.

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:

wget http://ozlabs.org/~anton/junkcode/context_switch.c
make context_switch
for i in `seq 0 63`; do taskset -c $i ./context_switch & done
vmstat 1

Signed-off-by: Anton Blanchard <anton@samba.org>
---

Note: ccing ia64 and s390 who have not yet added code to statically
initialise cputime_one_jiffy at boot. 
See a42548a18866e87092db93b771e6c5b060d78401 (cputime: Optimize
jiffies_to_cputime(1) for details). Adding this would help optimise not only
this patch but many other areas of the scheduler when
CONFIG_VIRT_CPU_ACCOUNTING is enabled.

Index: linux.trees.git/kernel/sched.c
===================================================================
--- linux.trees.git.orig/kernel/sched.c	2010-01-18 14:27:12.000000000 +1100
+++ linux.trees.git/kernel/sched.c	2010-01-18 15:21:59.000000000 +1100
@@ -10894,6 +10894,7 @@ static void cpuacct_update_stats(struct 
 		enum cpuacct_stat_index idx, cputime_t val)
 {
 	struct cpuacct *ca;
+	int batch;
 
 	if (unlikely(!cpuacct_subsys.active))
 		return;
@@ -10901,8 +10902,9 @@ static void cpuacct_update_stats(struct 
 	rcu_read_lock();
 	ca = task_ca(tsk);
 
+	batch = min_t(long, percpu_counter_batch * cputime_one_jiffy, INT_MAX);
 	do {
-		percpu_counter_add(&ca->cpustat[idx], val);
+		__percpu_counter_add(&ca->cpustat[idx], val, batch);
 		ca = ca->parent;
 	} while (ca);
 	rcu_read_unlock();