Re: [PATCH v3] cpuacct: VIRT_CPU_ACCOUNTING don't prevent percpu cputime count

KOSAKI Motohiro
2009-05-09T09:26:21Z

>
> * KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:
>
> >
> > Changelog:
> > since v2
> > - revert using percpu_counter_sum()
> >
> > since v1
> > - use percpu_counter_sum() instead percpu_counter_read()
> >
> >
> > -------------------------------------
> > Subject: [PATCH v3] cpuacct: VIRT_CPU_ACCOUNTING don't prevent percpu cputime count
> >
> > cpuacct_update_stats() is called at every tick updating. and it use percpu_counter
> > for avoiding performance degression.
> >
> > For archs which define VIRT_CPU_ACCOUNTING, every tick would result
> > in >1000 units of cputime updates and since this is much much greater
> > than percpu_batch_counter, we end up taking spinlock on every tick.
> >
> > This patch change batch rule. now, any cpu can store "percpu_counter_bach * jiffies"
> > cputime in per-cpu cache.
> > it mean this patch don't have behavior change if VIRT_CPU_ACCOUNTING=n.
> >
> > Cc: Bharata B Rao <bharata@linux.vnet.ibm.com>
> > Cc: Balaji Rao <balajirrao@gmail.com>
> > Cc: Dhaval Giani <dhaval@linux.vnet.ibm.com>
> > Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
> > Cc: Ingo Molnar <mingo@elte.hu>
> > Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
> > Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> > ---
> > kernel/sched.c | 6 +++++-
> > 1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > Index: b/kernel/sched.c
> > ===================================================================
> > --- a/kernel/sched.c 2009-04-30 11:37:47.000000000 +0900
> > +++ b/kernel/sched.c 2009-05-07 16:34:00.000000000 +0900
> > @@ -10221,6 +10221,7 @@ struct cpuacct {
> > };
> >
> > struct cgroup_subsys cpuacct_subsys;
> > +static s32 cpuacct_batch;
>
> should be __read_mostly i guess.
>
> >
> > /* return cpu accounting group corresponding to this container */
> > static inline struct cpuacct *cgroup_ca(struct cgroup *cgrp)
> > @@ -10250,6 +10251,9 @@ static struct cgroup_subsys_state *cpuac
> > if (!ca->cpuusage)
> > goto out_free_ca;
> >
> > + if (!cpuacct_batch)
> > + cpuacct_batch = jiffies_to_cputime(percpu_counter_batch);
>
> So this is a once per boot condition? Why not initialize it in
> sched_init() or so? (instead of incuring this ugly check all the
> time)

Thanks! fixed.

Changelog:
since V3
- rewirte patch description (thanks Bharata!)
- append read_mostly to cpuacct_batch
- cpuacct_batch is initialized by sched_init()

since v2
- revert using percpu_counter_sum()

since v1
- use percpu_counter_sum() instead percpu_counter_read()

---------------------------------------------------------
Subject: [PATCH v4] cpuacct: Use bigger percpu counter batch values for stats counters on archs that have VIRT_CPU_ACCOUNTING=y

percpu counters used to accumulate statistics in cpuacct controller use
the default batch value [max(2*nr_cpus, 32)] which can be too small for
archs that define VIRT_CPU_ACCOUNTING. In such archs, a tick could result in
cputime updates in the range of thousands. As a result, cpuacct_update_stats()
would end up acquiring the percpu counter spinlock on every tick which
is not good for performance.

Let those architectures to have a bigger batch threshold so that percpu counter
spinlock isn't taken on every tick. This change doesn't affect the archs which
don't define VIRT_CPU_ACCOUNTING and they continue to have the default
percpu counter batch value.

Cc: Balaji Rao <balajirrao@gmail.com>
Cc: Dhaval Giani <dhaval@linux.vnet.ibm.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
---
kernel/sched.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)

Index: b/kernel/sched.c
===================================================================
--- a/kernel/sched.c 2009-05-09 16:48:08.000000000 +0900
+++ b/kernel/sched.c 2009-05-09 18:16:00.000000000 +0900
@@ -824,8 +824,12 @@ static struct file_operations sched_feat
.release = single_release,
};

+static __read_mostly s32 cpuacct_batch;
+
static __init int sched_init_debug(void)
{
+ cpuacct_batch = jiffies_to_cputime(percpu_counter_batch);
+
debugfs_create_file("sched_features", 0644, NULL, NULL,
&sched_feat_fops);

@@ -10457,7 +10461,8 @@ 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, cpuacct_batch);
+
ca = ca->parent;
} while (ca);
rcu_read_unlock();