Re: [PATCH v2] cpuacct: VIRT_CPU_ACCOUNTING don't prevent percpu cputime count

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Thu, 2009-04-30 at 11:48 +0530, Balbir Singh wrote:

> > Subject: [PATCH v2] 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 |    8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> > 
> > Index: b/kernel/sched.c
> > ===================================================================
> > --- a/kernel/sched.c	2009-04-30 11:37:47.000000000 +0900
> > +++ b/kernel/sched.c	2009-04-30 14:17:00.000000000 +0900
> > @@ -10221,6 +10221,7 @@ struct cpuacct {
> >  };
> > 
> >  struct cgroup_subsys cpuacct_subsys;
> > +static s32 cpuacct_batch;
> > 
> >  /* 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);
> > +
> >  	for (i = 0; i < CPUACCT_STAT_NSTATS; i++)
> >  		if (percpu_counter_init(&ca->cpustat[i], 0))
> >  			goto out_free_counters;
> > @@ -10376,7 +10380,7 @@ static int cpuacct_stats_show(struct cgr
> >  	int i;
> > 
> >  	for (i = 0; i < CPUACCT_STAT_NSTATS; i++) {
> > -		s64 val = percpu_counter_read(&ca->cpustat[i]);
> > +		s64 val = percpu_counter_sum(&ca->cpustat[i]);
> >  		val = cputime64_to_clock_t(val);
> >  		cb->fill(cb, cpuacct_stat_desc[i], val);
> >  	}
> > @@ -10446,7 +10450,7 @@ 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, cpuacct_batch);
> >  		ca = ca->parent;
> >  	} while (ca);
> >  	rcu_read_unlock();
> > 
> >
> 
> What do the test results look like with this? I'll see if I can find
> some time to test this patch. On a patch read level this seems much better
> to me, Peter?

I don't really fancy percpu_counter_sum() usage. I'm thinking its ok to
degrate accuracy on larger machines and simply use
percpu_counter_read().