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

From: KOSAKI Motohiro
Date: Thu Apr 30 2009 - 04:29:00 EST


> * KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx> [2009-04-30 15:11:15]:
>
> >
> > Changelog:
> > since v1
> > - use percpu_counter_sum() instead percpu_counter_read()
> >
> >
> > -------------------------------------
> > 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@xxxxxxxxxxxxxxxxxx>
> > Cc: Balaji Rao <balajirrao@xxxxxxxxx>
> > Cc: Dhaval Giani <dhaval@xxxxxxxxxxxxxxxxxx>
> > Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> > Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> > Cc: Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxx>
> > Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
> > Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> > ---
> > 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?
>
> Acked-by: Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx>

this patch decrease slow down risk on large server. but this patch
doesn't have functional change. you can't make functional test.
AFAIK, percpu_counter_sum() don't make any performance degression,
but you have good stress test, please tell me it.





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