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Message-ID: <20090428073151.GC3825@in.ibm.com>
Date: Tue, 28 Apr 2009 13:01:51 +0530
From: Bharata B Rao <bharata@...ux.vnet.ibm.com>
To: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc: LKML <linux-kernel@...r.kernel.org>,
Balaji Rao <balajirrao@...il.com>,
Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Balbir Singh <balbir@...ux.vnet.ibm.com>,
Ingo Molnar <mingo@...e.hu>,
Martin Schwidefsky <schwidefsky@...ibm.com>,
seto.hidetoshi@...fujitsu.com
Subject: Re: [PATCH] cpuacct: VIRT_CPU_ACCOUNTING don't prevent percpu
cputime count
On Tue, Apr 28, 2009 at 03:53:32PM +0900, KOSAKI Motohiro wrote:
>
> I'm not cpuacct expert. please give me comment.
>
> ====================
> Subject: [PATCH] cpuacct: VIRT_CPU_ACCOUNTING don't prevent percpu cputime caching
>
> impact: little performance improvement
>
> cpuacct_update_stats() is called at every tick updating. and it use percpu_counter
> for avoiding performance degression.
>
> Unfortunately, it doesn't works on VIRT_CPU_ACCOUNTING=y environment properly.
> if VIRT_CPU_ACCOUNTING=y, every tick update much than 1000 cputime.
> Thus every percpu_counter_add() makes spinlock grabbing and update non-percpu-variable.
>
> This patch change the batch rule. now, every cpu can store "percpu_counter_bach x jiffies"
> cputime in percpu cache.
> it mean this patch don't have behavior change if VIRT_CPU_ACCOUNTING=n, but
> works well on VIRT_CPU_ACCOUNTING=y too.
Let me try to understand what you are saying...
For archs which define VIRT_CPU_ACCOUNTING, every tick would result
in around 1000 units of cputime updates and since this is much much greater
than percpu_batch_counter, we end up taking spinlock on every tick.
If my above reading of the problem is correct, please look at my below comments.
> Index: b/kernel/sched.c
> ===================================================================
> --- a/kernel/sched.c 2009-04-28 14:18:36.000000000 +0900
> +++ b/kernel/sched.c 2009-04-28 15:18:07.000000000 +0900
> @@ -10117,6 +10117,7 @@ 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)
> @@ -10146,6 +10147,9 @@ static struct cgroup_subsys_state *cpuac
> if (!ca->cpuusage)
> goto out_free_ca;
>
> + if (!cpuacct_batch)
> + cpuacct_batch = jiffies_to_cputime(percpu_counter_batch);
You essentially end up increasing the batch value from the default value
of max(32, nr_cpus*2).
> +
> for (i = 0; i < CPUACCT_STAT_NSTATS; i++)
> if (percpu_counter_init(&ca->cpustat[i], 0))
> goto out_free_counters;
> @@ -10342,7 +10346,7 @@ 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);
And you do this unconditionally which will affect all archs ? So you make
this behaviour default for archs which have VIRT_CPU_ACCOUNTING=n.
BTW, did you observe any real problem with the percpu counter spinlock ?
Regards,
Bharata.
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