linux-kernel - Re: [tip:sched/core] sched: cpuacct: Use bigger percpu counter batch values for stats counters

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Date:	Thu, 20 Aug 2009 15:10:38 +1000
From:	Anton Blanchard <anton@...ba.org>
To:	Bharata B Rao <bharata@...ux.vnet.ibm.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>, mingo@...hat.com,
	hpa@...or.com, linux-kernel@...r.kernel.org,
	a.p.zijlstra@...llo.nl, schwidefsky@...ibm.com,
	balajirrao@...il.com, dhaval@...ux.vnet.ibm.com,
	tglx@...utronix.de, kamezawa.hiroyu@...fujitsu.com,
	akpm@...ux-foundation.org
Subject: Re: [tip:sched/core] sched: cpuacct: Use bigger percpu counter
	batch values for stats counters


Hi,

Looks like this issue is still present. I tested on a 32 core box and 
the patch improved maximum context switch rate from from 76k/sec to 9.5M/sec.
Thats over 100x faster, or 50x per line of code. That's got to be some sort of
record :)

Any chance we can get a fix in for 2.6.31? Don't make me find an even bigger
box so I can break the 200x mark :)

Anton

> --
> 
> When CONFIG_VIRT_CPU_ACCOUNTING is 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.
> 
> Since reading of the CPU accounting cgroup counters is not performance
> critical, we can use a maximum size batch of INT_MAX and use
> percpu_counter_sum on the read side which will add all the percpu
> counters.
> 
> With this patch an 8 core POWER6 with CONFIG_VIRT_CPU_ACCOUNTING and
> CONFIG_CGROUP_CPUACCT shows an improvement in aggregate context switch rate of
> 397k/sec to 3.9M/sec, a 10x improvement.
> 
> Signed-off-by: Anton Blanchard <anton@...ba.org>
> ---
> 
> Index: linux.trees.git/kernel/sched.c
> ===================================================================
> --- linux.trees.git.orig/kernel/sched.c	2009-07-16 10:11:02.000000000 +1000
> +++ linux.trees.git/kernel/sched.c	2009-07-16 10:16:41.000000000 +1000
> @@ -10551,7 +10551,7 @@
>  	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);
>  	}
> @@ -10621,7 +10621,7 @@
>  	ca = task_ca(tsk);
>  
>  	do {
> -		percpu_counter_add(&ca->cpustat[idx], val);
> +		__percpu_counter_add(&ca->cpustat[idx], val, INT_MAX);
>  		ca = ca->parent;
>  	} while (ca);
>  	rcu_read_unlock();
--
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