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 07:52:59 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	balbir@...ux.vnet.ibm.com
Cc:	Anton Blanchard <anton@...ba.org>,
	Bharata B Rao <bharata@...ux.vnet.ibm.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>, mingo@...hat.com, hpa@...or.com,
	linux-kernel@...r.kernel.org, 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

On Thu, 2009-08-20 at 10:56 +0530, Balbir Singh wrote:

> Looks good overall, why not keep the batch size conditional on
> CONFIG_VIRT_CPU_ACCOUNTING? I'd still like to stick with
> percpu_counter_read() on the read side because My concern is that a
> bad user space application can read cpuacct.stat and bring the kernel
> to its knees.

Agreed, the first hunk is dangerous, on the second hunk, I'm not sure
about the INT_MAX thing, that's a 4s gap per cpu, that might be a tad
much.

> > > 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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