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

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Date:	Tue, 26 Jan 2010 19:07:48 +1100
From:	Anton Blanchard <anton@...ba.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	balbir@...ux.vnet.ibm.com,
	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, a.p.zijlstra@...llo.nl,
	schwidefsky@...ibm.com, balajirrao@...il.com,
	dhaval@...ux.vnet.ibm.com, tglx@...utronix.de,
	kamezawa.hiroyu@...fujitsu.com, Tony Luck <tony.luck@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>, linux390@...ibm.com
Subject: Re: [PATCH] sched: cpuacct: Use bigger percpu counter batch values
 for stats counters


Hi Andrew,

> > I guess a lot of the changelog and comments are in the email history,
> 
> Not a very useful location for it!

Good point, I'll work on a useful comment.

> > Why does it look quite wrong to you?
> 
> Because it computes the correct value and then if it's larger than
> INT_MAX, it inexplicably assigns INT_MAX to it, giving a wrong result!
>
> Does that code actually work, btw?  percpu_counter_batch has type `int'
> and cputime_one_jiffy has type `int' so their product has type `int'. 
> So by the time min_t performs its comparison, the upper 32 bits of the
> product are already lost.

On ppc64, s390 and ia64 cputime_one_jiffy is 64bit and I want to prevent us
creating too large a batch value:

void __percpu_counter_add(struct percpu_counter *fbc, s64 amount, s32 batch)

If we pass in something bigger than INT_MAX we could end up with 0 after
truncation which will turn the percpu counter into a single spinlock global
counter.

Anton
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