linux-kernel - Re: [PATCH 1/4] sched,time: remove non-power-of-two divides from __acct_update

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Message-ID: <56AF614C.4070702@redhat.com>
Date:	Mon, 1 Feb 2016 08:44:44 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Cc:	linux-kernel@...r.kernel.org, fweisbec@...il.com, mingo@...nel.org,
	luto@...capital.net, clark@...hat.com
Subject: Re: [PATCH 1/4] sched,time: remove non-power-of-two divides from
 __acct_update_integrals

On 02/01/2016 04:22 AM, Peter Zijlstra wrote:
> On Mon, Feb 01, 2016 at 09:37:00AM +0100, Thomas Gleixner wrote:
>> On Sun, 31 Jan 2016, riel@...hat.com wrote:
>>> @@ -93,9 +93,9 @@ void xacct_add_tsk(struct taskstats *stats, struct task_struct *p)
>>>  {
>>>  	struct mm_struct *mm;
>>>  
>>> -	/* convert pages-usec to Mbyte-usec */
>>> -	stats->coremem = p->acct_rss_mem1 * PAGE_SIZE / MB;
>>> -	stats->virtmem = p->acct_vm_mem1 * PAGE_SIZE / MB;
>>> +	/* convert pages-nsec/1024 to Mbyte-usec, see __acct_update_integrals */
>>> +	stats->coremem = p->acct_rss_mem1 * PAGE_SIZE / (1000 * KB);
>>> +	stats->virtmem = p->acct_vm_mem1 * PAGE_SIZE / (1000 * KB);
>>
>> You replace "/ (1024 * 1024)" by "/ (1000 * 1024). So that's introducing a non
>> power of 2 division instead of removing one and wont compile on systems which
>> do not have a 64/32 division in hardware.
> 
> Yep, so that needs to be fixed to use do_div(). But the reason for this
> is that this is the consumer side of these stats and therefore rarely
> executed.

Before I send in a v4 with do_div in that location, are there
any other changes you would like me to make to the code?

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