linux-kernel - Re: [PATCH] page-writeback.c: fix update bandwidth time judgment error

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Date:	Sun, 10 Jun 2012 12:54:03 +0800
From:	Wanpeng Li <liwp.linux@...il.com>
To:	Fengguang Wu <fengguang.wu@...el.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Jan Kara <jack@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	PeterZijlstra <a.p.zijlstra@...llo.nl>,
	Johannes Weiner <jweiner@...hat.com>, linux-mm@...ck.org,
	Gavin Shan <shangw@...ux.vnet.ibm.com>,
	Wanpeng Li <liwp@...ux.vnet.ibm.com>
Subject: Re: [PATCH] page-writeback.c: fix update bandwidth time judgment
 error

On Sun, Jun 10, 2012 at 12:36:41PM +0800, Fengguang Wu wrote:
>Wanpeng,
>
>Sorry this I won't take this: it don't really improve anything.  Even
>with the changed test, the real intervals are still some random values
>above (and not far away from) 200ms.. We are saying about 200ms
>intervals just for convenience.
>
But some parts like:

__bdi_update_bandwidth which bdi_update_bandwidth will call:

if(elapsed < BANDWIDTH_INTERVAL)
	return;

or

global_update_bandwidth:

if(time_before(now, update_time + BANDWIDTH_INTERVAL))
	return;

You me just ignore this disunion ?

Regards,
Wanpeng Li

>
>On Sun, Jun 10, 2012 at 12:20:05PM +0800, Wanpeng Li wrote:
>> From: Wanpneg Li <liwp@...ux.vnet.ibm.com>
>> 
>> Since bdi_update_bandwidth function  should estimate write bandwidth at 200ms intervals,
>> so the time is bdi->bw_time_stamp + BANDWIDTH_INTERVAL == jiffies, but
>> if use time_is_after_eq_jiffies intervals will be bdi->bw_time_stamp +
>> BANDWIDTH_INTERVAL + 1.
>> 
>> Signed-off-by: Wanpeng Li <liwp@...ux.vnet.ibm.com>
>> ---
>>  mm/page-writeback.c |    2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
>> index c833bf0..099e225 100644
>> --- a/mm/page-writeback.c
>> +++ b/mm/page-writeback.c
>> @@ -1032,7 +1032,7 @@ static void bdi_update_bandwidth(struct backing_dev_info *bdi,
>>  				 unsigned long bdi_dirty,
>>  				 unsigned long start_time)
>>  {
>> -	if (time_is_after_eq_jiffies(bdi->bw_time_stamp + BANDWIDTH_INTERVAL))
>> +	if (time_is_after_jiffies(bdi->bw_time_stamp + BANDWIDTH_INTERVAL))
>>  		return;
>>  	spin_lock(&bdi->wb.list_lock);
>>  	__bdi_update_bandwidth(bdi, thresh, bg_thresh, dirty,
>> -- 
>> 1.7.9.5
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