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

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Date:	Sun, 10 Jun 2012 15:41:15 +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>,
	Peter Zijlstra <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>,
	Wanpeng Li <liwp.linux@...il.com>
Subject: Re: [PATCH] page-writeback.c: fix update bandwidth time judgment
 error

On Sun, Jun 10, 2012 at 03:24:14PM +0800, Fengguang Wu wrote:
>On Sun, Jun 10, 2012 at 12:54:03PM +0800, Wanpeng Li wrote:
>> 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 ?
>
>Not a problem for me. But if that consistency makes you feel happy,
>you might revise the changelog and resend. But it's not that simple
>for the below reason..
>
>> >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,
>
>The above line represents a wrong assumption. It's normal for the
>re-estimate intervals to be >= 200ms.
>
>> >> 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.
>
>Strictly speaking, to ensure that ">= 200ms" is true, we'll have to
>skip the "jiffies == bw_time_stamp + BANDWIDTH_INTERVAL" case. For
>example, when HZ=100, the bw_time_stamp may actually be recorded in
>the very last ms of a 10ms range, and jiffies may be in the very first
>ms of the current 10ms range. So if using ">=" comparisons, it may
>actually let less than 200ms intervals go though.
>
>We can only reliably ensure "> 200ms", but no way for ">= 200ms".
>

static void global_update_bandwidth(unsigned long thresh,
				    unsigned long dirty,
					unsigned long now)
{
	static DEFINE_SPINLOCK(dirty_lock);
    static unsigned long update_time;

    /*
	 * check locklessly first to optimize away locking for the most time
     */
	if (time_before(now, update_time + BANDWIDTH_INTERVAL))
		return;
    
	spin_lock(&dirty_lock);
    if (time_after_eq(now, update_time + BANDWIDTH_INTERVAL)) {
		update_dirty_limit(thresh, dirty);
		update_time = now;
	}
	spin_unlock(&dirty_lock);
}

So time_after_eq in global_update_bandwidth function should also change
to time_after, or just ignore this disunion?

>
>> >> 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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