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

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Date:	Sun, 10 Jun 2012 15:24:14 +0800
From:	Fengguang Wu <fengguang.wu@...el.com>
To:	Wanpeng Li <liwp.linux@...il.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 12:54:03PM +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".

Thanks,
Fengguang

> >> 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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