linux-kernel - Re: [PATCH] kswapd: avoid unnecessary rebalance after an unsuccessful balancing

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Message-ID: <20110729154031.GV3010@suse.de>
Date:	Fri, 29 Jul 2011 16:40:31 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Alex Shi <alex.shi@...el.com>
Cc:	linux-mm@...ck.org, P@...igBrady.com, linux-kernel@...r.kernel.org,
	andrea@...share.com, tim.c.chen@...el.com, shaohua.li@...el.com,
	akpm@...ux-foundation.org, riel@...hat.com, luto@....edu
Subject: Re: [PATCH] kswapd: avoid unnecessary rebalance after an
 unsuccessful balancing

On Fri, Jul 29, 2011 at 11:23:10PM +0800, Alex Shi wrote:
> In commit 215ddd66, Mel Gorman said kswapd is better to sleep after a
> unsuccessful balancing if there is tighter reclaim request pending in
> the balancing. In this scenario, the 'order' and 'classzone_idx'
> that are checked for tighter request judgment is incorrect, since they
> aren't the one kswapd should read from new pgdat, but the last time pgdat
> value for just now balancing. Then kswapd will skip try_to_sleep func
> and rebalance the last pgdat request. It's not our expected behavior.
> 
> So, I added new variables to distinguish the returned order/classzone_idx
> from last balancing, that can resolved above issue in that scenario.
> 

I'm afraid this changelog is very difficult to read and I do not see
what problem you are trying to solve and I do not see what this patch
might solve.

When balance_pgdat() returns with a lower classzone or order, the values
stored in pgdat are not re-read and instead it tries to go to sleep
based on the starting request. Something like;

1. Read pgdat request A (classzone_idx, order)
2. balance_pgdat()
3.	During pgdat, a new pgdat request B (classzone_idx, order) is placed
4. balance_pgdat() returns but failed so classzone_idx is lower
5. Try to sleep based on pgdat request A

i.e. pgdat request B is not read and there is a comment explaining
why pgdat request B is not read after balance_pgdat() fails.

This patch adds some variables that might improve the readability
for some people but otherwise I can't see what problem is being
fixed. What did I miss?

-- 
Mel Gorman
SUSE Labs
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