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

["Alex,Shi" <alex.shi@intel.com>]

On Fri, 2011-07-29 at 23:40 +0800, Mel Gorman wrote:
> 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;

Thanks for your comments, I will use this comments style next time, list
request A, B etc. 

> 
> 1. Read pgdat request A (classzone_idx, order)

Assume the order of A > 0, like is 3.

> 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

Another balance_pgdat() failure indicate is returned order == 0, am I
right?  If so, the next step of kswapd is not trying to sleep, but do
request A balance again.  And I thought this behavior doesn't match the
comments in kswapd. 

> 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?
>