Re: [PATCH 3/3] mm: slub: Default slub_max_order to 0

Minchan Kim
2011-05-13T05:39:07Z

On Fri, May 13, 2011 at 7:58 AM, Minchan Kim <minchan.kim@gmail.com> wrote:
> On Fri, May 13, 2011 at 7:15 AM, Johannes Weiner <hannes@cmpxchg.org> wrote:
>> On Thu, May 12, 2011 at 05:04:41PM -0500, James Bottomley wrote:
>>> On Thu, 2011-05-12 at 15:04 -0500, James Bottomley wrote:
>>> > Confirmed, I'm afraid ... I can trigger the problem with all three
>>> > patches under PREEMPT.  It's not a hang this time, it's just kswapd
>>> > taking 100% system time on 1 CPU and it won't calm down after I unload
>>> > the system.
>>>
>>> Just on a "if you don't know what's wrong poke about and see" basis, I
>>> sliced out all the complex logic in sleeping_prematurely() and, as far
>>> as I can tell, it cures the problem behaviour.  I've loaded up the
>>> system, and taken the tar load generator through three runs without
>>> producing a spinning kswapd (this is PREEMPT).  I'll try with a
>>> non-PREEMPT kernel shortly.
>>>
>>> What this seems to say is that there's a problem with the complex logic
>>> in sleeping_prematurely().  I'm pretty sure hacking up
>>> sleeping_prematurely() just to dump all the calculations is the wrong
>>> thing to do, but perhaps someone can see what the right thing is ...
>>
>> I think I see the problem: the boolean logic of sleeping_prematurely()
>> is odd.  If it returns true, kswapd will keep running.  So if
>> pgdat_balanced() returns true, kswapd should go to sleep.
>>
>> This?
>
> Yes. Good catch.

In addition, I see some strange thing.
The comment in pgdat_balanced says
"Only zones that meet watermarks and are in a zone allowed by the
callers classzone_idx are added to balanced_pages"

It's true in case of balance_pgdat but it's not true in sleeping_prematurely.
This?

barrios@barrios-desktop:~/linux-mmotm$ git diff mm/vmscan.c
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 292582c..d9078cf 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2322,7 +2322,8 @@ static bool sleeping_prematurely(pg_data_t
*pgdat, int order, long remaining,
classzone_idx, 0))
all_zones_ok = false;
else
- balanced += zone->present_pages;
+ if (i <= classzone_idx)
+ balanced += zone->present_pages;
}

/*
@@ -2331,7 +2332,7 @@ static bool sleeping_prematurely(pg_data_t
*pgdat, int order, long remaining,
* must be balanced
*/
if (order)
- return pgdat_balanced(pgdat, balanced, classzone_idx);
+ return !pgdat_balanced(pgdat, balanced, classzone_idx);
else
return !all_zones_ok;
}

--
Kind regards,
Minchan Kim