Re: Commit f50de2d38 seems to be breaking my oom killer

[Will Newton <will.newton@gmail.com>]

On Thu, Jan 7, 2010 at 1:58 PM, Mel Gorman <mel@csn.ul.ie> wrote:
> On Thu, Jan 07, 2010 at 12:34:54PM +0000, Will Newton wrote:
>> Hi,
>>
>> I'm having some problems on a small embedded box with 24Mb of RAM and
>> no swap. If a process tries to use large amounts of memory and gets
>> OOM killed, with 2.6.32 it's fine, but with 2.6.33-rc2 kswapd gets
>> stuck and the system locks up.
>
> By stuck, do you mean it consumes 100% CPU and never goes to sleep?

I assume so. The system sems locked up so ctrl-c of the process
doesn't work and I can't get in via telnet. Looking where the pc and
return pointer are going via JTAG leads me to believe it's stuck in
kswapd.

>> The problem appears to have been
>> introduced with f50de2d38. If I change sleeping_prematurely to skip
>> the for_each_populated_zone test then OOM killing operates as
>> expected. I'm guessing it's caused by the new code not allowing kswapd
>> to schedule when it is required to let the killed task exit. Does that
>> sound plausible?
>>
>
> It's conceivable. The expectation was that the cond_resched() in
> balance_pgdat() should have been called at
>
>        if (!all_zones_ok) {
>                cond_resched();
>
> But it would appear that if all zones are unreclaimable, all_zones_ok == 1.
> It could be looping there indefinitly never calling schedule because it
> never reaches the points where cond_resched is called.
>
>> I'll try and investigate further into what's going on.
>>
>
> Can you try the following?
>
> ==== CUT HERE ====
> vmscan: kswapd should notice that all zones are not ok if they are unreclaimble
>
> In the event all zones are unreclaimble, it is possible for kswapd to
> never go to sleep because "all zones are ok even though watermarks are
> not reached". It gets into a situation where cond_reched() is not
> called.
>
> This patch notes that if all zones are unreclaimable then the zones are
> not ok and cond_resched() should be called.
>
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>

This fixes the problem, thanks for the quick response!

Tested-by: Will Newton <will.newton@gmail.com>

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