Re: OOM killer code in 2.6 kernel

[John Richard Moser <nigelenki@comcast.net>]

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So basically you discovered:

 - OOM activated due to say 512K free RAM (OOM happens before this IIRC
but eh)
 - OOM kills process to give 127M free RAM
 - Other CPU wants 4M RAM, so far 3M are free from killed process
 - ANOTHER OOM kill happens to free up another 35M of RAM

Looks like a race condition?

Petrovic, Boban wrote:
>   I have stumbled upon very interesting and in other hand very hard
> problem. I am trying to integrate code which introduces process
> protection from OOM killer action in cases when there is not enough RAM
> in the system. The code works fine on architectures like ppc32, ppc64
> and arm XScale. On Intel Xeon and Pentium M architectures with disabled
> SMP and HighMem support in the kernel the code also works fine. Problem
> begins if I turn on SMP and HighMem. I am using a test case which work
> following way:  when child is forked both parent and child malloc amount
> of memory which when summed together exceed amount of memory available
> on system; parent is than protected. On the Intel architectures with SMP
> and HighMem on I experience that OOM kills many processes in addition to
> child process like sshd, portmap, bash, xinetd, etc.  Number of killed
> processes vary from one test execution to other. As, I pointed out
> earlier, the code works fine only when SMP and HighMem are off. Any
> other combination of these options still causes the problem. I am using
> heavily patched Linux 2.6.10 kernel, with Kernel preempt turned off (the
> problem occurs with vanilla 2.6.14rc2 kernel also). The targets I have
> tested so far don't use swap device, and they have RAM in amounts
> starting from 1Gb.
>   What I discovered so far is that when OOM sends SIGKILL to process,
> memory owned by process is not released as quickly as system would like.
> Processes running on other processors jump into OOM and eventually more
> processes get killed. I introduced changes to mm/page_alloc.c ->
> __alloc_pages() function which suppress more than one processor to enter
> the OOM code, and also I reduced number of OOM calls to one call per
> second for allowed processor. The idea is to give enough time to process
> marked to be killed to release pages. The change works fine for one of
> the targets but isn't working for other Intel boards.
>   I need more insight on what might cause slow releasing of pages, when
> this release occurs and which part of kernel is in charge of that.
> Please CC me with your replies.
> 
>   Thanks,
>   Boban
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