Re: [PATCH (try #3)] mm: avoid unnecessary OOM kills

[Nick Piggin <nickpiggin@yahoo.com.au>]

Dave Peterson wrote:
> At 04:43 PM 5/23/2006, Nick Piggin wrote:

> > > I agree it's desirable to keep the OOM killing logic as encapsulated
> > > as possible.  However unless you are holding the oom kill semaphore
> > > when you make your final attempt to allocate memory it's a bit racy.
> > > Holding the OOM kill semaphore guarantees that our final allocation
> > > failure before invoking the OOM killer occurred _after_ any previous
> > > OOM kill victim freed its memory.  Thus we know we are not shooting
> > > another process prematurely (i.e. before the memory-freeing effects
> > > of our previous OOM kill have been felt).
> >
> > But there is so much fudge in it that I don't think it matters:
> > pages could be freed from other sources, some reclaim might happen,
> > the point at which OOM is declared is pretty arbitrary anyway, etc.
>
>
> There's definitely some fudge in it.  However the main scenario I'm
> concerned with is where one big process is hogging most of the memory
> (as opposed to a case where the collective memory-hogging effect of
> lots of little processes triggers the OOM killer).  In the first case
> we want to shoot the one big process and leave the little processes
> undisturbed.
>
> If the final allocation failure before invoking the OOM killer
> occurs when we don't yet hold the OOM kill semaphore then I'd
> be concerned about processes queueing up on the OOM kill semaphore
> after they fail their memory allocations.  If only one of these
> ends up getting awakened _after_ the death of the big memory hog,
> then that process will enter the OOM killer and shoot a little
> process unnecessarily.
>
> Alternately (perhaps less likely), if your kernel is preemptible,
> after the memory hog has been shot but not yet expired a process
> may get preempted between its final allocation failure and its
> choosing an OOM kill victim (with the memory hog expiring before
> the preempted process gets rescheduled).  Then the preempted
> process shoots a little process when rescheduled.

But just call into the oom killer, and let it queue up and/or do
nothing according to whether there is still a task being shot or
not.

page allocation would then just try again.

--
SUSE Labs, Novell Inc.
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