Messages in this thread | | | Date | Fri, 3 Dec 2004 00:35:00 +0100 | From | Andrea Arcangeli <> | Subject | Re: [PATCH] oom killer (Core) |
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On Thu, Dec 02, 2004 at 07:55:16PM +0100, Thomas Gleixner wrote: > On Thu, 2004-12-02 at 19:08 +0100, Andrea Arcangeli wrote: > > OTOH we must not forget 2.4(-aa) calls do_exit synchronously and it > > never sends signals. That might be why 2.4 doesn't kill more than one > > task by mistake, even without a callback-wakeup. > > I just run the same test on 2.4.27 and the behaviour is completely > different. > > The box seems to be stuck in a swap in/out loop for quite a long time. > During this time the box is not responsive at all. It finally stops the > forking after quite a long time of swapping with > fork() (error: resource temporarily not available).
Fork eventually failing is very reasonable if you're executing a fork loop.
> > There is no output in dmesg, but I'm not able to remove the remaining > hackbench processes as even a kill -SIGKILL returns with > fork() (error: resource temporarily not available) > > I'm not sure, which of the two scenarios I like better :)
Please try with 2.4.23aa3, I think there was some oom killer change after I had no resources to track 2.4 anymore. I'm not saying 2.4.23aa3 will work better though, but I would like to know if there's some corner case still open in 2.4-aa. Careful, 2.4.23aa3 has security bugs (only local security of course, i.e. normally not a big issue, sure good enough for a quick test).
I doubt your testcase simulates anything remotely realistic but anyway it's still informative.
What I'm simulating here is very real life scenario with a couple of apps allocating more memory than ram.
> FYI, I tried with 2.6 UP and PREEMPT=n. The result is more horrible. The > box just gets stuck in an endless swap in/swap out and does not respond > to anything else than SysRq-T and the reset button. > > With the callback the machine did not come back after 20 Minutes.
Was the oom killer invoked at all? If yes, and it works with preempt, that could mean a cond_resched is simply missing...
> You meant the one in badness() right ?
yes.
> Well it makes it better, but I was able to have a second invocation > before the first killed tasks exited. That's simple to explain. The task > is on the way out and releases resources, so the VM size is reduced and > the killer picks another process.
That's trivial to fix checking for PF_DEAD/PF_EXITING.
> > I'd rather fix this by removing buggy code, than by adding additional > > logics on top of already buggy code (i.e. setting PF_MEMDIE is a smp > > race and can corrupt other bitflags), but at least the > > oom-wakeup-callback from do_exit still makes a lot of sense (even if > > PF_MEMDIE must be dropped since it's buggy, and something else should be > > used instead). > > I think the callback is the only safe way to fix that. If PF_MEMDIE is > racy then I'm sure we will find a different indicator for that.
The callback adds overhead to the exit path. Plus strictly speaking it's not actually a callback, you're just "polling" for the bitflag :)
> Yep, but the reentrancy blocking with the callback makes the time, count > crap and the watermark check go away, as it is safe to reenable the > killer at this point because we definitely freed memory resources. So > the watermark comes for free.
You can get an I/O race where your program is about to finish a failing try_to_free_pages pass (note that a task exiting won't make try_to_free_pages work any easier, try_to_free_pages has to free allocated memory, it doesn't care if there's 1M or 100M of free memory). If you don't check the watermarks after waiting for I/O, you're going to generate a suprious oom-killing. Your changes can't help.
Note that even the watermark checks leaves a race window open, but at least it's not an I/O window. While try_to_free_pages can wait for I/O and then fail.
I'll add to my last patch the removal of the PF_MEMDIE check in oom_kill plus I'll fix the remaining race with PF_EXITING/DEAD, and I'll add a cond_resched. Then you can try again with my simple way (w/ and w/o PREEMPT ;).
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