Re: [PATCH] OOM killer: wait for tasks with pending SIGKILL to exit

[Sergey Dyasly]

It seems to me that we are going nowhere with this discussion...

If you are ok with the first change in my patch regarding fatal_signal_pending,
I can send new patch with just that change.


On Mon, 30 Sep 2013 15:08:25 -0700 (PDT)
David Rientjes <rientjes@xxxxxxxxxx> wrote:

> On Fri, 27 Sep 2013, Sergey Dyasly wrote:
> 
> > What you are saying contradicts current OOMk code the way I read it. Comment in
> > oom_kill_process() says:
> > 
> > "If the task is already exiting ... set TIF_MEMDIE so it can die quickly"
> > 
> > I just want to know the right solution.
> > 
> 
> That's a comment, not code.  The point of the PF_EXITING special handling 
> in oom_kill_process() is to avoid telling sysadmins that a process has 
> been killed to free memory when it has already called exit() and to avoid 
> sacrificing one of its children for the exiting process.
> 
> It may or may not need access to memory reserves to actually exit after 
> PF_EXITING depending on whether it needs to allocate memory for 
> coredumping or anything else.  So instead of waiting for it to recall the 
> oom killer, TIF_MEMDIE is set anyway.  The point is that PF_EXITING 
> processes can already get TIF_MEMDIE immediately when their memory 
> allocation fails so there's no reason not to set it now as an 
> optimization.
> 
> But we definitely want to avoid printing anything to the kernel log when 
> the process has already called exit() and issuing the SIGKILL at that 
> point would be pointless.
> 
> > You are mistaken, oom_kill_process() is only called from out_of_memory()
> > and mem_cgroup_out_of_memory().
> > 
> 
> out_of_memory() calls oom_kill_process() in two places, plus the call from 
> mem_cgroup_out_of_memory(), making three calls in the tree.  Not that this 
> matters in the slightest, though.
> 
> > > Read the comment about why we don't emit anything to the kernel log in 
> > > this case; the process is already exiting, there's no need to kill it or 
> > > make anyone believe that it was killed.
> > 
> > Yes, but there is already the PF_EXITING check in oom_scan_process_thread(),
> > and in this case oom_kill_process() won't be even called. That's why it's
> > redundant.
> > 
> 
> You apparently have no idea how long select_bad_process() runs on a large 
> system with thousands of processes.  Keep in mind that SGI requested the 
> addition of the oom_kill_allocating_task sysctl specifically because of 
> how long select_bad_process() runs.  The PF_EXITING check in 
> oom_kill_process() is simply an optimization to return early and with 
> access to memory reserves so it can exit as quickly as possible and 
> without the kernel stating it's killing something that has already called 
> exit().


-- 
Sergey Dyasly <dserrg@xxxxxxxxx>
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