Re: [patch 1/2] mm, memcg: avoid oom notification when current needsaccess to memory reserves

[Johannes Weiner]

On Mon, Dec 09, 2013 at 01:46:16PM -0800, David Rientjes wrote:
> On Mon, 9 Dec 2013, Michal Hocko wrote:
> 
> > > Google depends on getting memory.oom_control notifications only when they 
> > > are actionable, which is exactly how Documentation/cgroups/memory.txt 
> > > describes how userspace should respond to such a notification.
> > > 
> > > "Actionable" here means that the kernel has exhausted its capabilities of 
> > > allowing for future memory freeing, which is the entire premise of any oom 
> > > killer.
> > > 
> > > Giving a dying process or a process that is going to subsequently die 
> > > access to memory reserves is a capability the kernel users to ensure 
> > > progress is made in oom conditions.  It is not an exhaustion of 
> > > capabilities.
> > > 
> > > Yes, we all know that subsequent to the userspace notification that memory 
> > > may be freed and the kill no longer becomes required.  There is nothing 
> > > that can be done about that, and it has never been implied that a memcg is 
> > > guaranteed to still be oom when the process wakes up.
> > > 
> > > I'm referring to a siutation that can manifest in a number of ways: 
> > > coincidental process exit, coincidental process being killed, 
> > > VMPRESSURE_CRITICAL notification that results in a process being killed, 
> > > or memory threshold notification that results in a process being killed.  
> > > Regardless, we're talking about a situation where something is already 
> > > in the exit path or has been killed and is simply attempting to free its 
> > > memory.
> > 
> > You have already mentioned that. Several times in fact. And I do
> > understand what you are saying. You are just not backing your claims
> > with anything that would convince us that what you are trying to solve
> > is an issue in the real life. So show us it is real, please.
> > 
> 
> What exactly would you like to see?  It's obvious that the kernel has not 
> exhausted its capabilities of allowing for future memory freeing if the 
> notification happens before the check for current->flags & PF_EXITING or 
> fatal_signal_pending(current).  Does that conditional get triggered?  ALL 
> THE TIME.

We check for fatal signals during the repeated charge attempts and
reclaim.  Should we be checking for PF_EXITING too?

> We know it happens because I had to introduce it into both the 
> system oom killer and the memcg oom killer to fix mm->mmap_sem issues for 
> threads that were killed as part of the oom killer SIGKILL but weren't the 
> thread lucky enough to get TIF_MEMDIE set and they were in the allocation 
> path.



> 
> Are you asking me to patch our kernel, get it rolled out, and plot a graph 
> to show how often it gets triggered over time in our datacenters and that 
> it causes us to get unnecessary oom kill notifications?
> 
> I'm trying to support you in any way I can by giving you the information 
> you need, but in all honesty this seems pretty trivial and obvious to 
> understand.  I'm really quite stunned at this thread.  What exactly are 
> you arguing in the other direction for?  What does giving an oom 
> notification before allowing exiting processes to free its memory so the 
> memcg or system is no longer oom do?  Why can't you use memory thresholds 
> or vmpressure for such a situation?
> 
> > > Such a process simply needs access to memory reserves to make progress and 
> > > free its memory as part of the exit path.  The process waiting on 
> > > memory.oom_control does _not_ need to do any of the actions mentioned in 
> > > Documentation/cgroups/memory.txt: reduce usage, enlarge the limit, kill a 
> > > process, or move a process with charge migration.
> > > 
> > > It would be ridiculous to require anybody implementing such a process to 
> > > check if the oom condition still exists after a period of time before 
> > > taking such an action.
> > 
> > Why would you consider that ridiculous? If your memcg is oom already
> > then waiting few seconds to let racing tasks finish doesn't sound that
> > bad to me.
> > 
> 
> A few seconds?  Is that just handwaving or are you making a guarantee that 
> all processes that need access to memory reserves will wake up, try its 
> allocation, get the memcg's oom lock, get access to memory reserves, 
> allocate, return to handle its pending SIGKILL, proceed down the exit() 
> path, and free its memory by then?
> 
> Meanwhile, the userspace oom handler is doing its little sleep(3) that you 
> suggest, it checks the status of the memcg, finds it's still oom, but 
> doesn't realize because it didn't do a second blocking read() that its a 
> second oom condition for a different process attached to the memcg and 
> that process simply needs memory reserves to exit.
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