On Thu 21-04-16 20:49:16, Tetsuo Handa wrote: > Michal Hocko wrote: > > On Wed 20-04-16 06:55:42, Tetsuo Handa wrote: > > > Michal Hocko wrote: > > > > > This patch adds a timeout for handling corner cases where a TIF_MEMDIE > > > > > thread got stuck. Since the timeout is checked at oom_unkillable_task(), > > > > > oom_scan_process_thread() will not find TIF_MEMDIE thread > > > > > (for !oom_kill_allocating_task case) and oom_badness() will return 0 > > > > > (for oom_kill_allocating_task case). > > > > > > > > > > By applying this patch, the kernel will automatically press SysRq-f if > > > > > the OOM reaper cannot reap the victim's memory, and we will never OOM > > > > > livelock forever as long as the OOM killer is called. > > > > > > > > Which will not guarantee anything as already pointed out several times > > > > before. So I think this is not really that useful. I have said it > > > > earlier and will repeat it again. Any timeout based solution which > > > > doesn't guarantee that the system will be in a consistent state (reboot, > > > > panic or kill all existing tasks) after the specified timeout is > > > > pointless. > > > > > > Triggering the reboot/panic is the worst action. Killing all existing tasks > > > is the next worst action. Thus, I prefer killing tasks one by one. > > > > killing a task by task doesn't guarantee any convergence to a usable > > state. If somebody really cares about these highly unlikely lockups > > I am pretty sure he would really appreciate to have a _reliable_ and > > _guaranteed_ way out of that situation. Having a fuzzy mechanism to do > > something in a good hope of resolving that state is just unhelpful. > > Killing a task by task shall eventually converge to the kernel panic. I (as an admin) do not want to wait unbounded amount of time though. This is just not practical. > But since we now have the OOM reaper, the possibility of needing to kill > next task is very low. Killing a task by task via timeout is an insurance > for rare situations where the OOM reaper cannot reap the OOM-killed thread's > memory due to mmap_sem being held for write. You are changing one unlikely situation for another and that's why I think this is basically unusable in the real life and why I am so strongly opposing it. > (If TIF_MEMDIE were set to all > OOM-kiled thread groups, the OOM killer can converge to the kernel panic > more quickly by ignoring the rest of OOM-killed threads sharing the same > memory, but that is a different patch.) > > > > > If I was an admin and had a machine on the other side of the globe and > > that machine just locked up due to OOM I would pretty much wanted to > > force reboot as my other means of fixing that situation would be pretty > > much close to zero otherwise. > > I posted V2 of patch which also allows triggering the kernel panic via timeout. I have seen that patch. I didn't get to review it properly yet as I am still travelling. From a quick view I think it is conflating two things together. I could see arguments for the panic part but I do not consider the move-to-kill-another timeout as justified. I would have to see a clear indication this is actually useful for real life usecases. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>