Michal Hocko <mho...@kernel.org> writes: > On Thu 31-05-18 20:07:28, Eric W. Biederman wrote: >> Michal Hocko <mho...@kernel.org> writes: >> >> > On Thu 26-04-18 14:00:19, Kirill Tkhai wrote: >> >> This function searches for a new mm owner in children and siblings, >> >> and then iterates over all processes in the system in unlikely case. >> >> Despite the case is unlikely, its probability growths with the number >> >> of processes in the system. The time, spent on iterations, also growths. >> >> I regulary observe mm_update_next_owner() in crash dumps (not related >> >> to this function) of the nodes with many processes (20K+), so it looks >> >> like it's not so unlikely case. >> > >> > Did you manage to find the pattern that forces mm_update_next_owner to >> > slow paths? This really shouldn't trigger very often. If we can fallback >> > easily then I suspect that we should be better off reconsidering >> > mm->owner and try to come up with something more clever. I've had a >> > patch to remove owner few years back. It needed some work to finish but >> > maybe that would be a better than try to make non-scalable thing suck >> > less. >> >> Reading through the code I just found a trivial pattern that triggers >> this. Create a multi-threaded process. Have the thread group leader >> (the first thread) exit. > > Hmm, I thought that we try to iterate over threads in the same thread > group first. But we are not doing that. Anyway just CLONE_VM without > CLONE_THREAD would achieve the same pathological path but that should be > rare.
Yes, if the child exited. The code searches the children and siblings but the parents of the process that exited. > Group leader exiting early without tearing down the whole thread > group should be quite rare as well. No question that somebody might do > that on purpose though... The group leader exiting early is a completely legitimate and reasonable thing to do, even if it is rare. I think all it would take is one program like that in a work-load for the performance to descend into something unpleasant. Eric
