On Thu, 10 Dec 2020 15:09:10 +0100 Eric Dumazet <edumazet@xxxxxxxxxx> wrote: > On Thu, Dec 10, 2020 at 9:09 AM SeongJae Park <sjpark@xxxxxxxxxx> wrote: > > > > From: SeongJae Park <sjpark@xxxxxxxxx> > > > > On a few of our systems, I found frequent 'unshare(CLONE_NEWNET)' calls > > make the number of active slab objects including 'sock_inode_cache' type > > rapidly and continuously increase. As a result, memory pressure occurs. > > > > In more detail, I made an artificial reproducer that resembles the > > workload that we found the problem and reproduce the problem faster. It > > merely repeats 'unshare(CLONE_NEWNET)' 50,000 times in a loop. It takes > > about 2 minutes. On 40 CPU cores, 70GB DRAM machine, it reduced about > > 15GB of available memory in total. Note that the issue don't reproduce > > on every machine. On my 6 CPU cores machine, the problem didn't > > reproduce. > > OK, that is the number before the patch, but what is the number after > the patch ? No continuous memory reduction but some fluctuation observed. Nevertheless, the available memory reduction was only up to about 400MB. > > I think the idea is very nice, but this will serialize fqdir hash > tables destruction on one single cpu, > this might become a real issue _if_ these hash tables are populated. > > (Obviously in your for (i=1;i<50000;i++) unshare(CLONE_NEWNET); all > these tables are empty...) > > As you may now, frags are often used as vectors for DDOS attacks. > > I would suggest maybe to not (ab)use system_wq, but a dedicated work queue > with a limit (@max_active argument set to 1 in alloc_workqueue()) , to > make sure that the number of > threads is optimal/bounded. > > Only the phase after hash table removal could benefit from your > deferral to a single context, > so that a single rcu_barrier() is active, since the part after > rcu_barrier() is damn cheap and _can_ be serialized > > if (refcount_dec_and_test(&f->refcnt)) > complete(&f->completion); Good point, thanks for this kind suggestion. I will do so in next version. Thanks, SeongJae Park
