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. 'cleanup_net()' and 'fqdir_work_fn()' are functions that deallocate the relevant memory objects. They are asynchronously invoked by the work queues and internally use 'rcu_barrier()' to ensure safe destructions. 'cleanup_net()' works in a batched maneer in a single thread worker, while 'fqdir_work_fn()' works for each 'fqdir_exit()' call in the 'system_wq'. Therefore, 'fqdir_work_fn()' called frequently under the workload and made the contention for 'rcu_barrier()' high. In more detail, the global mutex, 'rcu_state.barrier_mutex' became the bottleneck. I tried making 'fqdir_work_fn()' batched and confirmed it works. The following patch is for the change. I think this is the right solution for point fix of this issue, but someone might blame different parts. 1. User: Frequent 'unshare()' calls >From some point of view, such frequent 'unshare()' calls might seem only insane. 2. Global mutex in 'rcu_barrier()' Because of the global mutex, 'rcu_barrier()' callers could wait long even after the callbacks started before the call finished. Therefore, similar issues could happen in another 'rcu_barrier()' usages. Maybe we can use some wait queue like mechanism to notify the waiters when the desired time came. I personally believe applying the point fix for now and making 'rcu_barrier()' improvement in longterm make sense. If I'm missing something or you have different opinion, please feel free to let me know. Patch History ------------- Changes from v1 (https://lore.kernel.org/netdev/20201208094529.23266-1-sjpark@xxxxxxxxxx/) - Keep xmas tree variable ordering (Jakub Kicinski) - Add more numbers (Eric Dumazet) - Use 'llist_for_each_entry_safe()' (Eric Dumazet) SeongJae Park (1): net/ipv4/inet_fragment: Batch fqdir destroy works include/net/inet_frag.h | 2 +- net/ipv4/inet_fragment.c | 28 ++++++++++++++++++++-------- 2 files changed, 21 insertions(+), 9 deletions(-) -- 2.17.1
