Re: [PATCH v2 0/1] net: Reduce rcu_barrier() contentions from 'unshare(CLONE_NEWNET)'

Eric Dumazet
2020-12-10T14:10:39Z

On Thu, Dec 10, 2020 at 9:09 AM SeongJae Park <sjpark@amazon.com> wrote:
>
> From: SeongJae Park <sjpark@amazon.de>
>
> 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 ?

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);

Thanks !

>
> '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@amazon.com/)
> - 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
>