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Date:   Tue, 8 Dec 2020 10:45:28 +0100
From:   SeongJae Park <sjpark@...zon.com>
To:     <davem@...emloft.net>
CC:     SeongJae Park <sjpark@...zon.de>, <kuba@...nel.org>,
        <kuznet@....inr.ac.ru>, <paulmck@...nel.org>,
        <netdev@...r.kernel.org>, <rcu@...r.kernel.org>,
        <linux-kernel@...r.kernel.orgor>
Subject: [PATCH 0/1] net: Reduce rcu_barrier() contentions from 'unshare(CLONE_NEWNET)'

From: SeongJae Park <sjpark@...zon.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.

'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 opinions, please feel free to let me
know.

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

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