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Thu, 09 Nov 2023 18:35:27 -0800 (PST) Received: from [10.54.24.52] ([143.92.118.3]) by smtp.gmail.com with ESMTPSA id jo18-20020a170903055200b001c0a4146961sm4207161plb.19.2023.11.09.18.35.24 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 09 Nov 2023 18:35:27 -0800 (PST) Message-ID: <0fda4dff-9683-48a8-9b5b-2fdec9daabc0@shopee.com> Date: Fri, 10 Nov 2023 10:35:23 +0800 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] boning: use a read-write lock in bonding_show_bonds() To: Stephen Hemminger Cc: j.vosburgh@gmail.com, andy@greyhouse.net, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, netdev@vger.kernel.org, linux-kernel@vger.kernel.org References: <20231108064641.65209-1-haifeng.xu@shopee.com> <20231109095502.5a03bfd5@hermes.local> From: Haifeng Xu In-Reply-To: <20231109095502.5a03bfd5@hermes.local> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 2023/11/10 01:55, Stephen Hemminger wrote: > On Wed, 8 Nov 2023 06:46:41 +0000 > Haifeng Xu wrote: > >> call stack: >> ...... >> PID: 210933 TASK: ffff92424e5ec080 CPU: 13 COMMAND: "kworker/u96:2" >> [ffffa7a8e96bbac0] __schedule at ffffffffb0719898 >> [ffffa7a8e96bbb48] schedule at ffffffffb0719e9e >> [ffffa7a8e96bbb68] rwsem_down_write_slowpath at ffffffffafb3167a >> [ffffa7a8e96bbc00] down_write at ffffffffb071bfc1 >> [ffffa7a8e96bbc18] kernfs_remove_by_name_ns at ffffffffafe3593e >> [ffffa7a8e96bbc48] sysfs_unmerge_group at ffffffffafe38922 >> [ffffa7a8e96bbc68] dpm_sysfs_remove at ffffffffb021c96a >> [ffffa7a8e96bbc80] device_del at ffffffffb0209af8 >> [ffffa7a8e96bbcd0] netdev_unregister_kobject at ffffffffb04a6b0e >> [ffffa7a8e96bbcf8] unregister_netdevice_many at ffffffffb046d3d9 >> [ffffa7a8e96bbd60] default_device_exit_batch at ffffffffb046d8d1 >> [ffffa7a8e96bbdd0] ops_exit_list at ffffffffb045e21d >> [ffffa7a8e96bbe00] cleanup_net at ffffffffb045ea46 >> [ffffa7a8e96bbe60] process_one_work at ffffffffafad94bb >> [ffffa7a8e96bbeb0] worker_thread at ffffffffafad96ad >> [ffffa7a8e96bbf10] kthread at ffffffffafae132a >> [ffffa7a8e96bbf50] ret_from_fork at ffffffffafa04b92 >> >> 290858 PID: 278176 TASK: ffff925deb39a040 CPU: 32 COMMAND: "node-exporter" >> [ffffa7a8d14dbb80] __schedule at ffffffffb0719898 >> [ffffa7a8d14dbc08] schedule at ffffffffb0719e9e >> [ffffa7a8d14dbc28] schedule_preempt_disabled at ffffffffb071a24e >> [ffffa7a8d14dbc38] __mutex_lock at ffffffffb071af28 >> [ffffa7a8d14dbcb8] __mutex_lock_slowpath at ffffffffb071b1a3 >> [ffffa7a8d14dbcc8] mutex_lock at ffffffffb071b1e2 >> [ffffa7a8d14dbce0] rtnl_lock at ffffffffb047f4b5 >> [ffffa7a8d14dbcf0] bonding_show_bonds at ffffffffc079b1a1 [bonding] >> [ffffa7a8d14dbd20] class_attr_show at ffffffffb02117ce >> [ffffa7a8d14dbd30] sysfs_kf_seq_show at ffffffffafe37ba1 >> [ffffa7a8d14dbd50] kernfs_seq_show at ffffffffafe35c07 >> [ffffa7a8d14dbd60] seq_read_iter at ffffffffafd9fce0 >> [ffffa7a8d14dbdc0] kernfs_fop_read_iter at ffffffffafe36a10 >> [ffffa7a8d14dbe00] new_sync_read at ffffffffafd6de23 >> [ffffa7a8d14dbe90] vfs_read at ffffffffafd6e64e >> [ffffa7a8d14dbed0] ksys_read at ffffffffafd70977 >> [ffffa7a8d14dbf10] __x64_sys_read at ffffffffafd70a0a >> [ffffa7a8d14dbf20] do_syscall_64 at ffffffffb070bf1c >> [ffffa7a8d14dbf50] entry_SYSCALL_64_after_hwframe at ffffffffb080007c >> ...... >> >> Problem description: >> >> Thread 210933 holds the rtnl_mutex and tries to acquire the kernfs_rwsem, >> but there are many readers which hold the kernfs_rwsem, so it has to sleep >> for a long time to wait the readers release the lock. Thread 278176 and any >> other threads which call bonding_show_bonds() also need to wait because >> they try to accuire the rtnl_mutex. >> >> bonding_show_bonds() uses rtnl_mutex to protect the bond_list traversal. >> However, the addition and deletion of bond_list are only performed in >> bond_init()/bond_uninit(), so we can intoduce a separate read-write lock >> to synchronize bond list mutation. >> >> What's the benefits of this change? >> >> 1) All threads which call bonding_show_bonds() only wait when the >> registration or unregistration of bond device happens. >> >> 2) There are many other users of rtnl_mutex, so bonding_show_bonds() >> won't compete with them. >> >> In a word, this change reduces the lock contention of rtnl_mutex. >> >> Signed-off-by: Haifeng Xu >> --- >> drivers/net/bonding/bond_main.c | 4 ++++ >> drivers/net/bonding/bond_sysfs.c | 6 ++++-- >> include/net/bonding.h | 3 +++ >> 3 files changed, 11 insertions(+), 2 deletions(-) > > Reader-writer locks are slower than spin locks and should be discouraged> Would it be possible to use RCU here instead? In most cases，there are many threads which want to iterate over the bond_list, the registration or unregistration of bond device rarely happens.