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Message-ID: <273e8c67-fbb0-edd8-600f-512c1a6812f3@linux.ibm.com>
Date: Mon, 6 Feb 2023 12:06:45 +0100
From: Wenjia Zhang <wenjia@...ux.ibm.com>
To: "D. Wythe" <alibuda@...ux.alibaba.com>, kgraul@...ux.ibm.com,
jaka@...ux.ibm.com
Cc: kuba@...nel.org, davem@...emloft.net, netdev@...r.kernel.org,
linux-s390@...r.kernel.org, linux-rdma@...r.kernel.org
Subject: Re: [net-next v7 0/4] net/smc: optimize the parallelism of SMC-R
connections
On 02.02.23 09:26, D. Wythe wrote:
> From: "D. Wythe" <alibuda@...ux.alibaba.com>
>
> This patch set attempts to optimize the parallelism of SMC-R connections,
> mainly to reduce unnecessary blocking on locks, and to fix exceptions that
> occur after thoses optimization.
>
> According to Off-CPU graph, SMC worker's off-CPU as that:
>
> smc_close_passive_work (1.09%)
> smcr_buf_unuse (1.08%)
> smc_llc_flow_initiate (1.02%)
>
> smc_listen_work (48.17%)
> __mutex_lock.isra.11 (47.96%)
>
>
> An ideal SMC-R connection process should only block on the IO events
> of the network, but it's quite clear that the SMC-R connection now is
> queued on the lock most of the time.
>
> The goal of this patchset is to achieve our ideal situation where
> network IO events are blocked for the majority of the connection lifetime.
>
> There are three big locks here:
>
> 1. smc_client_lgr_pending & smc_server_lgr_pending
>
> 2. llc_conf_mutex
>
> 3. rmbs_lock & sndbufs_lock
>
> And an implementation issue:
>
> 1. confirm/delete rkey msg can't be sent concurrently while
> protocol allows indeed.
>
> Unfortunately,The above problems together affect the parallelism of
> SMC-R connection. If any of them are not solved. our goal cannot
> be achieved.
>
> After this patch set, we can get a quite ideal off-CPU graph as
> following:
>
> smc_close_passive_work (41.58%)
> smcr_buf_unuse (41.57%)
> smc_llc_do_delete_rkey (41.57%)
>
> smc_listen_work (39.10%)
> smc_clc_wait_msg (13.18%)
> tcp_recvmsg_locked (13.18)
> smc_listen_find_device (25.87%)
> smcr_lgr_reg_rmbs (25.87%)
> smc_llc_do_confirm_rkey (25.87%)
>
> We can see that most of the waiting times are waiting for network IO
> events. This also has a certain performance improvement on our
> short-lived conenction wrk/nginx benchmark test:
>
> +--------------+------+------+-------+--------+------+--------+
> |conns/qps |c4 | c8 | c16 | c32 | c64 | c200 |
> +--------------+------+------+-------+--------+------+--------+
> |SMC-R before |9.7k | 10k | 10k | 9.9k | 9.1k | 8.9k |
> +--------------+------+------+-------+--------+------+--------+
> |SMC-R now |13k | 19k | 18k | 16k | 15k | 12k |
> +--------------+------+------+-------+--------+------+--------+
> |TCP |15k | 35k | 51k | 80k | 100k | 162k |
> +--------------+------+------+-------+--------+------+--------+
>
> The reason why the benefit is not obvious after the number of connections
> has increased dues to workqueue. If we try to change workqueue to UNBOUND,
> we can obtain at least 4-5 times performance improvement, reach up to half
> of TCP. However, this is not an elegant solution, the optimization of it
> will be much more complicated. But in any case, we will submit relevant
> optimization patches as soon as possible.
>
> Please note that the premise here is that the lock related problem
> must be solved first, otherwise, no matter how we optimize the workqueue,
> there won't be much improvement.
>
> Because there are a lot of related changes to the code, if you have
> any questions or suggestions, please let me know.
>
> Thanks
> D. Wythe
>
> v1 -> v2:
>
> 1. Fix panic in SMC-D scenario
> 2. Fix lnkc related hashfn calculation exception, caused by operator
> priority
> 3. Only wake up one connection if the lnk is not active
> 4. Delete obsolete unlock logic in smc_listen_work()
> 5. PATCH format, do Reverse Christmas tree
> 6. PATCH format, change all xxx_lnk_xxx function to xxx_link_xxx
> 7. PATCH format, add correct fix tag for the patches for fixes.
> 8. PATCH format, fix some spelling error
> 9. PATCH format, rename slow to do_slow
>
> v2 -> v3:
>
> 1. add SMC-D support, remove the concept of link cluster since SMC-D has
> no link at all. Replace it by lgr decision maker, who provides suggestions
> to SMC-D and SMC-R on whether to create new link group.
>
> 2. Fix the corruption problem described by PATCH 'fix application
> data exception' on SMC-D.
>
> v3 -> v4:
>
> 1. Fix panic caused by uninitialization map.
>
> v4 -> v5:
>
> 1. Make SMC-D buf creation be serial to avoid Potential error
> 2. Add a flag to synchronize the success of the first contact
> with the ready of the link group, including SMC-D and SMC-R.
> 3. Fixed possible reference count leak in smc_llc_flow_start().
> 4. reorder the patch, make bugfix PATCH be ahead.
>
> v5 -> v6:
>
> 1. Separate the bugfix patches to make it independent.
> 2. Merge patch 'fix SMC_CLC_DECL_ERR_REGRMB without smc_server_lgr_pending'
> with patch 'remove locks smc_client_lgr_pending and smc_server_lgr_pending'
> 3. Format code styles, including alignment and reverse christmas tree
> style.
> 4. Fix a possible memory leak in smc_llc_rmt_delete_rkey()
> and smc_llc_rmt_conf_rkey().
>
> v6 -> v7:
>
> 1. Discard patch attempting to remove global locks
> 2. Discard patch attempting make confirm/delete rkey process concurrently
>
> D. Wythe (4):
> net/smc: llc_conf_mutex refactor, replace it with rw_semaphore
> net/smc: use read semaphores to reduce unnecessary blocking in
> smc_buf_create() & smcr_buf_unuse()
> net/smc: reduce unnecessary blocking in smcr_lgr_reg_rmbs()
> net/smc: replace mutex rmbs_lock and sndbufs_lock with rw_semaphore
>
> net/smc/af_smc.c | 25 ++++++++++++++----
> net/smc/smc_core.c | 75 +++++++++++++++++++++++++++---------------------------
> net/smc/smc_core.h | 6 ++---
> net/smc/smc_llc.c | 34 ++++++++++++-------------
> 4 files changed, 78 insertions(+), 62 deletions(-)
>
This answer seems too late ;-)
I did some test as thoroughly as I can, it looks good to me.
Reviewed-by: Wenjia Zhang <wenjia@...ux.ibm.com>
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