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Date: Fri, 9 Sep 2022 08:59:41 +0200
From: Jan Karcher <jaka@...ux.ibm.com>
To: "D. Wythe" <alibuda@...ux.alibaba.com>, kgraul@...ux.ibm.com,
wenjia@...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: [PATCH net-next v2 00/10] optimize the parallelism of SMC-R
connections
On 26.08.2022 11:51, 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. Remove -EBUSY processing of rhashtable_insert_fast, see more details
> in comments around smcr_link_get_or_create_cluster().
> 4. Only wake up one connection if the link has not been active.
> 5. Delete obsolete unlock logic in smc_listen_work().
> 6. PATCH format, do Reverse Christmas tree.
> 7. PATCH format, change all xxx_lnk_xxx function to xxx_link_xxx.
> 8. PATCH format, add correct fix tag for the patches for fixes.
> 9. PATCH format, fix some spelling error.
> 10.PATCH format, rename slow to do_slow in smcr_lgr_reg_rmbs().
>
>
> D. Wythe (10):
> net/smc: remove locks smc_client_lgr_pending and
> smc_server_lgr_pending
> net/smc: fix SMC_CLC_DECL_ERR_REGRMB without smc_server_lgr_pending
> net/smc: allow confirm/delete rkey response deliver multiplex
> net/smc: make SMC_LLC_FLOW_RKEY run concurrently
> 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: Fix potential panic dues to unprotected
> smc_llc_srv_add_link()
> net/smc: fix application data exception
>
> net/smc/af_smc.c | 42 +++--
> net/smc/smc_core.c | 443 +++++++++++++++++++++++++++++++++++++++++++++++------
> net/smc/smc_core.h | 78 +++++++++-
> net/smc/smc_llc.c | 286 +++++++++++++++++++++++++---------
> net/smc/smc_llc.h | 6 +
> net/smc/smc_wr.c | 10 --
> net/smc/smc_wr.h | 10 ++
> 7 files changed, 725 insertions(+), 150 deletions(-)
>
D.,
I'm sorry.
I replied to the patch 01/10 with the test results and not the cover
letter. I have a filter on my inbox separating everything for "net/smc:"
and the keywords are missing on this cover letter.
Mea culpa.
https://lore.kernel.org/netdev/1767b6e4-0053-728b-9722-add68da13781@linux.ibm.com/
- Jan
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