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Subject: [PATCH v2 0/9] SUNRPC service thread scheduler optimizations
From:   Chuck Lever <cel@kernel.org>
To:     linux-nfs@vger.kernel.org
Cc:     Chuck Lever <chuck.lever@oracle.com>, lorenzo@kernel.org,
        neilb@suse.de, jlayton@redhat.com, david@fromorbit.com
Date:   Mon, 03 Jul 2023 20:07:29 -0400
Message-ID: <168842897573.139194.15893960758088950748.stgit@manet.1015granger.net>
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Walking a linked list to find an idle thread is not CPU cache-
friendly, and in fact I've noted palpable per-request latency
impacts as the number of nfsd threads on the server increases.

After discussing some possible improvements with Jeff at LSF/MM,
I've been experimenting with the following series. I've measured an
order of magnitude latency improvement in the thread lookup time,
and have managed to keep the whole thing lockless.

After some offline discussion with Neil, I tried out using just
the xarray plus its spinlock to mark threads idle or busy. This
worked as well as the bitmap for lower thread counts, but got
predictably bad as the thread count when past several hundred. For
the moment I'm sticking with the wait-free bitmap lookup.

Also, the maximum thread count is now 4096. I'm still willing to try
an RCU-based bitmap resizing mechanism if we believe this is still
to small of a maximum.


Changes since RFC:
* Add a counter for ingress RPC messages
* Add a few documenting comments
* Move the more controversial patches to the end of the series
* Clarify the refactoring of svc_wake_up() 
* Increase the value of RPCSVC_MAXPOOLTHREADS to 4096 (and tested with that many threads)
* Optimize the loop in svc_pool_wake_idle_thread()
* Optimize marking a thread "idle" in svc_get_next_xprt()

---

Chuck Lever (9):
      SUNRPC: Deduplicate thread wake-up code
      SUNRPC: Report when no service thread is available.
      SUNRPC: Split the svc_xprt_dequeue tracepoint
      SUNRPC: Count ingress RPC messages per svc_pool
      SUNRPC: Clean up svc_set_num_threads
      SUNRPC: Replace dprintk() call site in __svc_create()
      SUNRPC: Don't disable BH's when taking sp_lock
      SUNRPC: Replace sp_threads_all with an xarray
      SUNRPC: Convert RQ_BUSY into a per-pool bitmap


 fs/nfsd/nfssvc.c              |   3 +-
 include/linux/sunrpc/svc.h    |  18 ++--
 include/trace/events/sunrpc.h | 159 +++++++++++++++++++++++++++----
 net/sunrpc/svc.c              | 174 ++++++++++++++++++++++------------
 net/sunrpc/svc_xprt.c         | 114 +++++++++++-----------
 5 files changed, 328 insertions(+), 140 deletions(-)

--
Chuck Lever