netdev - Re: [PATCH RFC net-next v4 7/8] vsock: Add lockless sendmsg() support

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Date: Thu, 22 Jun 2023 18:37:21 +0200
From: Stefano Garzarella <sgarzare@...hat.com>
To: Bobby Eshleman <bobby.eshleman@...edance.com>
Cc: Stefan Hajnoczi <stefanha@...hat.com>, 
	"Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>, 
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, "K. Y. Srinivasan" <kys@...rosoft.com>, 
	Haiyang Zhang <haiyangz@...rosoft.com>, Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>, 
	Bryan Tan <bryantan@...are.com>, Vishnu Dasa <vdasa@...are.com>, 
	VMware PV-Drivers Reviewers <pv-drivers@...are.com>, Dan Carpenter <dan.carpenter@...aro.org>, 
	Simon Horman <simon.horman@...igine.com>, Krasnov Arseniy <oxffffaa@...il.com>, kvm@...r.kernel.org, 
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-hyperv@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH RFC net-next v4 7/8] vsock: Add lockless sendmsg() support

On Sat, Jun 10, 2023 at 12:58:34AM +0000, Bobby Eshleman wrote:
>Because the dgram sendmsg() path for AF_VSOCK acquires the socket lock
>it does not scale when many senders share a socket.
>
>Prior to this patch the socket lock is used to protect both reads and
>writes to the local_addr, remote_addr, transport, and buffer size
>variables of a vsock socket. What follows are the new protection schemes
>for these fields that ensure a race-free and usually lock-free
>multi-sender sendmsg() path for vsock dgrams.
>
>- local_addr
>local_addr changes as a result of binding a socket. The write path
>for local_addr is bind() and various vsock_auto_bind() call sites.
>After a socket has been bound via vsock_auto_bind() or bind(), subsequent
>calls to bind()/vsock_auto_bind() do not write to local_addr again. bind()
>rejects the user request and vsock_auto_bind() early exits.
>Therefore, the local addr can not change while a parallel thread is
>in sendmsg() and lock-free reads of local addr in sendmsg() are safe.
>Change: only acquire lock for auto-binding as-needed in sendmsg().
>
>- buffer size variables
>Not used by dgram, so they do not need protection. No change.
>
>- remote_addr and transport
>Because a remote_addr update may result in a changed transport, but we
>would like to be able to read these two fields lock-free but coherently
>in the vsock send path, this patch packages these two fields into a new
>struct vsock_remote_info that is referenced by an RCU-protected pointer.
>
>Writes are synchronized as usual by the socket lock. Reads only take
>place in RCU read-side critical sections. When remote_addr or transport
>is updated, a new remote info is allocated. Old readers still see the
>old coherent remote_addr/transport pair, and new readers will refer to
>the new coherent. The coherency between remote_addr and transport
>previously provided by the socket lock alone is now also preserved by
>RCU, except with the highly-scalable lock-free read-side.
>
>Helpers are introduced for accessing and updating the new pointer.
>
>The new structure is contains an rcu_head so that kfree_rcu() can be
>used. This removes the need of writers to use synchronize_rcu() after
>freeing old structures which is simply more efficient and reduces code
>churn where remote_addr/transport are already being updated inside RCU
>read-side sections.
>
>Only virtio has been tested, but updates were necessary to the VMCI and
>hyperv code. Unfortunately the author does not have access to
>VMCI/hyperv systems so those changes are untested.

@Dexuan, @Vishnu, @Bryan, can you test this?

>
>Perf Tests (results from patch v2)
>vCPUS: 16
>Threads: 16
>Payload: 4KB
>Test Runs: 5
>Type: SOCK_DGRAM
>
>Before: 245.2 MB/s
>After: 509.2 MB/s (+107%)
>
>Notably, on the same test system, vsock dgram even outperforms
>multi-threaded UDP over virtio-net with vhost and MQ support enabled.
>
>Throughput metrics for single-threaded SOCK_DGRAM and
>single/multi-threaded SOCK_STREAM showed no statistically signficant
>throughput changes (lowest p-value reaching 0.27), with the range of the
>mean difference ranging between -5% to +1%.
>

Quite nice. Did you see any improvements also on stream/seqpacket
sockets?

However this is a big change, maybe I would move it to another series,
because it takes time to be reviewed and tested properly.

WDYT?

Thanks,
Stefano