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

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Message-ID: <ZJTSSqXL+IXoRK3x@bullseye>
Date: Thu, 22 Jun 2023 22:59:22 +0000
From: Bobby Eshleman <bobbyeshleman@...il.com>
To: Simon Horman <simon.horman@...igine.com>
Cc: Bobby Eshleman <bobby.eshleman@...edance.com>,
	Stefan Hajnoczi <stefanha@...hat.com>,
	Stefano Garzarella <sgarzare@...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>,
	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 Mon, Jun 12, 2023 at 11:53:00AM +0200, Simon Horman wrote:
> 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.
> > 
> > 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
> 
> Hi Bobby,
> 
> a minor nit from checkpatch --codespell: signficant -> significant
> 

Thanks! I'll fix that and add that argument to my pre-commit hook.

> > throughput changes (lowest p-value reaching 0.27), with the range of the
> > mean difference ranging between -5% to +1%.
> > 
> > Signed-off-by: Bobby Eshleman <bobby.eshleman@...edance.com>
> 
> ...