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

[Stefano Garzarella]

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