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

[Bobby Eshleman]

On Thu, Jun 22, 2023 at 06:37:21PM +0200, Stefano Garzarella 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.
> 
> @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?
> 

The change seemed to be null for stream sockets. I assumed the same
would be for seqpacket too, but I'll run some numbers there too for the
next revision.

> 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?
> 

Sounds good to me, I'll lop it off and resend on its own.

> Thanks,
> Stefano
> 

Thanks!
Bobby