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Date: Wed, 15 Nov 2023 12:08:23 +0100
From: Stefano Garzarella <sgarzare@...hat.com>
To: Arseniy Krasnov <avkrasnov@...utedevices.com>
Cc: Stefan Hajnoczi <stefanha@...hat.com>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	"Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>, 
	Bobby Eshleman <bobby.eshleman@...edance.com>, kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org, 
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org, kernel@...rdevices.ru, 
	oxffffaa@...il.com
Subject: Re: [RFC PATCH v1 0/2] send credit update during setting SO_RCVLOWAT

On Wed, Nov 08, 2023 at 10:20:02AM +0300, Arseniy Krasnov wrote:
>Hello,
>
>                               DESCRIPTION
>
>This patchset fixes old problem with hungup of both rx/tx sides and adds
>test for it. This happens due to non-default SO_RCVLOWAT value and
>deferred credit update in virtio/vsock. Link to previous old patchset:
>https://lore.kernel.org/netdev/39b2e9fd-601b-189d-39a9-914e5574524c@sberdevices.ru/
>
>Here is what happens step by step:
>
>                                  TEST
>
>                            INITIAL CONDITIONS
>
>1) Vsock buffer size is 128KB.
>2) Maximum packet size is also 64KB as defined in header (yes it is
>   hardcoded, just to remind about that value).
>3) SO_RCVLOWAT is default, e.g. 1 byte.
>
>
>                                 STEPS
>
>            SENDER                              RECEIVER
>1) sends 128KB + 1 byte in a
>   single buffer. 128KB will
>   be sent, but for 1 byte
>   sender will wait for free
>   space at peer. Sender goes
>   to sleep.
>
>
>2)                                     reads 64KB, credit update not sent
>3)                                     sets SO_RCVLOWAT to 64KB + 1
>4)                                     poll() -> wait forever, there is
>                                       only 64KB available to read.
>
>So in step 4) receiver also goes to sleep, waiting for enough data or
>connection shutdown message from the sender. Idea to fix it is that rx
>kicks tx side to continue transmission (and may be close connection)
>when rx changes number of bytes to be woken up (e.g. SO_RCVLOWAT) and
>this value is bigger than number of available bytes to read.
>
>I've added small test for this, but not sure as it uses hardcoded value

Thanks for adding the test!

>for maximum packet length, this value is defined in kernel header and
>used to control deferred credit update. And as this is not available to
>userspace, I can't control test parameters correctly (if one day this
>define will be changed - test may become useless).

I see, I'll leave a comment in the patch!

Thanks,
Stefano

>
>Head for this patchset is:
>https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=ff269e2cd5adce4ae14f883fc9c8803bc43ee1e9
>
>Arseniy Krasnov (2):
>  virtio/vsock: send credit update during setting SO_RCVLOWAT
>  vsock/test: SO_RCVLOWAT + deferred credit update test
>
> drivers/vhost/vsock.c                   |   2 +
> include/linux/virtio_vsock.h            |   1 +
> net/vmw_vsock/virtio_transport.c        |   2 +
> net/vmw_vsock/virtio_transport_common.c |  31 ++++++
> net/vmw_vsock/vsock_loopback.c          |   2 +
> tools/testing/vsock/vsock_test.c        | 131 ++++++++++++++++++++++++
> 6 files changed, 169 insertions(+)
>
>-- 
>2.25.1
>


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