RE: Introduce xenwatch multithreading (mtwatch)

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
> From: Dongli Zhang [mailto:dongli.zhang@oracle.com]
> Sent: 14 September 2018 08:34
> To: xen-devel@lists.xenproject.org; linux-kernel@vger.kernel.org
> Cc: boris.ostrovsky@oracle.com; jgross@suse.com; Paul Durrant
> <Paul.Durrant@citrix.com>; Wei Liu <wei.liu2@citrix.com>;
> konrad.wilk@oracle.com; Roger Pau Monne <roger.pau@citrix.com>;
> srinivas.eeda@oracle.com
> Subject: Introduce xenwatch multithreading (mtwatch)
> 
> Hi,
> 
> This patch set introduces xenwatch multithreading (mtwatch) based on the
> below xen summit 2018 design session notes:

Ah, here is the cover letter... just not labelled 0/6.

> 
> https://lists.xenproject.org/archives/html/xen-devel/2018-07/msg00017.html
> 
> 
> xenwatch_thread is a single kernel thread processing the callback function
> for subscribed xenwatch events successively. The xenwatch is stalled in
> 'D'
> state if any of callback function is stalled and uninterruptible.
> 
> The domU 

I think you should use 'frontend'. It is perfectly possible that a frontend could run in dom0m if, say, the system were using a driver domain.

> create/destroy is failed if xenwatch is stalled in 'D' state as
> the paravirtual driver init/uninit cannot complete. Usually, the only
> option is to reboot dom0 server unless there is solution/workaround to

Similarly 'backend' instead of 'dom0', pointing out that this is bad because it is normal for a single backend domain to serve multiple frontend domains, and the PV protocol is not (yet) re-startable if a backend goes away.

  Paul

> move forward and complete the stalled xenwatch event callback function.
> Below is the output of 'xl create' when xenwatch is stalled (the issue is
> reproduced on purpose by hooking netif_receive_skb() to intercept an
> sk_buff sent out from vifX.Y on dom0 with patch at
> https://github.com/finallyjustice/patchset/blob/master/xenwatch-stall-by-
> vif.patch):
> 
> # xl create pv.cfg
> Parsing config from pv.cfg
> libxl: error: libxl_device.c:1080:device_backend_callback: Domain 2:unable
> to add device with path /local/domain/0/backend/vbd/2/51712
> libxl: error: libxl_create.c:1278:domcreate_launch_dm: Domain 2:unable to
> add disk devices
> libxl: error: libxl_device.c:1080:device_backend_callback: Domain 2:unable
> to remove device with path /local/domain/0/backend/vbd/2/51712
> libxl: error: libxl_domain.c:1073:devices_destroy_cb: Domain
> 2:libxl__devices_destroy failed
> libxl: error: libxl_domain.c:1000:libxl__destroy_domid: Domain 2:Non-
> existant domain
> libxl: error: libxl_domain.c:959:domain_destroy_callback: Domain 2:Unable
> to destroy guest
> libxl: error: libxl_domain.c:886:domain_destroy_cb: Domain 2:Destruction
> of domain failed
> 
> 
> The idea of this patch set is to create a per-domU xenwatch thread for
> each
> domid. The per-domid thread is created when the 1st pv backend device (for
> this domid and with xenwatch multithreading enabled) is created, while
> this
> thread is destroyed when the last pv backend device (for this domid and
> with xenwatch multithreading enabled) is removed. Per-domid xs_watch_event
> is never put on the default event list, but is put on the per-domid event
> list directly.
> 
> 
> For more details, please refer to the xen summit 2018 design session notes
> and presentation slides:
> 
> https://lists.xenproject.org/archives/html/xen-devel/2018-07/msg00017.html
> http://www.donglizhang.org/xenwatch_multithreading.pdf
> 
> ----------------------------------------------------------------
> 
> Dongli Zhang (6):
>     xenbus: prepare data structures and parameter for xenwatch
> multithreading
>     xenbus: implement the xenwatch multithreading framework
>     xenbus: dispatch per-domU watch event to per-domU xenwatch thread
>     xenbus: process otherend_watch event at 'state' entry in xenwatch
> multithreading
>     xenbus: process be_watch events in xenwatch multithreading
>     drivers: enable xenwatch multithreading for xen-netback and xen-
> blkback driver
> 
>  Documentation/admin-guide/kernel-parameters.txt |   3 +
>  drivers/block/xen-blkback/xenbus.c              |   3 +-
>  drivers/net/xen-netback/xenbus.c                |   1 +
>  drivers/xen/xenbus/xenbus_probe.c               |  24 +-
>  drivers/xen/xenbus/xenbus_probe_backend.c       |  32 +++
>  drivers/xen/xenbus/xenbus_xs.c                  | 357
> +++++++++++++++++++++++-
>  include/xen/xenbus.h                            |  70 +++++
>  7 files changed, 484 insertions(+), 6 deletions(-)
> 
> Thank you very much!
> 
> Dongli Zhang