Re: [Xen-devel] [PATCH 2/2] xen-bus: Avoid rewriting identical values to xenstore

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

> -----Original Message-----
> From: Anthony PERARD <anthony.perard@citrix.com>
> Sent: 22 August 2019 11:22
> To: Paul Durrant <Paul.Durrant@citrix.com>
> Cc: qemu-devel@nongnu.org; Stefano Stabellini <sstabellini@kernel.org>; xen-devel@lists.xenproject.org
> Subject: Re: [PATCH 2/2] xen-bus: Avoid rewriting identical values to xenstore
> 
> On Wed, Aug 21, 2019 at 04:40:05PM +0100, Paul Durrant wrote:
> > > -----Original Message-----
> > > From: Anthony PERARD <anthony.perard@citrix.com>
> > > Sent: 21 August 2019 10:20
> > > To: qemu-devel@nongnu.org
> > > Cc: Anthony Perard <anthony.perard@citrix.com>; Stefano Stabellini <sstabellini@kernel.org>; Paul
> > > Durrant <Paul.Durrant@citrix.com>; xen-devel@lists.xenproject.org
> > > Subject: [PATCH 2/2] xen-bus: Avoid rewriting identical values to xenstore
> > >
> > > When QEMU receive a xenstore watch event suggesting that the "state" or
> > > "online" status of the frontend or the backend changed, it record this
> > > in its own state but it also re-write the value back into xenstore even
> > > so there were no changed. This trigger an unnecessary xenstore watch
> > > event which QEMU will process again (and maybe the frontend as well).
> > >
> > > Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
> > > ---
> > >  hw/xen/xen-bus.c | 37 ++++++++++++++++++++++++-------------
> > >  1 file changed, 24 insertions(+), 13 deletions(-)
> > >
> > > diff --git a/hw/xen/xen-bus.c b/hw/xen/xen-bus.c
> > > index 982eca4533..c83f07424a 100644
> > > --- a/hw/xen/xen-bus.c
> > > +++ b/hw/xen/xen-bus.c
> > > @@ -481,20 +481,27 @@ static int xen_device_backend_scanf(XenDevice *xendev, const char *key,
> > >      return rc;
> > >  }
> > >
> > > -void xen_device_backend_set_state(XenDevice *xendev,
> > > -                                  enum xenbus_state state)
> > > +static bool xen_device_backend_record_state(XenDevice *xendev,
> > > +                                            enum xenbus_state state)
> > >  {
> > >      const char *type = object_get_typename(OBJECT(xendev));
> > >
> > >      if (xendev->backend_state == state) {
> > > -        return;
> > > +        return false;
> > >      }
> > >
> > >      trace_xen_device_backend_state(type, xendev->name,
> > >                                     xs_strstate(state));
> > >
> > >      xendev->backend_state = state;
> > > -    xen_device_backend_printf(xendev, "state", "%u", state);
> > > +    return true;
> > > +}
> > > +
> > > +void xen_device_backend_set_state(XenDevice *xendev,
> > > +                                  enum xenbus_state state)
> > > +{
> > > +    if (xen_device_backend_record_state(xendev, state))
> > > +        xen_device_backend_printf(xendev, "state", "%u", state);
> > >  }
> > >
> > >  enum xenbus_state xen_device_backend_get_state(XenDevice *xendev)
> > > @@ -502,7 +509,8 @@ enum xenbus_state xen_device_backend_get_state(XenDevice *xendev)
> > >      return xendev->backend_state;
> > >  }
> > >
> > > -static void xen_device_backend_set_online(XenDevice *xendev, bool online)
> > > +static void xen_device_backend_set_online(XenDevice *xendev, bool online,
> > > +                                          bool export)
> > >  {
> > >      const char *type = object_get_typename(OBJECT(xendev));
> > >
> > > @@ -513,7 +521,8 @@ static void xen_device_backend_set_online(XenDevice *xendev, bool online)
> > >      trace_xen_device_backend_online(type, xendev->name, online);
> > >
> > >      xendev->backend_online = online;
> > > -    xen_device_backend_printf(xendev, "online", "%u", online);
> > > +    if (export)
> > > +        xen_device_backend_printf(xendev, "online", "%u", online);
> > >  }
> > >
> >
> > Perhaps the behaviour of backend_set_state() and backend_set_online() could be the same? I.e. they
> both take an 'export' (or perhaps 'publish'?) parameter and only write xenstore if that is true. (I
> realise that would involve modifying xen-block to pass 'true' as the extra export/publish param, but I
> think it would be neater overall).
> 
> I've actually did it this way for backend_set_state() because the only
> reason to update internal states without writing that state into
> xenstore is because the xenstore state changed, so
> {front,back}end_changed() are the only function that don't want/need to
> write the new state into xenstore. I wanted to avoid misuse of the
> extra export/publish param in future backend drivers.
> 
> As for frontend_set_state() and backend_set_online(), they are only used
> in xen-bus.c, creating a new function didn't seems as needed.
> 
> I kind of think that maybe I should go further and also have
> frontend_record_state() is it could be possible to have frontend drivers
> in QEMU. (and maybe record_online so they all looks the same.)
> 

I guess I don't like the term 'record'... I'd really like to stick with 'set'...

> So, would you prefer to have the extra param to *_set_*() that should be
> "true" outside of *_changed(), or the extra functions like I did with
> backend_{set,record}_state() ?
> 

...so I prefer the extra param.

But, now I look at the code again without your patch applied I don't actually see the problem it is trying to fix. The functions xen_device_[back|front]end_set_state return early if the state being set matches the existing state and hence never get to the line where the state is written to xenstore.

  Paul

> Thanks,
> 
> --
> Anthony PERARD

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