Re: [PATCH v1 2/2] migration/multifd: Warn user when zerocopy not working

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Jun 28, 2022 at 09:32:04AM -0300, Leonardo Bras Soares Passos wrote:
> On Tue, Jun 28, 2022 at 4:53 AM Daniel P. Berrangé <berrange@redhat.com> wrote:
> >
> > On Mon, Jun 27, 2022 at 10:09:09PM -0300, Leonardo Bras wrote:
> > > Some errors, like the lack of Scatter-Gather support by the network
> > > interface(NETIF_F_SG) may cause sendmsg(...,MSG_ZEROCOPY) to fail on using
> > > zero-copy, which causes it to fall back to the default copying mechanism.
> >
> > How common is this lack of SG support ? What NICs did you have that
> > were affected ?
> 
> I am not aware of any NIC without SG available for testing, nor have
> any idea on how common they are.
> But since we can detect sendmsg() falling back to copying we should
> warn the user if this ever happens.
> 
> There is also a case in IPv6 related to fragmentation that may cause
> MSG_ZEROCOPY to fall back to the copying mechanism, so it's also
> covered.
> 
> >
> > > After each full dirty-bitmap scan there should be a zero-copy flush
> > > happening, which checks for errors each of the previous calls to
> > > sendmsg(...,MSG_ZEROCOPY). If all of them failed to use zero-copy, then
> > > warn the user about it.
> > >
> > > Since it happens once each full dirty-bitmap scan, even in worst case
> > > scenario it should not print a lot of warnings, and will allow tracking
> > > how many dirty-bitmap iterations were not able to use zero-copy send.
> >
> > For long running migrations which are not converging, or converging
> > very slowly there could be 100's of passes.
> >
> 
> I could change it so it only warns once, if that is too much output.

Well I'm mostly wondering what we're expecting the user todo with this
information. Generally a log file containing warnings ends up turning
into a bug report. If we think it is important for users and/or mgmt
apps to be aware of this info, then it might be better to actually
put a field in the query-migrate stats to report if zero-copy is
being honoured or not, and just have a trace point in this location
instead.

With regards,
Daniel
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