Re: [Qemu-devel] [PATCH] vl: rework smp_parse

[Andrew Jones <drjones@redhat.com>]

On Fri, Nov 07, 2014 at 10:16:06AM -0200, Eduardo Habkost wrote:
> On Fri, Nov 07, 2014 at 12:21:26PM +0100, Andrew Jones wrote:
> > On Fri, Nov 07, 2014 at 10:52:31AM +0100, Andrew Jones wrote:
> > > On Fri, Nov 07, 2014 at 10:40:14AM +0100, Paolo Bonzini wrote:
> > > > 
> > > > 
> > > > On 07/11/2014 10:29, Andrew Jones wrote:
> > > > >> > I think this would cause too many failures in the wild.  Perhaps error
> > > > >> > out if it is lower, and warn if sockets * cores * threads > max_cpus
> > > > >> > since we actually allow hot-plug a thread at a time?
> > > > > We'd still have more failures if we choose to error out when it's lower,
> > > > > since we currently silently adjust threads in some of those cases, or
> > > > > just don't care that the topology doesn't support up to maxcpus in other.
> > > > 
> > > > So I guess we need a decent fallback if it doesn't match.  Something
> > > > like (based also on the reply from Eduardo):
> > > > 
> > > > 1) always warn if max_cpus % (cores*threads) != 0 || smp_cpus %
> > > > (cores*threads) != 0
> > > > 
> > > > 2) if sockets*cpus*threads < max_cpus, adjust sockets to
> > > > DIV_ROUND_UP(max_cpus, cores*threads).  If we didn't warn in step 1, do
> > > > it now.  Give a different, less harsh warning if the cmdline
> > > > sockets*cpus*threads did match smp_cpus.  In the latter case, the user
> > > > _almost_ knows what he was doing.
> > > > 
> > > > Not perfect, but it could be something to start from.  Adjusting sockets
> > > > is better than adjusting threads.
> > > 
> > > OK. I can whip up a v2 that is less harsh (more warnings, less aborts).
> > > I'll also address the other issue I mentioned in the bottom of my reply
> > > to Eduardo, which is to make sure we consider machine_class->max_cpus.
> > 
> > After talking with Igor, it seems like the better approach is to get
> > smp parameters converted over to machine properties, allowing us to
> > use the old parser for old machine types, and the new for new. I'll
> > look into it.
> 
> While we work on that, I think we could at least change the existing
> code to abort if sockets*cores*threads < smp_cpus (when all 4 options
> are present in the command-line), and never adjust any option silently.

But then we'll end up with 3 different behaviors. If we do anything for
the short-term, then maybe it should only be to add warnings. The
machine property version will then convert those warnings to aborts for
new machines types.

drew