Re: [Qemu-devel] [PATCH] vl:c: make sure that sockets are calculated correctly in '-smp X' case

[Eduardo Habkost <ehabkost@redhat.com>]

On Thu, Aug 30, 2018 at 09:58:51AM +0200, Igor Mammedov wrote:
> On Wed, 29 Aug 2018 14:33:01 -0300
> Eduardo Habkost <ehabkost@redhat.com> wrote:
> 
> > On Wed, Aug 29, 2018 at 04:32:01PM +0200, Igor Mammedov wrote:
> > > commit
> > >   (5cdc9b76e3 vl.c: Remove dead assignment)
> > > removed sockets calculation when 'sockets' weren't provided on CLI
> > > since there wasn't any users for it back then. Exiting checks
> > > are neither reachable
> > >    } else if (sockets * cores * threads < cpus) {
> > > or nor triggable
> > >    if (sockets * cores * threads > max_cpus)
> > > so we weren't noticing wrong topology since then, since users
> > > recalculate sockets adhoc on their own.
> > > 
> > > However with deprecation check it becomes noticable, for example
> > >   -smp 2
> > > will start printing warning:
> > >   "warning: Invalid CPU topology deprecated: sockets (1) * cores (1) * threads (1) != maxcpus (2)"
> > > calculating sockets if they weren't specified.
> > > 
> > > Fix it by returning back sockets calculation if it's omited on CLI.
> > > 
> > > Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> > > ---
> > >  vl.c | 5 ++++-
> > >  1 file changed, 4 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/vl.c b/vl.c
> > > index 7fd700e..333d638 100644
> > > --- a/vl.c
> > > +++ b/vl.c
> > > @@ -1210,11 +1210,14 @@ static void smp_parse(QemuOpts *opts)
> > >  
> > >          /* compute missing values, prefer sockets over cores over threads */
> > >          if (cpus == 0 || sockets == 0) {
> > > -            sockets = sockets > 0 ? sockets : 1;
> > >              cores = cores > 0 ? cores : 1;
> > >              threads = threads > 0 ? threads : 1;
> > >              if (cpus == 0) {
> > > +                sockets = sockets > 0 ? sockets : 1;
> > >                  cpus = cores * threads * sockets;
> > > +            } else {
> > > +                max_cpus = qemu_opt_get_number(opts, "maxcpus", cpus);  
> > 
> > We already have a "max_cpus = qemu_opt_get_number(...)" line in
> > this function[1]:
> > 
> >         /* compute missing values, prefer sockets over cores over threads */
> >         if (cpus == 0 || sockets == 0) {
> >             sockets = sockets > 0 ? sockets : 1;
> >             cores = cores > 0 ? cores : 1;
> >             threads = threads > 0 ? threads : 1;
> >             if (cpus == 0) {
> >                 cpus = cores * threads * sockets;
> >             }
> >         } else if (cores == 0) {
> >             [...]
> >         } else if (threads == 0) {
> >             [...]
> >         } else if (sockets * cores * threads < cpus) {
> >             [...]
> >         }
> > 
> >         max_cpus = qemu_opt_get_number(opts, "maxcpus", cpus); /* [1] */
> > 
> >         /* [2] */
> > 
> >         if (max_cpus < cpus) {
> >             error_report("maxcpus must be equal to or greater than smp");
> >             exit(1);
> >         }
> > 
> > Why not move the sockets == 0 check to [2]?
> it won't work, since the rest 'else if' depends on sockets being non 0
> which is ensured by (cpus == 0 || sockets == 0) branch, I can split this in 2
> equivalent conditions. It's more code but it might be easier to read

I like your original patch better.  I wasn't thinking about the
"cpus == 0 || sockets == 0" check, but about this line:

   sockets = !sockets ? max_cpus / (cores * threads) : sockets;

Just moving this line to [2] would be enough, wouldn't it?

> 
[...]
> > 
> > 
> > > +                sockets = !sockets ? max_cpus / (cores * threads) : sockets;  
> > 
> > The two patches in this thread make QEMU print a warning on a
> > case that was never documented as invalid/deprecated: maxcpus now
> > needs to be a multiple of cores*threads.
> > 
> > However, the error message doesn't make it obvious:
> > 
> >   $ ./x86_64-softmmu/qemu-system-x86_64 -smp 7,cores=3
> >   qemu-system-x86_64: warning: Invalid CPU topology deprecated: sockets (2) * cores (3) * threads (1) != maxcpus (7)
> to me it look pretty obvious, fix one side of equation to make sure that VM
> starts in the future.
> 
> > I think this would make more sense:
> > 
> >   $ ./x86_64-softmmu/qemu-system-x86_64 -smp 7,cores=3
> >   qemu-system-x86_64: warning: Invalid CPU topology deprecated: maxcpus (7) is not a multiple of cores (3) * threads (1)
> I think it will add not necessary warning/code, but if you really
> think we need it, I can add it to 'vl.c deprecate incorrect CPUs
> topology' on respin.

My main worry is about insufficient documentation: we need to
clarify that the s*c*t == maxcpus rule applies even if "sockets"
is omitted in the command-line, and that this indirectly requires
that maxcpus be a multiple of c*6.

That said, I won't mind if you keep the existing message in this
patch for code simplicity, as long as the documentation is
updated.

> 
> > 
> > 
> > >              }
> > >          } else if (cores == 0) {
> > >              threads = threads > 0 ? threads : 1;
> > > -- 
> > > 2.7.4
> > >   
> > 
> 

-- 
Eduardo