On Thu, Aug 30, 2018 at 09:58:51AM +0200, Igor Mammedov wrote:
> On Wed, 29 Aug 2018 14:33:01 -0300
> Eduardo Habkost <ehabk...@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 <imamm...@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
