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

[Igor Mammedov <imammedo@redhat.com>]

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

diff --git a/vl.c b/vl.c
index 7fd700e..6320bdc 100644
--- a/vl.c
+++ b/vl.c
@@ -1208,14 +1208,18 @@ static void smp_parse(QemuOpts *opts)
         unsigned cores   = qemu_opt_get_number(opts, "cores", 0);
         unsigned threads = qemu_opt_get_number(opts, "threads", 0);
 
+        max_cpus = qemu_opt_get_number(opts, "maxcpus", cpus);
         /* compute missing values, prefer sockets over cores over threads */
-        if (cpus == 0 || sockets == 0) {
+        if (cpus == 0) {
             sockets = sockets > 0 ? sockets : 1;
             cores = cores > 0 ? cores : 1;
             threads = threads > 0 ? threads : 1;
-            if (cpus == 0) {
-                cpus = cores * threads * sockets;
-            }
+            cpus = cores * threads * sockets;
+            max_cpus = max_cpus > 0 ? max_cpus : cpus;
+        } else if (sockets == 0) {
+            cores = cores > 0 ? cores : 1;
+            threads = threads > 0 ? threads : 1;
+            sockets = max_cpus / (cores * threads);
         } else if (cores == 0) {
             threads = threads > 0 ? threads : 1;
             cores = cpus / (sockets * threads);
@@ -1231,8 +1235,6 @@ static void smp_parse(QemuOpts *opts)
             exit(1);
         }
 
-        max_cpus = qemu_opt_get_number(opts, "maxcpus", cpus);
-
         if (max_cpus < cpus) {
             error_report("maxcpus must be equal to or greater than smp");
             exit(1);


> 
> 
> > +                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.

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