Re: [PATCH] sched: print information about scheduler granularity

[Jürgen Groß <jgross@xxxxxxxx>]

On 16.04.20 18:43, Dario Faggioli wrote:
> On Thu, 2020-04-16 at 09:33 +0100, Sergey Dyasli wrote:
> > Currently it might be not obvious which scheduling mode is being used
> > by the scheduler. Alleviate this by printing additional information
> > about the selected granularity.
> I like the idea. However, I don't like how verbose and long that line
> becomes.
>
> >   Messages now look like these:
> >
> > 1. boot
> > (XEN) [00089808f0ea7496] Using scheduler: SMP Credit Scheduler
> > (credit) in core-scheduling mode
> >
> > 2. xl debug-keys r
> > (XEN) [   45.914314] Scheduler: SMP Credit Scheduler (credit) in 2-
> > way core-scheduling mode
> What about adding an entry, just below these ones. Something looking
> like, for instance (both at boot and in the debug-key dump):
>
> "Scheduling granularity: cpu"
>
> (or "core", or "socket")
>
> Also
>
> > --- a/xen/common/sched/cpupool.c
> > +++ b/xen/common/sched/cpupool.c
> > @@ -38,7 +38,35 @@ static cpumask_t cpupool_locked_cpus;
> >   static DEFINE_SPINLOCK(cpupool_lock);
> >   
> >   static enum sched_gran __read_mostly opt_sched_granularity =
> > SCHED_GRAN_cpu;
> > -static unsigned int __read_mostly sched_granularity = 1;
> > +static unsigned int __read_mostly sched_granularity;
> > +
> > +char *sched_gran_str(char *str, size_t size)
> > +{
> > +    char *mode = "";
> > +
> > +    switch ( opt_sched_granularity )
> > +    {
> > +    case SCHED_GRAN_cpu:
> > +        mode = "cpu";
> > +        break;
> > +    case SCHED_GRAN_core:
> > +        mode = "core";
> > +        break;
> > +    case SCHED_GRAN_socket:
> > +        mode = "socket";
> > +        break;
> > +    default:
> > +        ASSERT_UNREACHABLE();
> > +        break;
> > +    }
> > +
> > +    if ( sched_granularity )
> > +        snprintf(str, size, "%u-way %s", sched_granularity, mode);
> I'm not sure about using the value of the enum like this.
enum? sched_granularity holds the number of cpus per scheduling
resource. opt_sched_granularity is the enum.

> E.g., in a system with 4 threads per core, enabling core scheduling
> granularity would mean having 4 vCPUs in the scheduling units. But this
> will still print "2-way core-scheduling", which I think would sound
> confusing.
It would print "4-way", of course.

> So I'd just go with "cpu", "core" and "socket" strings.
No, this is not a good idea. With e.g. smt=0 you'll be able to have
"1-way core" which is much more informative than "core".


Juergen