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

[Dario Faggioli <dfaggioli@xxxxxxxx>]

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.

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.

So I'd just go with "cpu", "core" and "socket" strings.

Regards
-- 
Dario Faggioli, Ph.D
http://about.me/dario.faggioli
Virtualization Software Engineer
SUSE Labs, SUSE https://www.suse.com/
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