Re: [Xen-devel] [PATCH v5 20/19] docs: add "sched-gran" boot parameter documentation

[Jan Beulich <jbeulich@xxxxxxxx>]

On 30.09.2019 12:09, Juergen Gross wrote:
> Add documentation for the new "sched-gran" hypervisor boot parameter.
> 
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
>  docs/misc/xen-command-line.pandoc | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/docs/misc/xen-command-line.pandoc 
> b/docs/misc/xen-command-line.pandoc
> index fc64429064..c855246050 100644
> --- a/docs/misc/xen-command-line.pandoc
> +++ b/docs/misc/xen-command-line.pandoc
> @@ -1782,6 +1782,27 @@ Set the timeslice of the credit1 scheduler, in 
> milliseconds.  The
>  default is 30ms.  Reasonable values may include 10, 5, or even 1 for
>  very latency-sensitive workloads.
>  
> +### sched-gran (x86)
> +> `= cpu | core | socket`
> +
> +> Default: `sched-gran=cpu`
> +
> +Set the scheduling granularity. In case the granularity is larger than 1 
> (e.g.
> +`core`on a SMT-enabled system, or `socket`) multiple vcpus are assigned
> +statically to a "scheduling unit" which will then be subject to scheduling.
> +This assignment of vcpus to scheduling units is fixed.
> +
> +`cpu`: Vcpus will be scheduled individually on single cpus.
> +
> +`core`: As many vcpus as there are hyperthreads on a physical core are
> +scheduled together on a physical core.
> +
> +`socket`: As many vcpus as there are hyperthreads on a physical sockets are
> +scheduled together on a physical socket.

I'd prefer if this didn't end up Intel-centric; ideally it also wouldn't be
x86-specific. AMD has introduced hyperthreading in Fam17 only; Fam15 used
"compute units", grouping together "cores". Internally the Intel side
"core vs hyperthread" is represented in the same variables (cpu_sibling_mask
in particular) as the AMD side "compute unit vs core".

Therefore it may be better to talk here about e.g. "smallest topological
sub-unit" and only say "e.g. a hyperthread to make a connection to common
x86 / Intel terminology". Of course the AMD side alternative use of the
variables also renders the actual command line option "sched-gran=core"
not overly fortunate. Perhaps we'd want to also use more abstract terms
here, e.g. topological "levels"?

> +Note: a value other than `cpu` will result in rejecting a runtime 
> modification
> +of the "smt" setting.

Perhaps add "attempt" here?

Jan

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