Re: [Xen-devel] [PATCH 2/4] xen: Add 'synthetic' preemption check parameter

[Jan Beulich <jbeulich@xxxxxxxx>]

On 23.12.2019 17:43, George Dunlap wrote:
> --- a/docs/misc/xen-command-line.pandoc
> +++ b/docs/misc/xen-command-line.pandoc
> @@ -636,13 +636,29 @@ Available alternatives, with their meaning, are:
>  Specify the USB controller to use, either by instance number (when going
>  over the PCI busses sequentially) or by PCI device (must be on segment 0).
>  
> -### debug_stack_lines
> +### debug\_stack\_lines
>  > `= <integer>`
>  
>  > Default: `20`
>  
>  Limits the number lines printed in Xen stack traces.
>  
> +### debug-synthetic-preemption
> +> `= <integer>`
> +
> +> Default: `0`
> +
> +Artificially increases rate at which `hypercall_preempt_check()`
> +returns `true`, for debugging purposes, to a rate of one in `N`. (The
> +default, `0`, disables the feature.)
> +
> +When promoting pagetables, for instance, `hypercall_preempt_check()`
> +is called before processing each PTE.  Since there are 512 PTEs per
> +page, a value of `1024` should result in pagetable promotion being
> +interrupted every other page on average.

If this is meant to be x86 only, then text here should state this.
Otherwise I think it would help if the example described in the
last sentence would mention its x86 relationship.

> --- a/xen/arch/x86/time.c
> +++ b/xen/arch/x86/time.c
> @@ -43,6 +43,17 @@
>  static char __initdata opt_clocksource[10];
>  string_param("clocksource", opt_clocksource);
>  
> +#ifndef NDEBUG
> +int debug_synthetic_preemption = 0;

static unsigned int __read_mostly?

> +integer_param("debug-synthetic-preemption", debug_synthetic_preemption);

Perhaps allow changing at runtime?

> +bool synthetic_preemption_check(void) {

Brace placement.

> +    if ( debug_synthetic_preemption == 0 )
> +        return false;
> +    return !(rdtsc() % debug_synthetic_preemption);

Please consistently use either ! or "== 0".

Jan

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