Re: [Xen-devel] [PATCHv1] xen: increase default number of PIRQs for hardware domains

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 03.12.14 at 17:04, <david.vrabel@xxxxxxxxxx> wrote:
> The default limit for the number of PIRQs for hardware domains (dom0)
> is not sufficient for some (x86) systems.
> 
> Since the pirq structures are individually and dynamically allocated,
> the limit for hardware domains may be increased to the number of
> possible IRQs.

I nevertheless disagree to moving the bound up to the Xen internal
limit unconditionally: What use does it have to allow hwdom to use
thousands of MSIs? If a system got that many, the main purpose of
running Xen on it I would expect to be to hand various of the
respective devices to guests. Hence no need for hwdom to have
that many by default, even if this doesn't result in any extra
resource consumption.

That said, I can see the current default of 256 being too low though.
Quite likely in the absence of a user specified value the default
ought to be derived from nr_irqs - nr_static_irqs rather than being
any fixed number. Considering the default used for nr_irqs, I'd think
along the lines of sqrt(num_present_cpus()) * NR_DYNAMIC_VECTORS
or dom0->max_vcpus * NR_DYNAMIC_VECTORS (or the minimum of
the two) for x86.

Jan

> The extra_guest_irqs command line option now only allows changes to
> the domU value.  Any argument for dom0 is ignored.
> 
> Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
> ---
>  docs/misc/xen-command-line.markdown |   11 ++++-------
>  xen/common/domain.c                 |    7 +------
>  2 files changed, 5 insertions(+), 13 deletions(-)
> 
> diff --git a/docs/misc/xen-command-line.markdown 
> b/docs/misc/xen-command-line.markdown
> index 0866df2..d352031 100644
> --- a/docs/misc/xen-command-line.markdown
> +++ b/docs/misc/xen-command-line.markdown
> @@ -594,15 +594,12 @@ except for debugging purposes.
>  Force or disable use of EFI runtime services.
>  
>  ### extra\_guest\_irqs
> -> `= [<domU number>][,<dom0 number>]`
> +> `= [<number>]`
>  
> -> Default: `32,256`
> +> Default: `32`
>  
> -Change the number of PIRQs available for guests.  The optional first number 
> is
> -common for all domUs, while the optional second number (preceded by a comma)
> -is for dom0.  Changing the setting for domU has no impact on dom0 and vice
> -versa.  For example to change dom0 without changing domU, use
> -`extra_guest_irqs=,512`
> +Change the number of PIRQs available for guests. This limit does not
> +apply to hardware domains (dom0).
>  
>  ### flask\_enabled
>  > `= <integer>`
> diff --git a/xen/common/domain.c b/xen/common/domain.c
> index 4a62c1d..a88d829 100644
> --- a/xen/common/domain.c
> +++ b/xen/common/domain.c
> @@ -231,14 +231,11 @@ static int late_hwdom_init(struct domain *d)
>  #endif
>  }
>  
> -static unsigned int __read_mostly extra_dom0_irqs = 256;
>  static unsigned int __read_mostly extra_domU_irqs = 32;
>  static void __init parse_extra_guest_irqs(const char *s)
>  {
>      if ( isdigit(*s) )
>          extra_domU_irqs = simple_strtoul(s, &s, 0);
> -    if ( *s == ',' && isdigit(*++s) )
> -        extra_dom0_irqs = simple_strtoul(s, &s, 0);
>  }
>  custom_param("extra_guest_irqs", parse_extra_guest_irqs);
>  
> @@ -324,10 +321,8 @@ struct domain *domain_create(
>          atomic_inc(&d->pause_count);
>  
>          if ( !is_hardware_domain(d) )
> -            d->nr_pirqs = nr_static_irqs + extra_domU_irqs;
> +            d->nr_pirqs = min(nr_static_irqs + extra_domU_irqs, nr_irqs);
>          else
> -            d->nr_pirqs = nr_static_irqs + extra_dom0_irqs;
> -        if ( d->nr_pirqs > nr_irqs )
>              d->nr_pirqs = nr_irqs;
>  
>          radix_tree_init(&d->pirq_tree);
> -- 
> 1.7.10.4




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