Re: [Xen-devel] [PATCH v4 15/15] docs: add MBA description in docs

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Sat, Sep 23, 2017 at 09:48:24AM +0000, Yi Sun wrote:
> This patch adds MBA description in related documents.
> 
> Signed-off-by: Yi Sun <yi.y.sun@xxxxxxxxxxxxxxx>
> Acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>

There's still a 'closed-loop' mention that I think I've already
pointed out in the last review, but anyway:

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

> ---
> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> CC: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> CC: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> 
> v4:
>     - modify description of MBA in 'xl.pod.1.in' to be same as feature doc.
>       (suggested by Roger Pau Monné)
>     - fix words issue.
>       (suggested by Roger Pau Monné)
> v2:
>     - state the value type shown by 'psr-mba-show'. For linear mode,
>       it shows decimal value. For non-linear mode, it shows hexadecimal
>       value.
>       (suggested by Chao Peng)
> ---
>  docs/man/xl.pod.1.in      | 33 +++++++++++++++++++++++++
>  docs/misc/xl-psr.markdown | 63 
> +++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 96 insertions(+)
> 
> diff --git a/docs/man/xl.pod.1.in b/docs/man/xl.pod.1.in
> index 3d5f2f7..64d87ba 100644
> --- a/docs/man/xl.pod.1.in
> +++ b/docs/man/xl.pod.1.in
> @@ -1803,6 +1803,39 @@ processed.
>  
>  =back
>  
> +=head2 Memory Bandwidth Allocation
> +
> +Intel Skylake and later server platforms offer capabilities to configure and
> +make use of the Memory Bandwidth Allocation (MBA) mechanisms, which provides
> +OS/VMMs the ability to slow misbehaving apps/VMs by using a credit-based
> +throttling mechanism. In the Xen implementation, MBA is used to control 
> memory
> +bandwidth on VM basis. To enforce bandwidth on a specific domain, just set
> +throttling value (THRTL) for the domain.
> +
> +=over 4
> +
> +=item B<psr-mba-set> [I<OPTIONS>] I<domain-id> I<thrtl>
> +
> +Set throttling value (THRTL) for a domain. For how to specify I<thrtl>
> +please refer to L<http://xenbits.xen.org/docs/unstable/misc/xl-psr.html>.
> +
> +B<OPTIONS>
> +
> +=over 4
> +
> +=item B<-s SOCKET>, B<--socket=SOCKET>
> +
> +Specify the socket to process, otherwise all sockets are processed.
> +
> +=back
> +
> +=item B<psr-mba-show> [I<domain-id>]
> +
> +Show MBA settings for a certain domain or all domains. For linear mode, it
> +shows the decimal value. For non-linear mode, it shows hexadecimal value.
> +
> +=back
> +
>  =head1 IGNORED FOR COMPATIBILITY WITH XM
>  
>  xl is mostly command-line compatible with the old xm utility used with
> diff --git a/docs/misc/xl-psr.markdown b/docs/misc/xl-psr.markdown
> index 04dd957..08c06dc 100644
> --- a/docs/misc/xl-psr.markdown
> +++ b/docs/misc/xl-psr.markdown
> @@ -186,6 +186,69 @@ Setting data CBM for a domain:
>  Setting the same code and data CBM for a domain:
>  `xl psr-cat-set <domid> <cbm>`
>  
> +## Memory Bandwidth Allocation (MBA)
> +
> +Memory Bandwidth Allocation (MBA) is a new feature available on Intel
> +Skylake and later server platforms that allows an OS or Hypervisor/VMM to
> +slow misbehaving apps/VMs or create advanced closed-loop control system via
> +exposing control over a credit-based throttling mechanism. To enforce 
> bandwidth
> +on a specific domain, just set throttling value (THRTL) into Class of Service
> +(COS). MBA provides two THRTL mode. One is linear mode and the other is
> +non-linear mode.
> +
> +In the linear mode the input precision is defined as 100-(THRTL_MAX). Values
> +not an even multiple of the precision (e.g., 12%) will be rounded down (e.g.,
> +to 10% delay by the hardware).
> +
> +If linear values are not supported then input delay values are powers-of-two
> +from zero to the THRTL_MAX value from CPUID. In this case any values not a 
> power
> +of two will be rounded down the next nearest power of two.
> +
> +For example, assuming a system with 2 domains:
> +
> + * A THRTL of 0x0 for every domain means each domain can access the whole 
> cache
> +   without any delay. This is the default.
> +
> + * Linear mode: Giving one domain a THRTL of 0xC and the other domain's 0 
> means
> +   that the first domain gets 10% delay to access the cache and the other one
> +   without any delay.
> +
> + * Non-linear mode: Giving one domain a THRTL of 0xC and the other domain's 0
> +   means that the first domain gets 8% delay to access the cache and the 
> other
> +   one without any delay.
> +
> +For more detailed information please refer to Intel SDM chapter
> +"Introduction to Memory Bandwidth Allocation".
> +
> +In Xen's implementation, THRTL can be configured with libxl/xl interfaces but
> +COS is maintained in hypervisor only. The cache partition granularity is per
> +domain, each domain has COS=0 assigned by default, the corresponding THRTL is
> +0, which means all the cache resource can be accessed without delay.
> +
> +### xl interfaces
> +
> +System MBA information such as maximum COS and maximum THRTL can be obtained 
> by:
> +
> +`xl psr-hwinfo --mba`
> +
> +The simplest way to change a domain's THRTL from its default is running:
> +
> +`xl psr-mba-set  [OPTIONS] <domid> <thrtl>`
> +
> +In a multi-socket system, the same thrtl will be set on each socket by 
> default.
> +Per socket thrtl can be specified with the `--socket SOCKET` option.
> +
> +Setting the THRTL may not be successful if insufficient COS is available. In
                                             ^ not enough COS are available.
> +such case unused COS(es) may be freed by setting THRTL of all related 
> domains to
> +its default value(0).
> +
> +Per domain THRTL settings can be shown by:
> +
> +`xl psr-mba-show [OPTIONS] <domid>`
> +
> +For linear mode, it shows the decimal value. For non-linear mode, it shows
> +hexadecimal value.
> +
>  ## Reference
>  
>  [1] Intel SDM
> -- 
> 1.9.1
> 

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