Re: [PATCH v1 06/20] x86/resctrl: Switch over to the resctrl mbps_val list

[Reinette Chatre <reinette.chatre@intel.com>]

Hi James,

On 7/29/2021 3:35 PM, James Morse wrote:
> Updates to resctrl's software controller follow the same path as
> other configuration updates, but they don't modify the hardware state.
> rdtgroup_schemata_write() uses parse_line() and the resource's
> ctrlval_parse function to stage the configuration.
> resctrl_arch_update_domains() then updates the mbps_val[] array
> instead, and resctrl_arch_update_domains() skips the rdt_ctrl_update()
> call that would update hardware.
> 
> This complicates the interface between resctrl's filesystem parts
> and architecture specific code. It should be possible for mba_sc
> to be completely implemented by the filesystem parts of resctrl. This
> would allow it to work on a second architecture with no additional code.
> 
> Change parse_bw() to write the configuration value directly to the
> mba_sc[] array in the domain structure. Change rdtgroup_schemata_write()
> to skip the call to resctrl_arch_update_domains(), meaning all the
> mba_sc specific code in resctrl_arch_update_domains() can be removed.
> On the read-side, show_doms() and update_mba_bw() are changed to read
> the mba_sc[] array from the domain structure. With this,
> resctrl_arch_get_config() no longer needs to consider mba_sc resources.
> 
> Change parse_bw() to write these values directly, meaning
> rdtgroup_schemata_write() never needs to call update_domains()
> for mba_sc resources.

The above paragraph seems to contain duplicate information from the 
paragraph that precedes it.

> 
> Get show_doms() to test is_mba_sc() and retrieve the value
> directly, instead of using get_config() for the hardware value.
> 
> This means the arch code's resctrl_arch_get_config() and
> resctrl_arch_update_domains() no longer need to be aware of
> mba_sc, and we can get rid of the update_mba_bw() code that
> reaches into the hw_dom to get the msr value.
> 
> Signed-off-by: James Morse <james.morse@arm.com>
> ---

...
> @@ -406,6 +406,14 @@ ssize_t rdtgroup_schemata_write(struct kernfs_open_file *of,
>   
>   	list_for_each_entry(s, &resctrl_schema_all, list) {
>   		r = s->res;
> +
> +		/*
> +		 * Writes to mba_sc resources update the software controller,
> +		 * not the control msr.
> +		 */
> +		if (is_mba_sc(r))
> +			continue;
> +

A few resources can be updated in a single write to the schemata file. 
It is thus possible to update the cache allocation resource as well as 
memory bandwidth allocation in a single write. As I understand this 
change in this scenario all configuration updates will be skipped, not 
just the memory bandwidth allocation ones.

>   		ret = resctrl_arch_update_domains(r, rdtgrp->closid);
>   		if (ret)
>   			goto out;

Reinette