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

[Reinette Chatre]

Hi James,

On 9/17/2021 9:57 AM, James Morse wrote:
> Hi Reinette,
>
> On 01/09/2021 22:25, Reinette Chatre wrote:
> > 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.
>
> Looks like two commit messages got combined. I've removed this, and the below paragraphs
> as its already covered.
>
>
> > > 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.
>
> > > @@ -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.
>
> i.e. echo "L3:0=7ff;1=7ff\nMB:0=100;1=50" > schemata

I do not think something like the above would show the issue. If you 
want to test this via the shell you need to use ANSI-C quoting. 
Adjusting what you show to something like:

echo -n $'L3:0=7ff;1=7ff\nMB:0=100;1=50\n'

> > As I understand this change in this scenario all configuration updates will be
> > skipped, not just the memory bandwidth allocation ones.
>
> The loop is per-schema, so its not a problem for L2/L3. This would only be a problem if
> the is_mba_sc() resource had multiple schema. Only CDP does this, which the MBA controls
> don't support.

The loop iterates through the entire buffer provided to the schemata 
file and the buffer could contain multiple schema. This is more typical 
when interacting with the schemata file with a SDK perhaps.

Reinette