Date:   Fri, 17 Sep 2021 17:57:54 +0100
From:   James Morse <james.morse@....com>
To:     Reinette Chatre <reinette.chatre@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Babu Moger <Babu.Moger@....com>,
        shameerali.kolothum.thodi@...wei.com,
        Jamie Iles <jamie@...iainc.com>,
        D Scott Phillips OS <scott@...amperecomputing.com>,
        lcherian@...vell.com, bobo.shaobowang@...wei.com
Subject: Re: [PATCH v1 06/20] x86/resctrl: Switch over to the resctrl mbps_val
 list

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

> 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.


Thanks,

James

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