linux-kernel - Re: [PATCH v1 06/20] x86/resctrl: Switch over to the resctrl mbps

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Message-ID: <b211c9c0-5f4c-75b2-2966-2bc1b848ab31@intel.com>
Date:   Fri, 17 Sep 2021 11:20:18 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     James Morse <james.morse@....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 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