linux-kernel - Re: [PATCH v5 06/40] x86/resctrl: Remove data

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Message-ID: <3649810f-5f52-4af8-ab8c-503ddb594691@arm.com>
Date: Fri, 18 Oct 2024 18:07:14 +0100
From: James Morse <james.morse@....com>
To: Tony Luck <tony.luck@...el.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
 Fenghua Yu <fenghua.yu@...el.com>,
 Reinette Chatre <reinette.chatre@...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,
 D Scott Phillips OS <scott@...amperecomputing.com>,
 carl@...amperecomputing.com, lcherian@...vell.com,
 bobo.shaobowang@...wei.com, tan.shaopeng@...itsu.com,
 baolin.wang@...ux.alibaba.com, Jamie Iles <quic_jiles@...cinc.com>,
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 David Hildenbrand <david@...hat.com>, Rex Nie <rex.nie@...uarmicro.com>,
 Dave Martin <dave.martin@....com>, Shaopeng Tan <tan.shaopeng@...fujitsu.com>
Subject: Re: [PATCH v5 06/40] x86/resctrl: Remove data_width and the tabular
 format

Hi Tony,

On 16/10/2024 00:29, Tony Luck wrote:
> On Fri, Oct 04, 2024 at 06:03:13PM +0000, James Morse wrote:
>> The resctrl architecture code provides a data_width for the controls of
>> each resource. This is used to zero pad all control values in the schemata
>> file so they appear in columns. The same is done with the resource names
>> to complete the visual effect. e.g.
>> | SMBA:0=2048
>> |   L3:0=00ff
>>
>> AMD platforms discover their maximum bandwidth for the MB resource from
>> firmware, but hard-code the data_width to 4. If the maximum bandwidth
>> requires more digits - the tabular format is silently broken.
>> If new schema are added resctrl will need to be able to determine the
>> maximum width. The benefit of this pretty-printing is questionable.
> 
> Agreed. It's particularly non-useful for L2 resources on systems with
> hundred+ cores. The L2 line in schemata is very long and doesn't look
> "pretty" at all. Padding may make it even longer.


> It never worked with the mba_MBps mount option because the field
> width wasn't updated for prettiness. E.g.
> 
> $ cat schemata
> MB:0=4294967295;1=4294967295
> L3:0=fff;1=fff

Good point - I'll add that the commit message.


>> Instead of handling runtime discovery of the data_width for AMD platforms,
>> remove the feature. These fields are always zero padded so should be
>> harmless to remove if the whole field has been treated as a number.
>> In the above example, this would now look like this:
> 
> Huzzah!
> 
> Reviewed-by: Tony Luck <tony.luck@...el.com>

Thanks!

James