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

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Message-ID: <Zw76wLvdGax2eSiP@agluck-desk3.sc.intel.com>
Date: Tue, 15 Oct 2024 16:29:04 -0700
From: Tony Luck <tony.luck@...el.com>
To: James Morse <james.morse@....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>,
	Xin Hao <xhao@...ux.alibaba.com>, peternewman@...gle.com,
	dfustini@...libre.com, amitsinght@...vell.com,
	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

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

> 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>

-Tony