linux-kernel - Re: [PATCH v8 3/7] x86/resctrl: Refactor mbm

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Message-ID: <ZyVcy-7BkqO6Vcr6@agluck-desk3>
Date: Fri, 1 Nov 2024 15:57:15 -0700
From: Tony Luck <tony.luck@...el.com>
To: Fenghua Yu <fenghua.yu@...el.com>
Cc: Reinette Chatre <reinette.chatre@...el.com>,
	Peter Newman <peternewman@...gle.com>,
	Jonathan Corbet <corbet@....net>,
	Shuah Khan <skhan@...uxfoundation.org>, x86@...nel.org,
	James Morse <james.morse@....com>,
	Jamie Iles <quic_jiles@...cinc.com>,
	Babu Moger <babu.moger@....com>,
	Randy Dunlap <rdunlap@...radead.org>,
	"Shaopeng Tan (Fujitsu)" <tan.shaopeng@...itsu.com>,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	patches@...ts.linux.dev
Subject: Re: [PATCH v8 3/7] x86/resctrl: Refactor mbm_update()

On Fri, Nov 01, 2024 at 03:08:27PM -0700, Fenghua Yu wrote:
> > -		/*
> > -		 * Call the MBA software controller only for the
> > -		 * control groups and when user has enabled
> > -		 * the software controller explicitly.
> > -		 */
> 
> Same comment was added in patch 2 but is removed in the helper. Maybe it's
> better to add back the comment in the helper.
> 
> > -		if (is_mba_sc(NULL))
> > -			mbm_bw_count(closid, rmid, &rr);

In patch 2 I cut & pasted the comment along with the code from
the if (is_mbm_local()) {...}  clause into the if (is_mbm_total()) { ... }
clause. Maybe to make it more obvious that the code was duplicated.

But I had doubts about the usefulness/accuracy when making the
helper function.

Breaking the comment into pieces:

    "Call the MBA software controller"

This code isn't calling the "controller". To me that's the
update_mba_bw() function that adjusts the MBA values. The
call to mbm_bw_count() is simply computing the bandwidth.

    "only for the control groups"

while this is true, the test in the code here "if (is_mba_sc(NULL))"
isn't making any checks about control groups.

    "and when user has enabled the software controller"

Accurate.

    "explicitly"

Redundant (or confusing, there is no "implicit" way to enable
the s/w controller).

If a comment is needed (and I'm not convinced that it is)
maybe something like:

	/*
	 * If the software controller controller is enabled, compute the
	 * bandwidth for this event id.
	 */

-Tony