linux-kernel - Re: [PATCH] x86/resctrl: mba_MBps: Fall back to total b/w if local b/w unavailable

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Date:   Wed, 25 Oct 2023 11:01:22 -0500
From:   "Moger, Babu" <babu.moger@....com>
To:     "Luck, Tony" <tony.luck@...el.com>, "Moger, Babu" <bmoger@....com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        Peter Newman <peternewman@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Shuah Khan <skhan@...uxfoundation.org>,
        "x86@...nel.org" <x86@...nel.org>
Cc:     Shaopeng Tan <tan.shaopeng@...itsu.com>,
        James Morse <james.morse@....com>,
        Jamie Iles <quic_jiles@...cinc.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "patches@...ts.linux.dev" <patches@...ts.linux.dev>
Subject: Re: [PATCH] x86/resctrl: mba_MBps: Fall back to total b/w if local
 b/w unavailable

Hi Tony,

On 10/24/23 18:43, Luck, Tony wrote:
>> Is this customer requirement ?
> 
> Any customer using the mba_MBps feedback mount option will need this
> on platforms that don't support local bandwidth measurement.
> 
>> What do you mean by " If local bandwidth measurement is not available" ?
>> Is the hardware supports only total bandwidth and not local?
> 
> There's going to be an Intel CPU that will only provide "total" bandwidth.

ok.

Why dont you use get_mbm_state which is already available instead of
writing another function(get_mbm_data).

You can pass evtid, rmid, domain information. Decide the evtid based on
what is available. I think that will make code simpler.

> 
> The CPUID enumeration in (CPUID.(EAX=0FH, ECX=1H) ).EDX{2}
> will be "0" indicating that the local mbm monitor event is not supported.
> 
>> It can get real ugly if we try to handle one special case.
> 
> Hard to predict the future (I didn't see this coming, or I'd have had Vikas
> implement the fallback in the original mba_MBps code). But I don't believe
> this will be a one-off special case.
> 
> I'm also wondering why this feedback loop picked "local" rather than "total".
> I dug into the e-mail archives, and I don't see any discussion. There's just
> an RFC series, and then the v2 series was applied with a few small suggestions
> from Thomas to make things cleaner..

May be MSR write which feedback loop does only has local effect. This will
be interesting to know.
-- 
Thanks
Babu Moger