linux-kernel - Re: [RFC PATCH 04/19] fs/resctrl: Add the documentation for Global Memory Bandwidth Allocation

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <e0c79c53-489d-47bf-89b9-f1bb709316c6@amd.com>
Date: Tue, 3 Feb 2026 10:38:35 -0600
From: Babu Moger <bmoger@....com>
To: "Luck, Tony" <tony.luck@...el.com>, Babu Moger <babu.moger@....com>
Cc: corbet@....net, reinette.chatre@...el.com, Dave.Martin@....com,
 james.morse@....com, tglx@...nel.org, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
 peterz@...radead.org, juri.lelli@...hat.com, vincent.guittot@...aro.org,
 dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
 mgorman@...e.de, vschneid@...hat.com, akpm@...ux-foundation.org,
 pawan.kumar.gupta@...ux.intel.com, pmladek@...e.com,
 feng.tang@...ux.alibaba.com, kees@...nel.org, arnd@...db.de,
 fvdl@...gle.com, lirongqing@...du.com, bhelgaas@...gle.com,
 seanjc@...gle.com, xin@...or.com, manali.shukla@....com,
 dapeng1.mi@...ux.intel.com, chang.seok.bae@...el.com,
 mario.limonciello@....com, naveen@...nel.org, elena.reshetova@...el.com,
 thomas.lendacky@....com, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, kvm@...r.kernel.org, peternewman@...gle.com,
 eranian@...gle.com, gautham.shenoy@....com
Subject: Re: [RFC PATCH 04/19] fs/resctrl: Add the documentation for Global
 Memory Bandwidth Allocation

Hi Tony,

On 2/2/26 18:00, Luck, Tony wrote:
> On Wed, Jan 21, 2026 at 03:12:42PM -0600, Babu Moger wrote:
>> +Global Memory bandwidth Allocation
>> +-----------------------------------
>> +
>> +AMD hardware supports Global Memory Bandwidth Allocation (GMBA) provides
>> +a mechanism for software to specify bandwidth limits for groups of threads
>> +that span across multiple QoS domains. This collection of QOS domains is
>> +referred to as GMBA control domain. The GMBA control domain is created by
>> +setting the same GMBA limits in one or more QoS domains. Setting the default
>> +max_bandwidth excludes the QoS domain from being part of GMBA control domain.
> I don't see any checks that the user sets the *SAME* GMBA limits.
>
> What happens if the user ignores the dosumentation and sets different
> limits?

Good point. Adding checks could be challenging when users update each 
schema individually with different values. We don't know which one value 
is the one he is intending to keep.

> ... snip ...
>
> +  # cat schemata
> +    GMB:0=2048;1=2048;2=2048;3=2048
> +     MB:0=4096;1=4096;2=4096;3=4096
> +     L3:0=ffff;1=ffff;2=ffff;3=ffff
> +
> +  # echo "GMB:0=8;2=8" > schemata
> +  # cat schemata
> +    GMB:0=   8;1=2048;2=   8;3=2048
> +     MB:0=4096;1=4096;2=4096;3=4096
> +     L3:0=ffff;1=ffff;2=ffff;3=ffff
>
> Can the user go on to set:
>
>     # echo "GMB:1=10;3=10" > schemata
>
> and have domains 0 & 2 with a combined 8GB limit,
> while domains 1 & 3 run with a combined 10GB limit?
> Or is there a single "GMBA domain"?

In that case, it  is still treated as a single GMBA domain, but the 
behavior becomes unpredictable. The hardware expert mentioned that it 
will default to the lowest value among all inputs in this case, 8GB.


> Will using "2048" as the "this domain isn't limited
> by GMBA" value come back to haunt you when some
> system has much more than 2TB bandwidth to divide up?

It is actually 4096 (4TB). I made a mistake in the example.  I am 
assuming it may not an issue in the current generation.

It is expected to go up in next generation.

GMB:0=4096;1=4096;2=4096;3=4096;
    MB:0=8192;1=8192;2=8192;3=8192;
     L3:0=ffff;1=ffff;2=ffff;3=ffff


>
> Should resctrl have a non-numeric "unlimited" value
> in the schemata file for this?

The value 4096 corresponds to 12th bit set.  It is called U-bit. If the 
U bit is set then that domain is not part of the GMBA domain.

I was thinking of displaying the "U" in those cases.  It may be good 
idea to do something like this.

GMB:0=      8;1=      U;2=     8 ;3=      U;
    MB:0=8192;1=8192;2=8192;3=8192;
     L3:0=ffff;1=ffff;2=ffff;3=ffff


>
> The "mba_MBps" feature used U32_MAX as the unlimited
> value. But it looks somewhat ugly in the schemata
> file:
Yes, I agree. Non-numeric would have been better.

Thanks

Babu