linux-kernel - Re: [PATCH] x86/resctrl: Implement new MBA

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Date: Wed, 17 Jan 2024 16:26:01 -0800
From: Reinette Chatre <reinette.chatre@...el.com>
To: Tony Luck <tony.luck@...el.com>, Fenghua Yu <fenghua.yu@...el.com>, "Peter
 Newman" <peternewman@...gle.com>, <x86@...nel.org>
CC: James Morse <james.morse@....com>, Jamie Iles <quic_jiles@...cinc.com>,
	Babu Moger <babu.moger@....com>, Xiaochen Shen <xiaochen.shen@...el.com>,
	<linux-kernel@...r.kernel.org>, <patches@...ts.linux.dev>
Subject: Re: [PATCH] x86/resctrl: Implement new MBA_mbps throttling heuristic

Hi Tony,

Please note in the subject and changelog that the mount option
is mba_MBps. The subject and first sentence of changelog seems
to fuse the mount option with the software controller enabled by it,
but I do not think doing so dilutes the message.

On 1/9/2024 2:00 PM, Tony Luck wrote:
> The MBA_mbps feedback loop increases throttling when a group is using
> more bandwidth than the target set by the user in the schemata file, and
> decreases throttling when below target.
> 
> To avoid possibly stepping throttling up and down on every poll a
> flag "delta_comp" is set whenever throttling is changed to indicate
> that the actual change in bandwidth should be recorded on the next
> poll in "delta_bw". Throttling is only reduced if the current bandwidth
> plus delta_bw is below the user target.
> 
> This algorithm works well if the workload has steady bandwidth needs.
> But it can go badly wrong if the workload moves to a different phase
> just as the throttling level changed. E.g. if the workload becomes
> essentially idle right as throttling level is increased, the value
> calculated for delta_bw will be more or less the old bandwidth level.
> If the workload then resumes, Linux may never reduce throttling because
> current bandwidth plu delta_bw is above the target set by the user.

"plu" -> "plus"

> 
> Implement a simpler heuristic by assuming that in the worst case the
> currently measured bandwidth is being controlled by the current level of
> throttling. Compute how much it may increase if throttling is relaxed to
> the next higher level. If that is still below the user target, then it
> is ok to reduce the amount of throttling.
> 
> Fixes: ba0f26d8529c ("x86/intel_rdt/mba_sc: Prepare for feedback loop")
> Reported-by: Xiaochen Shen <xiaochen.shen@...el.com>
> Signed-off-by: Tony Luck <tony.luck@...el.com>
> ---

Thank you very much for catching and fixing this.

With the issues ("MBA_mbps" -> "mba_MBps", "plu" -> "plus") addressed:

Reviewed-by: Reinette Chatre <reinette.chatre@...el.com>

Reinette