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

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Date: Tue, 16 Jan 2024 11:55:07 -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 Xiaochen,

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.
> 
> 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>
> ---
> 
> This patch was previously posted in:
> 
>   https://lore.kernel.org/lkml/ZVU+L92LRBbJXgXn@agluck-desk3/#t
> 
> as part of a proposal to allow the mba_MBps mount option to base its
> feedback loop input on total bandwidth instead of local bandwidth.
> Sending it now as a standalone patch because Xiaochen reported that
> real systems have experienced problems when delta_bw is incorrectly
> calculated.
> 

Does this new heuristic fix the problems you observe? If so, would you be
willing to provide a "Tested-by" tag?

Thank you.

Reinette