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

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Date: Wed, 17 Jan 2024 11:36:57 +0800
From: Xiaochen Shen <xiaochen.shen@...el.com>
To: Reinette Chatre <reinette.chatre@...el.com>,
 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>, linux-kernel@...r.kernel.org,
 patches@...ts.linux.dev, xiaochen.shen@...el.com
Subject: Re: [PATCH] x86/resctrl: Implement new MBA_mbps throttling heuristic

Hi Reinette and Tony,

On 1/17/2024 3:55, Reinette Chatre wrote:
> 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?

Yes. The patch fixed the problem. I will reply to the original thread to 
add "Tested-by" tag.
Thank you very much for help!

>
> Thank you.
>
> Reinette

Best regards,
Xiaochen