linux-kernel - Re: [PATCH v8 6/7] x86/resctrl: Add write option to "mba_MBps

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Message-ID: <7f5f1f66-df3e-bebd-4786-7fe8a8115f05@intel.com>
Date: Fri, 1 Nov 2024 17:57:44 -0700
From: Fenghua Yu <fenghua.yu@...el.com>
To: "Luck, Tony" <tony.luck@...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: James Morse <james.morse@....com>, Jamie Iles <quic_jiles@...cinc.com>,
	Babu Moger <babu.moger@....com>, Randy Dunlap <rdunlap@...radead.org>,
	"Shaopeng Tan (Fujitsu)" <tan.shaopeng@...itsu.com>,
	"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 v8 6/7] x86/resctrl: Add write option to "mba_MBps_event"
 file

Hi, Tony,

On 11/1/24 16:55, Luck, Tony wrote:
>>> +   if (!strcmp(buf, "mbm_local_bytes")) {
>>> +           if (is_mbm_local_enabled())
>>> +                   rdtgrp->mba_mbps_event = QOS_L3_MBM_LOCAL_EVENT_ID;
>>> +           else
>>> +                   ret = -ENXIO;
>>> +   } else if (!strcmp(buf, "mbm_total_bytes")) {
>>> +           if (is_mbm_total_enabled())
>>> +                   rdtgrp->mba_mbps_event = QOS_L3_MBM_TOTAL_EVENT_ID;
>>
>>
>> User may think each time toggling the local/total event will effect MBA.
>> And they may create usage case like frequently changing the events to
>> maintain/adjust both total and local within bw boundary.
>>
>> But toggling mba_mbps_event faster than 1sec doesn't have any effect on
>> MBA SC because MBA SC is called every one second.
>>
>> Maybe need to add a ratelimit of 1 second on calling this function? And
>> adding info in the document that toggling speed should be slower than 1
>> second?
> 
> The limit would need to be per ctrl_mon group, not on calls to this function.
> It's perfectly ok to switch multiple groups in a short interval.

Agree.

> 
> I'm not sure how to rate limit here. I could add a delay so that the write()
> call blocks until enough time passes before making the change. But
> what should I do if a user submits more writes to the file? Queue them
> all and apply at one second intervals?

Maybe define "mba_mbps_last_time" in rdtgroup. Then

if (time_before(jiffies, rdtgrp->mba_mbps_last_time + HZ) {
	rdt_last_cmd_printf("Too fast (>1/s) mba_MBps event change)\n");
         rdtgroup_kn_unlock(of->kn);
	return -EAGAIN;
}
rdtgrp->mba_mbps_last_time = jiffies;

> 
> Maybe it would be better to just to add some additional text to the
> documentation pointing out that resctrl only checks bandwidth once
> per second to make throttling adjustments. So changes to the event
> will only have effect after some seconds have passed?


Add additional text would be great.

Thanks.

-Fenghua