linux-kernel - Re: [PATCH v2 00/17] x86/resctrl : Support AMD Assignable Bandwidth Monitoring Counters (ABMC)

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Date: Fri, 8 Mar 2024 09:13:03 -0800
From: Reinette Chatre <reinette.chatre@...el.com>
To: Peter Newman <peternewman@...gle.com>
CC: <babu.moger@....com>, James Morse <james.morse@....com>, <corbet@....net>,
	<fenghua.yu@...el.com>, <tglx@...utronix.de>, <mingo@...hat.com>,
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	<hpa@...or.com>, <paulmck@...nel.org>, <rdunlap@...radead.org>,
	<tj@...nel.org>, <peterz@...radead.org>, <yanjiewtw@...il.com>,
	<kim.phillips@....com>, <lukas.bulwahn@...il.com>, <seanjc@...gle.com>,
	<jmattson@...gle.com>, <leitao@...ian.org>, <jpoimboe@...nel.org>,
	<rick.p.edgecombe@...el.com>, <kirill.shutemov@...ux.intel.com>,
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	<daniel.sneddon@...ux.intel.com>, <pbonzini@...hat.com>,
	<sandipan.das@....com>, <ilpo.jarvinen@...ux.intel.com>,
	<maciej.wieczor-retman@...el.com>, <linux-doc@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <eranian@...gle.com>
Subject: Re: [PATCH v2 00/17] x86/resctrl : Support AMD Assignable Bandwidth
 Monitoring Counters (ABMC)

Hi Peter,

On 3/7/2024 3:14 PM, Peter Newman wrote:
> On Thu, Mar 7, 2024 at 2:53 PM Reinette Chatre
> <reinette.chatre@...el.com> wrote:
>> On 3/7/2024 2:33 PM, Peter Newman wrote:
>>> On Thu, Mar 7, 2024 at 12:41 PM Reinette Chatre
>>> <reinette.chatre@...el.com> wrote:
>>>> I understood the proposed interface appeared to focus on one use case
>>>> while the goal is to find an interface to support all requirements.
>>>> With this proposed interface it it possible to make large scale changes
>>>> with a single sysfs write.
>>>
>>> Ok I see you requested[1] one such example earlier.
>>>
>>> From what I've read, is this what you had in mind of reassigning 32
>>> counters from the first 16 groups to the next?
>>>
>>> I had found that it's hard to get a single write() syscall out of a
>>> string containing newlines, so I'm using one explicit call:
>>
>> Apologies but this is not clear to me, could you please elaborate?
>>
>> If you are referring to testing via shell you can try ANSI-C Quoting like:
>>         echo -n $'c1/m1/00=_\nc2/m2/00=_\n'
> 
> The echo command uses buffered output through printf() and
> putchar()[1]. The behavior of the buffering seems to be a write() call
> after each newline, causing the kernel to see the request below as 32
> individual commands.

I see different behavior. Just to confirm I added a printk() in
rdtgroup_schemata_write():

diff --git a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
index 7471f6b747b6..00d9809a1bac 100644
--- a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
+++ b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
@@ -384,6 +384,7 @@ ssize_t rdtgroup_schemata_write(struct kernfs_open_file *of,
 
 	rdt_staged_configs_clear();
 
+	printk("%s:%d Parsing %s\n", __func__, __LINE__, buf);
 	while ((tok = strsep(&buf, "\n")) != NULL) {
 		resname = strim(strsep(&tok, ":"));
 		if (!tok) {

I believe the behavior you are referring to is when user does something
like:
	# echo -e "MB:0=90\nL3:0=7ff0" > schemata

Then, indeed it is two separate writes:
[  636.391304] rdtgroup_schemata_write:387 Parsing MB:0=90
[  636.397773] rdtgroup_schemata_write:387 Parsing L3:0=7ff0

When using ANSI-C Quoting I see a single write:

	# echo -n $'MB:0=90\nL3:0=7ff0\n' > schemata

[  655.879331] rdtgroup_schemata_write:387 Parsing MB:0=90
               L3:0=7ff0

Reinette