linux-kernel - Re: [PATCH v17 25/33] fs/resctrl: Provide interface to update the event configurations

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Message-ID: <e2ce443e-f26c-498b-a539-4c907d12a905@amd.com>
Date: Wed, 3 Sep 2025 17:03:01 -0500
From: "Moger, Babu" <babu.moger@....com>
To: Reinette Chatre <reinette.chatre@...el.com>, corbet@....net,
 tony.luck@...el.com, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com
Cc: Dave.Martin@....com, james.morse@....com, x86@...nel.org, hpa@...or.com,
 akpm@...ux-foundation.org, rostedt@...dmis.org, paulmck@...nel.org,
 pawan.kumar.gupta@...ux.intel.com, kees@...nel.org, arnd@...db.de,
 fvdl@...gle.com, seanjc@...gle.com, thomas.lendacky@....com,
 yosry.ahmed@...ux.dev, xin@...or.com, sohil.mehta@...el.com,
 kai.huang@...el.com, xiaoyao.li@...el.com, peterz@...radead.org,
 mario.limonciello@....com, xin3.li@...el.com, perry.yuan@....com,
 chang.seok.bae@...el.com, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, peternewman@...gle.com, eranian@...gle.com,
 gautham.shenoy@....com
Subject: Re: [PATCH v17 25/33] fs/resctrl: Provide interface to update the
 event configurations

Hi Reinette,

On 9/3/2025 3:52 PM, Reinette Chatre wrote:
> Hi Babu,
>
> On 9/3/25 11:32 AM, Moger, Babu wrote:
>> Hi Reinette,
>>
>> On 9/3/25 12:55, Reinette Chatre wrote:
>>> Hi Babu,
>>>
>>> On 9/3/25 10:38 AM, Moger, Babu wrote:
>>>> On 9/2/25 21:41, Reinette Chatre wrote:
>>>>> On 8/14/25 7:25 PM, Babu Moger wrote:
>>>>>> +
>>>>>> +ssize_t event_filter_write(struct kernfs_open_file *of, char *buf, size_t nbytes,
>>>>>> +			   loff_t off)
>>>>>> +{
>>>>>> +	struct mon_evt *mevt = rdt_kn_parent_priv(of->kn);
>>>>>> +	struct rdt_resource *r;
>>>>>> +	u32 evt_cfg = 0;
>>>>>> +	int ret = 0;
>>>>>> +
>>>>>> +	/* Valid input requires a trailing newline */
>>>>>> +	if (nbytes == 0 || buf[nbytes - 1] != '\n')
>>>>>> +		return -EINVAL;
>>>>>> +
>>>>>> +	buf[nbytes - 1] = '\0';
>>>>>> +
>>>>>> +	cpus_read_lock();
>>>>>> +	mutex_lock(&rdtgroup_mutex);
>>>>>> +
>>>>>> +	rdt_last_cmd_clear();
>>>>>> +
>>>>>> +	r = resctrl_arch_get_resource(mevt->rid);
>>>>>> +	if (!resctrl_arch_mbm_cntr_assign_enabled(r)) {
>>>>>> +		rdt_last_cmd_puts("mbm_event counter assignment mode is not enabled\n");
>>>>>> +		ret = -EINVAL;
>>>>>> +		goto out_unlock;
>>>>>> +	}
>>>>>> +
>>>>>> +	ret = resctrl_parse_mem_transactions(buf, &evt_cfg);
>>>>>> +	if (!ret && mevt->evt_cfg != evt_cfg) {
>>>>> ... is evt_cfg of 0 (a) a valid value (that will not cause hardware to fault) and
>>>>> (b) a reasonable value to allow?
>>>>>
>>>> The value evt_cfg = 0 is valid and permitted for both ABMC and BMEC. I
>>>> have confirmed here through verification and testing. In that case, the
>>>> event counter will not be monitoring anything.
>>> Thank you for checking.
>>>
>>> I do not know what a use case for this would be but I also do not see why kernel
>>> should prevent user space from doing this. Looks like event_filter_show() will then
>>> just print a '\n' that is expected. Are counters expected to always return 0 in this
>>> case or will they return an error? I am not clear on what qualifies as "invalid counter
>>> configuration" for ABMC that results in RMID_VAL_ERROR.
>> The event counters return 0 in this case. I would not think this as an
>> invalid event configuration.
>>
>> One option is to add a text in last_cmd_status from event_filter_show().
>>
>> rdt_last_cmd_printf("Event %s is not configured to monitor any memory
>> transactions\n", mevt->name);
>
> event_filter_show() does not return an error in this case so a user may not
> check last_cmd_status since the doc specifies that "If the command was successful,
> it will read as "ok"." Since the write() succeeded we should not fail the associated
> show() to prompt the user to check last_cmd_status though.
>
> Even so, an additional message does not seem necessary to me because as I understand
> event_filter_show() returns an empty (just a '\n') file that I think
> already reflects that no memory transactions are configured. It remains an awkward
> use case to me but from what you stated this is valid for the hardware, it is what the
> user requested, and the user can use interface to confirm the state. It does not seem
> like changes are needed for this scenario.

Sure.  Sounds good.

Thanks

Babu