Re: [PATCH v9 10/13] x86/resctrl: Add sysfs interface to write mbm_total_bytes_config

["Moger, Babu" <babu.moger@amd.com>]

Hi Reinette,

On 12/20/22 11:32, Reinette Chatre wrote:
> Hi Babu,
>
> On 12/19/2022 11:28 AM, Moger, Babu wrote:
>> [AMD Official Use Only - General]
>>
>> Hi Reinette,
>>
>>> -----Original Message-----
>>> From: Reinette Chatre <reinette.chatre@intel.com>
>>> Sent: Thursday, December 15, 2022 12:25 PM
>>> To: Moger, Babu <Babu.Moger@amd.com>; corbet@lwn.net;
>>> tglx@linutronix.de; mingo@redhat.com; bp@alien8.de
>>> Cc: fenghua.yu@intel.com; dave.hansen@linux.intel.com; x86@kernel.org;
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>>> peternewman@google.com
>>> Subject: Re: [PATCH v9 10/13] x86/resctrl: Add sysfs interface to write
>>> mbm_total_bytes_config
>>>
>>> Hi Babu,
>>>
>>> On 12/1/2022 7:37 AM, Babu Moger wrote:
> ...
>
>>>> +	/*
>>>> +	 * When an Event Configuration is changed, the bandwidth counters
>>>> +	 * for all RMIDs and Events will be cleared by the hardware. The
>>>> +	 * hardware also sets MSR_IA32_QM_CTR.Unavailable (bit 62) for
>>>> +	 * every RMID on the next read to any event for every RMID.
>>>> +	 * Subsequent reads will have MSR_IA32_QM_CTR.Unavailable (bit 62)
>>>> +	 * cleared while it is tracked by the hardware. Clear the
>>>> +	 * mbm_local and mbm_total counts for all the RMIDs.
>>>> +	 */
>>>> +	resctrl_arch_reset_rmid_all(r, d);
>>> If I understand correctly the expectation is that when user space read counters
>>> (via mon_data files) right after the configuration was changed then this read
>>> will return "Unavailable" and then the next read will return data.
>>>
>>> If this is the case then I think a snippet about this user experience would be
>>> helpful to add to the documentation.
>> Ok.  How about this in the documentation?
>>
>> "When an event configuration is changed, the bandwidth counters for all the RMIDs and the events will be cleared for that domain.
>> The next read for every RMID will report "Unavailable" and subsequent reads will report the valid value."
>>
>>
> Thinking about this more ... why are the counters for all eventids cleared
> when only one eventid's configuration is changed?

Its because of the way U-Bit tracking is implemented. The U-bit is tracked
per RMID basis, not on a Per-Event + Per RMID basis. Therefore, resetting
the U-Bit for 1 will reset it for both.

This is what I got from h/w team.

Thanks

Babu

>
> Reinette
>
>
-- 
Thanks
Babu Moger