Re: [PATCH v2] x86/resctrl: Fix buggy overflow when reactivating previously Unavailable RMID

[Babu Moger <bmoger@amd.com>]

Hi Reinette,

On 10/8/25 21:00, Reinette Chatre wrote:
> Hi Babu,
>
> On 10/8/25 12:39 PM, Babu Moger wrote:
>> Users can create as many monitoring groups as the number of RMIDs supported
>> by the hardware. However, on AMD systems, only a limited number of RMIDs
>> are guaranteed to be actively tracked by the hardware. RMIDs that exceed
>> this limit are placed in an "Unavailable" state. When a bandwidth counter
>> is read for such an RMID, the hardware sets MSR_IA32_QM_CTR.Unavailable
>> (bit 62).
> To make this context complete I think you can append something like:
> 	When such an RMID starts being tracked again the hardware counter is
> 	reset to zero. MSR_IA32_QM_CTR.Unavailable remains set on first read after
> 	tracking re-starts and is clear on all subsequent reads as long as the
> 	RMID is tracked.
Sure. Looks good.
>
>> The problem occurs when an RMID transitions from the “Unavailable” state
> Which problem? (Please let changelog stand on its own and not be continuation of subject)

Sure.


>
>> back to the active state. When this happens, the hardware resets the
>> counter to zero, but the kernel compares this new smaller value with the
>> previously saved MSR value and mistakenly interprets it as an overflow.
> I do not think this is just about overflow. Certainly this is the
> most visible symptom but the stored counter value may also be smaller than the new
> counter value resulting in undercounting of bandwidth? (ignoring that not
> counting at all while RMID is unavailable is technically also undercounting).
Yes. That can also happen during that window.
>
> Would something like below be accurate?
>
> 	resctrl miscounts the bandwidth events after an RMID transitions
> 	from the "Unavailable" state back to being tracked. This happens
> 	because when the hardware starts counting again after resetting the counter to
> 	zero, resctrl in turn compares the new count against the counter value
> 	stored from the previous time the RMID was tracked. This results in resctrl
> 	computing an event value that is either undercounting (when new counter is more than
> 	stored counter)	or a mistaken overflow (when new counter is less than stored counter).
Sure,
>
> If you agree with the summary then please update the subject to match. For example,
> "x86/resctrl: Fix miscount of bandwidth event when reactivating previously Unavailable RMID"
Sure.
>
> I think Dave's feedback about changelog length is valid. The changelog can present the
> fix at this point and leave the detailed description of the overflow scenario to the end of
> changelog with a heading that reader can use to decide to skip over if problem is clear or use as
> reference to see the problem in action.
>
> I also recommend that the fix be specific and avoid vague statement like "to resolve the issue".
> For example,
>
> 	Reset the stored value (arch_mbm_state::prev_msr) of MSR_IA32_QM_CTR to zero
> 	whenever the RMID is in the "Unavailable" state to ensure accurate
> 	counting after the RMID resets to zero when it starts to be tracked again

Looks good.


>
>> Problem scenario:
> The portion below can have a heading to help reader identify its purpose. For example,
>
> Example scenario that results in mistaken overflow
> ==================================================
>
Sure.
>> 1. The resctrl filesystem is mounted, and a task is assigned to a
>>     monitoring group.
>>
>>     $mount -t resctrl resctrl /sys/fs/resctrl
>>     $mkdir /sys/fs/resctrl/mon_groups/test1/
>>     $echo 1234 > /sys/fs/resctrl/mon_groups/test1/tasks
>>
>>     $cat /sys/fs/resctrl/mon_groups/test1/mon_data/mon_L3_*/mbm_total_bytes
>>     21323            <- Total bytes on domain 0
>>     "Unavailable"    <- Total bytes on domain 1
>>
>>     Task is running on domain 0. Counter on domain 1 is "Unavailable".
>>
>> 2. The task runs on domain 0 for a while and then moves to domain 1. The
>>     counter starts incrementing on domain 1.
>>
>>     $cat /sys/fs/resctrl/mon_groups/test1/mon_data/mon_L3_*/mbm_total_bytes
>>     7345357          <- Total bytes on domain 0
>>     4545             <- Total bytes on domain 1
>>
>>
>> 3. At some point, the RMID in domain 0 transitions to the "Unavailable"
>>     state because the task is no longer executing in that domain.
>>
>>     $cat /sys/fs/resctrl/mon_groups/test1/mon_data/mon_L3_*/mbm_total_bytes
>>     "Unavailable"    <- Total bytes on domain 0
>>     434341           <- Total bytes on domain 1
>>
>> 4.  Since the task continues to migrate between domains, it may eventually
>>      return to domain 0.
>>
>>      $cat /sys/fs/resctrl/mon_groups/test1/mon_data/mon_L3_*/mbm_total_bytes
>>      17592178699059  <- Overflow on domain 0
>>      3232332         <- Total bytes on domain 1
>>
> Is below intended to be indented?
Removed the indentation.
>>      In this case, the RMID on domain 0 transitions from “Unavailable”
>>      state to the active state. The hardware sets MSR_IA32_QM_CTR.Unavailable
> "active state" -> "tracked state" (to be consistent with terminology - not sure what
> is preferred between "active" and "tracked" but please be consistent)
Changed it to active state.
>
>>      (bit 62) when the counter is read and begins tracking the RMID counting
>>      from 0. Subsequent reads succeed but may return a value smaller than the
> "may return" -> "returns"
Sure.
>>      previously saved MSR value (7345357). Consequently, the kernel’s overflow
> "the kernel’s" -> "resctrl's"?
Sure.
>>      logic is triggered—it compares the previous value (7345357) with the new,
>>      smaller value and incorrectly interprets this as a counter overflow,
>>      adding a large delta. In reality, this is a false positive: the counter
>>      did not overflow but was simply reset when the RMID transitioned from
>>      “Unavailable” back to active.
> Here is what I do to check for non-ascii characters:
> $ b4 am <message ID>
> $ grep -P '[^\t\n\x20-\x7E]' <downloaded patch>
>
> Could you please try it out on this patch and fix the matches?

Yes. Now I see. Thanks fixed it.


>> Reset the stored value (arch_mbm_state::prev_msr) of MSR_IA32_QM_CTR, used
>> for handling counter overflows, whenever the RMID transitions to the
>> “Unavailable” state to resolve the issue.
>>
>> Here is the text from APM [1] available from [2].
>>
>> "In PQOS Version 2.0 or higher, the MBM hardware will set the U bit on the
>> first QM_CTR read when it begins tracking an RMID that it was not
>> previously tracking. The U bit will be zero for all subsequent reads from
>> that RMID while it is still tracked by the hardware. Therefore, a QM_CTR
>> read with the U bit set when that RMID is in use by a processor can be
>> considered 0 when calculating the difference with a subsequent read."
>>
>> [1] AMD64 Architecture Programmer's Manual Volume 2: System Programming
>>      Publication # 24593 Revision 3.41 section 19.3.3 Monitoring L3 Memory
>>      Bandwidth (MBM).
>>
>> Cc: stable@vger.kernel.org # needs adjustments for <= v6.17
> Tag ordering guide "Ordering of commit tags" found in
> Documentation/process/maintainer-tip.rst places the "Cc" just before
> the "Link:" tag.

Sure.

Thanks

Babu