Re: [PATCH 07/18] x86/resctrl: Move CLOSID/RMID matching and setting to use helpers

[James Morse]

Hi Valentin,

On 18/11/2022 15:49, Valentin Schneider wrote:
> On 21/10/22 13:11, James Morse wrote:
>> When switching tasks, the CLOSID and RMID that the new task should
>> use are stored in struct task_struct. For x86 the CLOSID known by resctrl,
>> the value in task_struct, and the value written to the CPU register are
>> all the same thing.
>>
>> MPAM's CPU interface has two different PARTID's one for data accesses
>> the other for instruction fetch. Storing resctrl's CLOSID value in
>> struct task_struct implies the arch code knows whether resctrl is using
>> CDP.
>>
>> Move the matching and setting of the struct task_struct properties
>> to use helpers. This allows arm64 to store the hardware format of
>> the register, instead of having to convert it each time.
>>
>> __rdtgroup_move_task()s use of READ_ONCE()/WRITE_ONCE() ensures torn
>> values aren't seen as another CPU may schedule the task being moved
>> while the value is being changed. MPAM has an additional corner-case
>> here as the PMG bits extend the PARTID space. If the scheduler sees a
>> new-CLOSID but old-RMID, the task will dirty an RMID that the limbo code
>> is not watching causing an inaccurate count. x86's RMID are independent
>> values, so the limbo code will still be watching the old-RMID in this
>> circumstance.
>> To avoid this, arm64 needs both the CLOSID/RMID WRITE_ONCE()d together.
>> Both values must be provided together.

> I think I remember something about this... Is that about having them
> union'd and read/written as one? (just for my own curiosity)

Yup. Many moons ago you pointed out that storing partid/pmg separately lets the remote
CPUs load torn values. arm64 not only needs the values as a pair, it needs to read/write
them as a pair.


>> Because MPAM's RMID values are not unique, the CLOSID must be provided
>> when matching the RMID.
>>
>> CC: Valentin Schneider <vschneid@xxxxxxxxxx>

(CC here also stands for celebrate-contribution!)


>> Signed-off-by: James Morse <james.morse@xxxxxxx>
> 
> Reviewed-by: Valentin Schneider <vschneid@xxxxxxxxxx>

Thanks!

James