Messages in this thread | | | Date | Thu, 24 Nov 2022 14:21:25 +0000 | Subject | Re: [PATCH 07/18] x86/resctrl: Move CLOSID/RMID matching and setting to use helpers | From | James Morse <> |
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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@redhat.com>
(CC here also stands for celebrate-contribution!)
>> Signed-off-by: James Morse <james.morse@arm.com> > > Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Thanks!
James
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