Re: [PATCH v8 13/24] x86/resctrl: Queue mon_event_read() instead of sending an IPI

[James Morse]

Hi Babu,

On 03/01/2024 19:43, Moger, Babu wrote:
> On 12/15/23 11:43, James Morse wrote:
>> Intel is blessed with an abundance of monitors, one per RMID, that can be
>> read from any CPU in the domain. MPAMs monitors reside in the MMIO MSC,
>> the number implemented is up to the manufacturer. This means when there are
>> fewer monitors than needed, they need to be allocated and freed.
>>
>> MPAM's CSU monitors are used to back the 'llc_occupancy' monitor file. The
>> CSU counter is allowed to return 'not ready' for a small number of
>> micro-seconds after programming. To allow one CSU hardware monitor to be
>> used for multiple control or monitor groups, the CPU accessing the
>> monitor needs to be able to block when configuring and reading the
>> counter.
>>
>> Worse, the domain may be broken up into slices, and the MMIO accesses
>> for each slice may need performing from different CPUs.
>>
>> These two details mean MPAMs monitor code needs to be able to sleep, and
>> IPI another CPU in the domain to read from a resource that has been sliced.
>>
>> mon_event_read() already invokes mon_event_count() via IPI, which means
>> this isn't possible. On systems using nohz-full, some CPUs need to be
>> interrupted to run kernel work as they otherwise stay in user-space
>> running realtime workloads. Interrupting these CPUs should be avoided,
>> and scheduling work on them may never complete.
>>
>> Change mon_event_read() to pick a housekeeping CPU, (one that is not using
>> nohz_full) and schedule mon_event_count() and wait. If all the CPUs
>> in a domain are using nohz-full, then an IPI is used as the fallback.
>>
>> This function is only used in response to a user-space filesystem request
>> (not the timing sensitive overflow code).
>>
>> This allows MPAM to hide the slice behaviour from resctrl, and to keep
>> the monitor-allocation in monitor.c. When the IPI fallback is used on
>> machines where MPAM needs to make an access on multiple CPUs, the counter
>> read will always fail.

> Reviewed-by: Babu Moger <babu.moger@xxxxxxx>


Thanks!

James