Re: [PATCH v7 13/24] x86/resctrl: Queue mon_event_read() instead of sending an IPI

[Reinette Chatre <reinette.chatre@intel.com>]

Hi James,

On 10/25/2023 11:03 AM, 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.
> 
> Tested-by: Shaopeng Tan <tan.shaopeng@fujitsu.com>
> Tested-by: Peter Newman <peternewman@google.com>
> Reviewed-by: Shaopeng Tan <tan.shaopeng@fujitsu.com>
> Reviewed-by: Peter Newman <peternewman@google.com>
> Signed-off-by: James Morse <james.morse@arm.com>
> ---

Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>

Reinette