Re: [RFC PATCH v6] sched: Fix performance regression introduced by mm_cid

[Aaron Lu]

On Thu, Apr 13, 2023 at 06:33:56PM -0400, Mathieu Desnoyers wrote:
> Introduce per-mm/cpu current concurrency id (mm_cid) to fix a PostgreSQL
> sysbench regression reported by Aaron Lu.
> 
> Keep track of the currently allocated mm_cid for each mm/cpu rather than
> freeing them immediately on context switch. This eliminates most atomic
> operations when context switching back and forth between threads
> belonging to different memory spaces in multi-threaded scenarios (many
> processes, each with many threads). The per-mm/per-cpu mm_cid values are
> serialized by their respective runqueue locks.
> 
> Thread migration is handled by introducing a task-work executed
> periodically, similarly to NUMA work, which delays reclaim of cid
> values when they are unused for a period of time.
> 
> Keep track of the allocation time for each per-cpu cid, and let the task
> work clear them when they are observed to be older than
> SCHED_MM_CID_PERIOD_NS and unused.
> 
> This fix is going for a task-work and delayed reclaim approach rather
> than adding hooks to migrate-from and migrate-to because migration
> happens to be a hot path for various real-world workloads.
> 
> Because we want to ensure the mm_cid converges towards the smaller
> values as migrations happen, the prior optimization that was done when
> context switching between threads belonging to the same mm is removed,
> because it could delay the lazy release of the destination runqueue
> mm_cid after it has been replaced by a migration. Removing this prior
> optimization is not an issue performance-wise because the introduced
> per-mm/per-cpu mm_cid tracking also covers this more specific case.

I was wondering, if a thread was migrated to all possible cpus in the
SCHED_MM_CID_PERIOD_NS window, its mm_cidmask will be full. For user
space, if cid can be the full set of cpus, then it will have to prepare
storage for the full set. Then what's the point of doing compaction? Or
do I understand it wrong?