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

[Peter Zijlstra <peterz@infradead.org>]

Sorry for being a little late to the party.

On Mon, Apr 03, 2023 at 02:13:42PM -0400, Mathieu Desnoyers wrote:

> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 0722859c3647..21466fdc4dc6 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -609,6 +609,14 @@ struct mm_struct {
>  		 * were being concurrently updated by the updaters.
>  		 */
>  		raw_spinlock_t cid_lock;
> +		/**
> +		 * @pcpu_cid: Per-cpu current cid.
> +		 *
> +		 * Keep track of the currently allocated mm_cid for each cpu.
> +		 * The per-cpu mm_cid values are serialized by their respective
> +		 * runqueue locks.
> +		 */
> +		int __percpu *pcpu_cid;

This *might* be a problem... Consider running *many* single threaded
processes, this could exhaust the limited per-cpu storage (32bit) or use
excessive memory on large CPU systems.

Consider having to allocate per-cpu storage on a 4K CPU system while
only running single threaded processes -- but *LOTS* of them..