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Date:   Tue, 4 Apr 2023 10:57:49 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     linux-kernel@vger.kernel.org, Aaron Lu <aaron.lu@intel.com>
Subject: Re: [RFC PATCH] sched: Fix performance regression introduced by
 mm_cid
Message-ID: <20230404085749.GA284733@hirez.programming.kicks-ass.net>
References: <20230403181342.210896-1-mathieu.desnoyers@efficios.com>
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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..