linux-kernel - Re: [patch V3 20/20] sched/mmcid: Switch over to the new mechanism

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Message-ID: <72cc0b5f-b1e6-43d4-aaac-ef3ff2e20a01@efficios.com>
Date: Thu, 30 Oct 2025 12:07:22 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Thomas Gleixner <tglx@...utronix.de>, LKML <linux-kernel@...r.kernel.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
 Gabriele Monaco <gmonaco@...hat.com>, Michael Jeanson
 <mjeanson@...icios.com>, Jens Axboe <axboe@...nel.dk>,
 "Paul E. McKenney" <paulmck@...nel.org>,
 "Gautham R. Shenoy" <gautham.shenoy@....com>,
 Florian Weimer <fweimer@...hat.com>, Tim Chen <tim.c.chen@...el.com>,
 Yury Norov <yury.norov@...il.com>, Shrikanth Hegde <sshegde@...ux.ibm.com>
Subject: Re: [patch V3 20/20] sched/mmcid: Switch over to the new mechanism

On 2025-10-29 09:09, Thomas Gleixner wrote:

> @@ -10702,10 +10758,43 @@ void sched_mm_cid_exit(struct task_struc
>   
>   	if (!mm || !t->mm_cid.active)
>   		return;
> +	/*
> +	 * Ensure that only one instance is doing MM CID operations within
> +	 * a MM. The common case is uncontended. The rare fixup case adds
> +	 * some overhead.
> +	 */
> +	scoped_guard(mutex, &mm->mm_cid.mutex) {

When exiting from a mm where mm->mm_cid.users == 1 (read with
READ_ONCE()), can we do this without holding the mutex as an
optimization ?

> +	 * Synchronize eventally pending work to ensure that there are no

eventually

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com