linux-kernel - Re: [patch V3 13/20] sched/mmcid: Provide precomputed maximal value

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Message-ID: <36caa866-1f22-4825-aaf5-6e4d2629a4e2@efficios.com>
Date: Thu, 30 Oct 2025 10:23:09 -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 13/20] sched/mmcid: Provide precomputed maximal value

On 2025-10-29 09:09, Thomas Gleixner wrote:
> Reading mm::mm_users and mm:::mm_cid::nr_cpus_allowed everytime to compute

every time

> + * @max_cids:		The exclusive maximum CID value for allocation and convergance

convergence

> + * @users:		The number of tasks sharing this MM. Seperate from mm::mm_users

Separate

> + *			as that is modified by mmget()/mm_put() by other entities which
> + *			do not actually share the MM.
>    * @lock:		Spinlock to protect all fields except @pcpu. It also protects
>    *			the MM cid cpumask and the MM cidmask bitmap.
>    */
>   struct mm_mm_cid {
>   	struct mm_cid_pcpu	__percpu *pcpu;
> +	unsigned int		max_cids;
>   	unsigned int		nr_cpus_allowed;
> +	unsigned int		users;

I suspect this reintroduces false-sharing between the "users"
and "lock" fields (updated every time a thread is forked/exits)
and load of the pcpu pointer which is pretty much immutable.
This will slow down accesses to the percpu data in the scheduler
fast path.

>   	raw_spinlock_t		lock;
>   }____cacheline_aligned_in_smp;
>   #else /* CONFIG_SCHED_MM_CID */

Thanks,

Mathieu

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