linux-kernel - Re: [patch V4 16/20] sched/mmcid: Provide new scheduler CID mechanism

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Message-ID: <1a531839-8380-4f62-8299-fd7556448cca@efficios.com>
Date: Mon, 17 Nov 2025 14:40:47 -0500
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 V4 16/20] sched/mmcid: Provide new scheduler CID mechanism

On 2025-11-16 15:49, Thomas Gleixner wrote:
[...]
> The current upstream implementation tries to keep the CID with the task
> even in overcommit situations, which complicates task migration. It also
> has to do the CID space consolidation work from a task work in the exit to
> user space path. As that work is assigned to a random task related to a MM
> this can inflict unwanted exit latencies.
> 
> Implement the context switch parts of a strict ownership mechanism to
> address this.
> 
> This removes most of the work from the task which schedules out. Only
> during transitioning from per CPU to per task ownership it is required to
> drop the CID when leaving the CPU to prevent CID space exhaustion. Other
> than that scheduling out is just a single check and branch.

Same comment as for prior patch.

[...]

>   struct mm_mm_cid {
> +	/* Hotpath read mostly members */
>   	struct mm_cid_pcpu	__percpu *pcpu;
> +	unsigned int		percpu;
> +	unsigned int		transit;

[...]

> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -10495,6 +10495,7 @@ void mm_init_cid(struct mm_struct *mm, s
>   		per_cpu_ptr(pcpu, cpu)->cid = MM_CID_UNSET;
>   
>   	mm->mm_cid.max_cids = 0;
> +	mm->mm_cid.percpu = 0;

I'm unsure where it appears (maybe later in the series ?), but
I would have expected:

         mm->mm_cid.transit = 0;

here.

Other than those comments:

Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>

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