linux-kernel - Re: [patch V3 03/20] sched/mmcid: Cacheline align MM CID storage

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Message-ID: <996b1e7a-59df-4638-9dd8-ff6edbe8d1d4@efficios.com>
Date: Wed, 29 Oct 2025 11:39:45 -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 03/20] sched/mmcid: Cacheline align MM CID storage

On 2025-10-29 09:08, Thomas Gleixner wrote:
[...]
>   struct mm_cid_pcpu {
>   	unsigned int	cid;
> -};
> +}____cacheline_aligned_in_smp;

What's the point in cacheline aligning this per-CPU variable ?
Should we expect other accesses to per-CPU variables sharing the
same cache line to update them frequently from remote CPUs ?

I did not cacheline align it expecting that per-CPU variables are
typically updated from their respective CPUs. So perhaps reality
don't match my expectations, but that's news to me.

> @@ -126,7 +126,7 @@ struct mm_mm_cid {
[...]
> -};
> +}____cacheline_aligned_in_smp;

OK for this cacheline align.

Thanks,

Mathieu

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