linux-kernel - Re: [patch 4/4] sched/mmcid: Optimize transitional CIDs when scheduling out

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Message-ID: <50542cbe-8867-47fb-878e-0cff4b926eef@efficios.com>
Date: Fri, 30 Jan 2026 10:50:38 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Thomas Gleixner <tglx@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Cc: Ihor Solodrai <ihor.solodrai@...ux.dev>,
 Shrikanth Hegde <sshegde@...ux.ibm.com>,
 Peter Zijlstra <peterz@...radead.org>,
 Michael Jeanson <mjeanson@...icios.com>
Subject: Re: [patch 4/4] sched/mmcid: Optimize transitional CIDs when
 scheduling out

On 2026-01-29 16:20, Thomas Gleixner wrote:
> During the investigation of the various transition mode issues
> instrumentation revealed that the amount of bitmap operations can be
> significantly reduced when a task with a transitional CID schedules out
> after the fixup function completed and disabled the transition mode.
> 
> At that point the mode is stable and therefore it is not required to drop
> the transitional CID back into the pool. As the fixup is complete the
> potential exhaustion of the CID pool is not longer possible, so the CID can
> be transferred to the scheduling out task or to the CPU depending on the
> current ownership mode. This is now possible because mm_cid::mode contains
> both the ownership state and the transition bit so the racy snapshot is
> valid under all circumstances because a subsequent modification of the
> mode is serialized by the corresponding runqueue lock.

AFAIU the mc->mode updates are serialized by the mm->mm_cid.lock
and not the runqueue locks. What am I missing ?

[...]

> +	/*
> +	 * If transition mode is done, transfer ownership when the CID is
> +	 * within the convergion range. Otherwise the next schedule in will

convergence

> +	 * have to allocate or converge

add final ".".

Thanks,

Mathieu

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