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

[Mathieu Desnoyers]

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